NEED HELP !!!! Mallory takes a dirt nap and damage to my front universals

[csutton7]

I think what might be killing some of these pumps is the start/stop/start thing they do--electric motors like to run not start and stop anyway it can't be that great for them---how may electric pumps do you know of that start/stop/start(run/stop/run) and how many at a reduced voltage for one of the run cycles--just doesn't make sense--yea Dodge had a reason, but probably some goof off trying to overthink something--I don't think you can hurt the ECM at all with the relay setup-well there's always a way, but it'd be bad luck---and I think the same goes for the VP44 with the pressure--look the pressure isn't sustained-the pump turns off after a couple of seconds--anyway that's my take on the whole thing---chris

 

[Un-Hemi]

Fellas, remember that until Scott's pump is autopsied, all this comment is purely speculation!

Dave, what a great idea! An external Mosfet could be used as a driver for the pump. I hadn't considered that, mostly because the ECM is probably using a Mosfet to pulse-width-modulate the drive to the pump. With an external Mosfet, at least you would know its operational parameters. You could use a whacking great big one! In pulse control systems, a Mosfet can control amazing voltages and currents relative to its size. Remember, however, that if the manual can be trusted #ad
the ECM lift pump driver is capable of supplying 12 Amps.

Tractornut, most every variable-speed drill uses pulse width speed control. It may be hard on the motor, but it appears to be popular proven technology. Proper circuit design prevents generation of switching transients and/or electrical noise.

Blew the schnozz out of a 30 amp fuse, eh? No comment!!!! With an 7. 5-amp fuse in line with the pump, you KNOW you're protected below the factory's alledged maximum of 12 Amps. Dave is bang-on about putting the fuse as close to the power source as possible... By the way, did you know that the fuse is there and sized to protect the WIRING, not the load?

Underdriving the pump could very well be the problem too. I haven't made any remarks about it, because I don't know what happens to the current draw when the pump voltage is too low. Could it be that the current goes up at lower voltage? Excess current could possibly fry the brushes and overheat the motor. Or does the current go down with the voltage?? Another research project for the Carter transfer-pump/mule/victim on Sunday!

Scott, From your description of your relay, I extrapolate that it has a metal can and screw terminals on it. If I'm incorrect, then ignore the next couple of sentences. I'm sure it's got lots of capacity and should work, *but* while you're picking up your diodes at Radio Shack (see below), pick up one of those plastic Bosch-style relays they sell. It may be more practical than that chunk you've got there, particularly regarding the coil current.

Some advice for the relay guys. Run right now to Radio Shack and buy some 1N400X (the X could be 1,2,3,4,5,6,7) diodes (cheap like Borscht) to install across the relay's coil. The striped end of the diode goes to the + (positive) terminal of the relay coil; the other end of the diode goes to the negative (or ground) terminal of the relay. Don't get it backwards, or it will look like a short across the coil. This diode will suppress the back EMF (bad thing) that is generated when the drive voltage is removed from the coil and the magnetic field collapses. This collapse generates a substantial reverse voltage that *could* damage other (expensive) electonic components and *will* generate electrical noise. The diode also allows the relay to release faster. This is a good thing.

Hey, B. Gill, what kinda pump are you running?

Definitions:
Mosfet; Metal Oxide Semiconductor Field Effect Transistor
EMF: Electromotive Force (Voltage)

Nowel, I WISH I still had the plastic seat covers on the Turbosaurus. Unfortunately, it's been rode hard and put away wet, then was discarded as soon as the warranty was up, poor thing. It's bled my slush money dry bringing it up to like-new status, and it ain't over yet! It's big, it's ugly, it's stinky, it's noisy, but it's powerful and fun and I love it!

And what a great bunch of guys on this website with whom to share the pleasure!

Thanks for the kind comments about my posts.

Tom


------------------
Tom Glover - VE7DQ, C-GWTG
99 QC SLT, LWB 4x2, ETC/DGP, Wht/Agate, etc.

 

[B. Gill]

no sweat big saint,thanks for the reply
Tom T i am a cheap SOB as some one else had said a while back on this thread regarding spending money during R & D phase lol
this is what i have,it is a bosch electric fuel pump from a 420 SEL that i got from a friend who is a service manager at a benz dealer ship,i asked him few things about the pump and found that it is rated at a much higher pressure and it has an integral regulater with it, i requested if he can get me one and he said some times when the regulater is bad the whole unit gets replaced so he got me one,i took the pump off the unit, it is a cylinderical device about 7 inches long and 2 inches in diameter, fuel enters from one side thats where the pump section is then it goes thru the armature section of the motor cooling it then it goes thru the commutater and brush section cleaning and lubricating it and then it exits. it has a generous GPH i have not measured that but just looking at it when i was testing it i could tell,and is capable of quite high pressures,i regulated the flow at 12 psi using a regulater also from the same friend, a very simple device consisting of a banjo bolt that is drilled from the hexagonal side and then another bolt is screwed in that keeps the spring and a ball in there. the tension of the spring determines the pressure so it is adjustable. excess fuel gets shunted back to tank from there after the desired pressure is reached. so far it is working fine and i drive about 80 miles one way to work and i ahve checked when i reach my place of work at times, i have felt the pump and it is just warm not hot at all. also i am using a bosch relay and fuse thats near the power source. do you think those diodes are needed tom? as the relay is really not part of any electronic circuit? we have other realays firing on and off all the time in other circuits of the truck that do not have that type of diodes, but then i could be wrong also as to what you said as there is no harm in using them, just extra protection. i am planning to have the stock pump which i left in its place, in parallel with bosch and controlled thru a switch and check valves at the tank outlet so that in the case either one fails the other comes on when the switch is flipped to the other side.
after the evaluation phase is over and if this pump works to my satisfaction i am going to get a brand new one and put it in there.
you are quite right, a fuse protects the wiring,load is what makes it blow.
and thanks again Tom,your posts are quite informative.
sorry for the long post
with respect
bob

[This message has been edited by B. Gill (edited 05-26-2001). ]

 

[dresslered]

Tom!

Excellent reminder about the diode across the coil! Not necessary when running a relay from a toggle switch, but for anything else, IT IS A MUST DO!

Looking forward to Sunday's experiement.

 

[csutton7]

TT---wow--you're way over my head on this wiring--I just go by what's on the diagram--but anyway this resistor thing between the pos/neg--why is there no mention of it on the diagram, if it's so important--I'm just asking cuz I know nothing about it--2nd--all the mail order catalogs and most of the pump companies make a 30Amp relay setup and they don't include the fuse after or this diode thing-please clue me in--I do understand the fuse after the power source but shouldn't it be at least a 15amp--or is my common sense all goofed up on this--what will the resistor help with across the pos/neg---maybe a little diagram---or now that I'm thinking about what you posted is it so that if something fries inside the relay this will prevent further electrical backflow??? I think that's what you said---yes variable speeds drills can run at different speeds, but it's the run stop run that kills all electrical motors--or it used to---chris

 

[Archive]

Chris. . the diode is install to lease any back flowing current. There is almost always a clamping diode installed on the AC compressor. . When a coil is shut off/opened it can cause a serious Voltage spike. (If you have ever been in an old house and turned a closed a water valve, the flowing water stops, but it causes a pressure spike. . which causes the pipe to bang. This is not the best Example... ) Of course the relay has a small coil in it. . U might be able to find a relay with this diode already installed.
I have been questioning this... I don't think that the relay would cause any bigger spike than the motor. . unless the motor has a clamping diode build in... Maybe there is a diode already in the system??? Anyone know???
Bryan

 

[dresslered]

Hey Scott-
I looked through my stash and I am all out of diodes. Try radio shack. The 1N400x (or equivalent) diode is very common. If you can't find one, I can grab one from work on Tuesday.

The reason for the diode is that when a coil is switched off, a large BACK EMF appears across the coil. This back emf may be several hundred volts in value which may or may not be detrimental to the ECM. The diode, which is installed reverse biased, is forward biased by the back emf. The diode then shorts the voltage spike back to ground saving any transients from ever hitting the ECM.

It is quite common to find these diodes already installed in modern day relays. However, if one is using a "headlight" relay such as Scott, I can guarantee that they didn't install one for that application.


[This message has been edited by dresslered (edited 05-30-2001). ]

 

[Bigsaint]

Thanks, David. I perfomed the autopsy tonite, but before I did, I checked two things, as per your instructions. I checked the Omh. 's on the DVM and received a reading of 00. 8, that's good, right ? Then I went out to my's van and touched it off for a 1/2 second, okay, maybe a second tops, on the battery. The pump immediately started and gave my shirt a good dousing of diesel #ad
, like I didn't get enough on me already while installing the new pump. I only left it on for 1 second tops, as I had no diesel running through it for lubrication.

I then proceeded with the surgery, pic. 's below. I found no sediment on the inlet screen, appeared to be very clean in fact. I did find a small amount of sediment in the bowl of the pump, not enough in my opinion to jam the gerator, though. I cleaned everything and put the pump back together. I don't have any spare diesel on hand right now, but will pick some up and try running the "failed" pump with diesel as soon as time permits. The new pump is already installed and the fuel lines are attached. I have only the wiring left to complete, which I'll do tomorrow, weather permitting as the truck is still parked outside and the rain has been never ending this weekend.

So... . what did cause the Mallory to fail ? It's obvious that the motor is still good, which leaves me three options for failure.

1) The hot wire that ran the failed unit was the wire that ran the original Carter pump and goes through the ECM (no relay was installed with the first Mallory). I did test it for power the day of the failure, but I only had a test light on hand, which indicated that I had power when the starter was bumped, as the light was lite for about 25 seconds, then when out. I performed this test at the front splice, where the Carter was and at the splice to the Mallory. Is it possible that I'm receiving below 12 v and still showing power with the test light lit ? I'll check that wire with the DVM tomorrow, but this seems unlikely, no ?

2) I checked the ground, visually, at the time of the failure and it "appeared" to be secure. The ground for the Mallory is connected to a bolt which holds the sediment filter to the frame. Again, I doubt this was to blame.

3) Debris... I did find some in the bowl of the pump, very fine and not allot. However, when I was removing the Mallory today and had all but one bolt out, the pump tipped to the right (outlet side) and dumped it's remaining diesel on my shirt. So it's very possible that there was quite a bit more sediment in the pump, which exited before I was able to remove same. This maybe the only logical solution that I can find for the failure.

One last question... as I stated above, I plan on wiring the relay in tomorrow. What's the general consensus, should I wire in the relay without the diode or should I stick with my original wiring through the ECM ? I know that the relay is a safer way to go, but I now have the added concern of whether my relay will be safe without a diode. Any pro's that can respond before tomorrow, as I have a very small window of opportunity in the A. M. hours of Sunday, to do the install. I believe all the local stores will be closed tomorrow, so purchasing a diode before Tuesday, seems unlikely. Anyway, on to the pic. s.

This first one is of the Mallory in the pre-surgery stage, prepped with a good wipe down of Windex.

#ad


The second pic. is of the Mallory, again pre-surgery, with the "scapel" and gloves #ad


#ad


The last pic. is of the Mallory, completely apart (except for the motor, of course).

#ad


Thanks again for all the help and please respond a. s. a. p. if you have any ideas on the diode topic V. ECM wiring.

Scott W.

 

[dresslered]

Somebody please correct me if I am wrong, but I don't think that you need to worry about running the Mallory without diesel for lubrication. I think that only the injection pump requires this consideration. I am somewhat stumped as to your findings as I thought that the fault would definitely be found with the Mallory. I believe (again, I'm speculating) that your preliminary findings with the Mallory are all fine indeed.

The next step would be to check the flow and pressure output of the suspected unit. Addtionally, I would be curious to see the ECM output under load. I know that you have limited resources, but these are things that you are going to have to find out. Are you sure that the Mallory had a good ground? Definitely check the outout voltage of the ECM while it is under load (hooked up to the pump). Perhaps you are not receiving the full voltage (current) as required by the Mallory. I would also like to know what kind of pressure it is providing. Is there any chance a fuel line is kinked or clogged? I think that you also indicated that you could hear the pump before and now you can't. Is this true? Could the SPA just be bad? Sorry, boss, I am running out of ideas.

Maybe, given your situtation, you may want to gamble and hook the new one up the same way. Hopefully, this will at least tell you whether or not that you've found the culprit. Once you've found the cure, then go about hooking it up the correct way. I would say that if you have driven like that for 3000 miles, "CHANCES" are another couple isn't going to do anything new. Just remember, you are your own warrantee station now. #ad


[This message has been edited by dresslered (edited 05-27-2001). ]

 

[Bigsaint]

Thanks David. I will check the ECM wire before I do anything with the wiring. All I can do is check for volts though.

After having the pump apart, it sure looks like it needs diesel to run through it for lubrication, very close tolerences, to the naked eye.

The ground wire appears that it was intact. The bolt it was attached to was tight, as was the crimp connection and I could see the ground wires extended ever so slightly from the crimp towards the ring connection.

Scott W.

[This message has been edited by Bigsaint (edited 05-27-2001). ]

 

[dresslered]

Be sure to check the voltages while under load (have the pump connected). My concern is that you said the Mallory was running (making noise) when you hooked it up to the mini van. This is NOT what you were finding when it was in your truck. Why the difference???? This could be the problem!

 

[Bigsaint]

David, you updated as I updated. The fuel lines do not appear to be kinked and I beleive the SPA sender is working properly. When the pump failed, the truck was shut down and I bumped the started, no noise from the Mallory (I would normally hear the Mallory motor at this point, until the 25 or seconds would pass) and zero psi's on the SPA. If you recall from my past posts, when I first found the problem with the truck at idle, the SPA was reading -11 ( that's negative 11 psi's) at idle. Would this indicate that the VP44 is drawing the fuel rather than the Mallory pushing it forward ? I'm spent and need some sleep.

Scott W.

 

[TXRam]

Scott,

Yes, -11psi would indicate that the VP44 is pulling fueling THROUGH the sediment trap, then the lift pump (big restriction), and then the stock filter. Most guys' gauges only read down to zero. That's why I keep giving my $0. 02 worth on the other topics - if you see 0psi, it's probably very negative just like what you experienced, so you need to ensure that there's at least a couple of psi at the inlet to the VP44. That much vacuum (negative pressure) can cause the diesel to vaporize (2 ways to vaporize liquid - increase temperature or decrease pressure) - this is my theory on how the VP44 gets destroyed (I have seen many industrial pumps, with much "looser" clearances, tolerances, etc. , get destroyed by this same mechanism). What you saw was what happens when any lift pump goes bad - the VP44 keeps sucking fuel (which it was designed for per the Bosch CD that I received), but the bad lift pump is too much restriction for it and kills the VP44.

 

[B. Gill]

almost forgot that above observation is based on the fact that your ECM on the truck was supplying current to the pump as David has mentioned.
David as i had mentioned in the previous post no you dont need diesel to run the mallory just for a minute or so but it is needed for prolonged operation as other wise it will just wear it self out and then jam. it is quite tight toleranced as Scott has obeserved.
bob

 

[Bigsaint]

Thanks again to all that responded, here, e-mails and phone calls from as far away as CA. I did get the new Mallory wired today, but didn't use the relay yet due to time constraints and the weather. Once I had it wired to the existing ECM "hot line", I did a voltage check at the pump with the truck running and received a reading of 12. 4 v, give or take a few . 01's (digital meter). The "failed" pump is going back to Mallory this week for analysis and repair, should be interesting to see what they find and I will post the results as soon as I have same. The relay will go in sometime this week. One thing is for sure, I learned more than I could imagine about the Mallory, relays,diodes, etc. and it's all good #ad


Thanks again !!!!!

Scott W.

[This message has been edited by Bigsaint (edited 05-27-2001). ]

 

[B. Gill]

scott,this is what i think is wrong with mallory.
did you take the motor section of the pump apart?
if not then if and when you do i bet that the brushes are worn,when that happenes and the brushes have worn to the point that they hardly make contact to the commutater,at this stage the motor does run, the next stage is that they dont make contact any more,there is a hair line air gap between one or both of the brushes and the commutater,at that point the motor quits. seems like what happened is that when you had the pump removed,in the process it may have been knocked a few times and the brushes moved a little to make contact with the commutater again and when you appplied current it ran and sprayed the best perfume in the world on your shirt #ad
sorry brother #ad

DC motors by design are not intended for continuous and prolonged use unless certain other conditions are met.
yes david ,diesel is not needed just to test the pump but you cant run it like that for long period of time.
just my HO,as ther are better minds her than me.
respectfully
bob

[This message has been edited by B. Gill (edited 05-27-2001). ]

 

[Un-Hemi]

DOH! I CAN'T BELIEVE what I did this morning at 2AM. Had a long dissertation on diodes, current draw and wire sizing, a real rambling thing all ready to go, had a bunch of different windows open, and... ... . CLOSED THEM ALL!!!!!!! Lost everything, including my mind. Went to bed.

First of all, I did some checking on the ol' Turbosaurus this afternoon. The stock lift pump draws 3. 0 Amps while the Cummins is running. I don't know where the factory gets the 12 Amp rating from. The operating voltage appeared to be just over 12 Volts; I measured it with an analog meter while the digital meter was measuring current. I hooked up the trusty Tektronics 2236 oscilloscope to the hot wire to the pump and found that the duty cycle on start, on my truck at least, is not 25%, but 50%. During the start sequence the ECM was supplying voltage for 40 ms (milliseconds), and shut off for 40 ms, and so on. That's 12. 5 cycles per second. I know that smaller relays can operate at this frequency, but I am not sure about larger ones. FWIW.

Dang! I forgot to do the low-voltage test on my Carter transfer pump. By the way, it's a 7psi 'competition' pump, part number P4594, meant to drive carburetors without a regulator. I use it to pump fuel into and out of the various pieces of rolling stock I have laying around. Physically, it's identical to the stock factory lift pump. I'll try to do this test later this week.

Bob Gill, I'm impressed with your resourcefulness with the Bosch pump and hand-crafted regulator. You're onto a good thing there. Keep up the good work. I see where you are coming from about DC motors not being durable, but heater motors and wiper motors seem to soldier on for years, for the most part. You would think that the manufacturers would be able to specify the MTBF (mean time between failure) on these pumps. I guess they would prefer that we didn't know. By the way, has anyone else noticed that the stock lift pump is 'upside down' compared to the way most people (including me) would mount it? The motor's down.

David, yes, a diode isn't necessarily necessary (hmmmm) when using a switch, but it will eliminate switch arcing on turn-off, extending the operating life of the switch, and will also reduce electrical noise on the system that could interfere with other electronic stuff.

Fellas, I'm gonna modify what I said about the fuse protecting the wiring. That's what it's supposed to do, and in a perfect world, the load is supposed to include its own protection. Unfortunately, in our pumps there is no overcurrent protection built in, and the cost-conscious (cheap) manufacturers rely on the circuit fuse to protect their a$$-ets as well.

As far as sizing of fuses, as long as the wiring is *more* than adequate to handle the operating current of the load, then the fuse rating should be only somewhat higher than the normal operating current of the load.

The rating of the relays Chris mentioned is only the maximum current that can flow through its contacts without excessive heating. A 30 amp relay could certainly be used to switch a 5 amp load, but it would be unwise to use a 30 amp fuse in this case. Under certain circumstances, a 5-amp load (perhaps a pump) could fail with a partial short and could draw 30 amps. A 30 amp fuse wouldn't melt open right away, and the wiring may not be able to carry 30 amps without getting hot and melting. That would be a bad thing. Current draw of the lift pumps we are discussing appears to be 3 to 7 amps. A fuse with a rating 50 to 100% higher than our load current would prevent nuisance failures, and give good protection in case of circuit fault *as long as the wiring can safely handle the overcurrent*. Again, it's the load that determines the amount of current that flows in the circuit. In this discussion, the load is the lift pump.

Diodes! Why do we need them? Think about how an ignition coil operates. Points (yeah, I know, but humour me. ) are closed, current flows in the primary winding of the coil. A magnetic field builds in the windings. Points open. Current flow stops, magnetic field collapses. Huge voltage is generated in secondary winding by collapsing field. Large voltage is also generated in primary winding, but is suppressed by condenser in distributor so the points don't get fried, which they eventually do anyway. System was developed in the dark ages, before diodes!

A relay coil has the same characteristics as our ignition coil; it's magnetic field is used to pull in an armature to make a contact, instead of generating sparks. When the activating voltage is removed from the relay's coil (by opening a switch or by turning off a transistor) the magnetic field collapses and produces a back EMF (reverse voltage) that can top hundreds of volts in certain relays. Sometimes this collapsing magnetic field can hold the relay's armature in longer than desired. To prevent this reverse voltage from damaging whatever shut off the operating current, a diode is added across the winding of the relay to short the high reverse voltage to ground (usually). This back EMF is a normal product of a collapsing magnetic field in any coil. It can't be prevented, it can only be minimized. Diodes, which allow current flow only one way, are ideal for this application. The operating voltage sees the diode as a (close to infinite) resistance across the coil. The back EMF sees it as a short circuit. Everbody's happy. #ad
Diodes are often not necessary, but are still good operating practice.

It's my bet that the ECM has all kinds of protection (including protection diodes) built in. Every motor and relay that it runs can generate back EMF, but the thing is designed to operate in this environment. It also has to tolerate the huge fluctuations in battery voltage and starter transients that are generated at startup. For the most part, it appears to be pretty rugged. Yes, occasionally one fails, but when you consider how many of them there are in service, they seem pretty reliable.

It's unfortunate that I don't have the ability to put up diagrams on here. I feel the need for more technology!

You are welcome to email me directly if I have added to the confusion on any of this stuff.

I appreciate that I take up a lot of bandwidth at times, and for that I apologize.

Tom

------------------
Tom Glover - VE7DQ, C-GWTG
99 QC SLT, LWB 4x2, ETC/DGP, Wht/Agate, etc.

[This message has been edited by TommyTurbosaurus (edited 05-28-2001). ]

 

[dresslered]

Tom Turbo-
Excellent explanations! Maybe we'll see a reprint in the TDR. #ad

 

[csutton7]

TT--so you know I am just asking questions - beacause I don't know--
1) How long have diodes been around
2) My automotive electrical handbook doesn't even mention diodes
3) I'm guessing you have to tear apart the relay to add a diode--maybe put the whole she-bang in a bigger box
4) Are diodes sometimes built into automotive relays
5) Why do all the manufacturers of these relays not show the diode in their wiring diagrams
6) why do they make them 30 amps if they don't need to be
7) Are you saying if you have a 12 amp draw you should use a 25 amp fuse(basically double it)
8) So, since I have a 30 amp fuse between the battery and the relay, should I add a lesser size fuse between the relay and motor?? What size or how would you figure out the size?
9) My 30 amp fuse, between the battery and relay, blew when my BG pump fried, so if you answer yes to #8 and I had a smaller fuse in line between the relay and pump would it have blown before the 30amp fuse, thus saving the 30 amp and possibly more---although nothing else was toasted--it did what it was designed for-I think

Sorry for the questions, but it will help me understand more than my pea brain knows now--thanks... ... ... chris

 

[Un-Hemi]

Just for fun, I went to Radio Shack and bought a cheap 30 Amp Bosch-type relay, P/N 275-0226 to play with. Brought it back to the shop here (PLEEEEEZE don't tell my boss!! #ad
. ) and hooked my storage oscilloscope across the coil.

First of all, the relay coil draws 180 ma @ 12. 2 volts. The coil resistance is calculated to be 68 ohms. I took the cover off it and it DOES NOT have a diode inside.

I operated the relay by unplugging the banana plug to the power supply. I measured *in excess* of -400 Volts spiking off the relay. I was surprised to see that the voltage spiked in a positive direction as well, to over +200 Volts. This indicates that the coil is 'ringing'. The voltage across the coil didn't stabilize near 0 volts for 200 or 300 microseconds. I stuck a 1N4148 diode across the coil as described in the previous post. It's just a little sucker, not a power diode like a 1N4007, and it clamped the back EMF to about -6 volts. If there was any positive excursion, it was negligible. IMHO, a 1N4148 diode (cheaper than Borscht) is adequate for suppressing a Bosch-type relay. Incidentally, without the diode, the voltage transients were loud enough to be heard on the speaker I use on the bench.

Mick2500, while an ignition coil is designed to produce thousands of volts on the secondary winding, the primary winding is also within its own magnetic field, and when that field collapses, there most definitely is a back EMF generated. It won't be as great as the engineered back EMF that is generated by the secondary, but it will be there. The condenser across the points in the distributor is there to manage that back EMF from the primary winding. My experiment with the relay gives an example of what back EMF can be generated by just a little coil. Oh, by the way, the amount of actual energy in the relay's back EMF is pretty small, but it's voltage that arcs and sparks and damages solid state stuff.

Mick, thanks much for the information on the Bosch Aux Light Relays. I wasn't aware that they had diodes and fuses built-in. Chris's question 4 has just been answered! Fellas, take note! This is why we pay more for good quality products!!

Bob, as noted above, the voltage transients occur so quickly that a voltmeter won't pick register them. An oscilloscope is the preferred way to observe these voltage spikes. Yeah, about the motors, I was on a bit of a fishing trip... I wonder what the answer is? Replaceable brushes?? My buddy at the dealership has more experience with the lift pumps than I do. I'll ask him if the motors are wet, but, in my opinion, they are not; the shafts are sealed going into the pump housing. At least, my Holley, which could be disassembled, had a shaft seal and the motor ran dry. I don't expect the Carters to be very different, except that they can't be disassembled. By the way, remember that the VP-44's bypass regulator is a banjo style regulator.


My history of diodes is a study in generalities: I'm gonna ignore vacuum tube diodes, cuz they are of no consequence to this discussion. Diodes date back probably to the 1800s. The first practical small power diodes were probably produced in the 50s. Selenium rectifier diodes were used in early TVs but were big, ugly, and tended to expire often, releasing a smell of rotten eggs or skunks. Silicon diodes became popular in the early 60s because they didn't smell like skunks. Back then, they were encased in metal and looked like a miniature top hat, with a wire out of each end. Later on, they were replaced with molded epoxy encased diodes that were much cheaper to produce. The 1N4007 fits into this category. There are also glass-encased diodes, and they are normally used for small signal and electronic switching applications. The 1N4148 I used for my relay experiment is an example of that type, and appears to be useful for our Bosch-type relay application.

The 1N4148 diode I used is smaller than a wooden match head. I could easily add it internally to the Radio Shack relay and probably will. If I do that, I will have to remember which terminal is which on the relay coil, because the polarity of the diode is important. The two coil tabs on the relay are labeled 85 and 86, and I just know that there is a documented standard for hooking them up. Anybody with that knowledge can just step right up!

On the other hand, it wouldn't be difficult to crimp the diode's leads into the 1/4" push-on flat connectors that most people use to attach wires to the relays. Just note that the diodes are somewhat fragile physically due to their small size. The diode's wires will require some slip-on insulation to prevent accidental shorts also. I've used this technique myself on many occasions. Again, I stress that the polarity of the diode is important. Get it right the first time, or the fuse will blow, or, in the case of a 1N4148, it will fail in either open or shorted mode, depending upon how much current was forced through it.

Back before cars became computerized, there was little automotive application for transient protection (including diodes) in cars. Switches didn't care if they arced a little; they could be cleaned or replaced cheaply, and the radios had tubes, and contained their own type of protection. There just wasn't much need for it. When computers were introduced to cars, transient protection became very important. Computer failures in cars are *very* unpopular due to their inconvenience and expense!

Chris, I don't know why your handbook doesn't show protection diodes, or transient protection, considering how important this is on modern computerized cars.

As I mentioned above, there is probably a prescribed method of hooking these relays up, probably to some DIN standard. As long as the wiring polarity to the coil is correct, there would be no obligation to show whether the internal diode was there or not. Outside of suppressing back-EMF, the diode is 'invisible' to the user, as it simply eliminates an undesirable trait of the relay. Does anyone have a relay that shows a diode across the coil on the little stamped-in illustration on the relay? Mick, what does your genuine Bosch relay show?

Relays are often rated so that they are universal in their applications. A 30 Amp rated relay, the Bosch-type for instance, can be used in *any* application up to a maximum of 30 Amps, whether it is Aux lighting, driving fuel pumps, or whatever. I could use a relay capable of switching 10,000 amps for my driving lights, if that was all that was available, but I might have to mount it in the pickup bed because of its size. A relay rated at 30 Amps will cover *most* of the typical automotive applications that any of us mere mortals can think of. Also, one part number gives the manufacturers economy of production, cuz they only need to build one part number. (Yeah, I know, they make *all kindsa* relays. Gimme a break!) The Bosch-style relay is the Volkwagen of relays. (sorry, I almost said ch****let) #ad


Well, if I were running a load with a 12 Amp draw, I would tend to run toward the lower end of the overcurrent protection range. Remember what I said about 50 to 100% overrating? Well, I put that in there after considering that a motor's startup current might be high enough to blow a fuse with a 50% overrating. Maybe it would, and maybe it wouldn't. I regret that I may have added to the confusion here. A 25 Amp fuse wouldn't be out of the question with a 12 Amp load, but I would be more comfortable with a fuse with an 18 to 20 amp rating. A 15 Amp fuse might work too. You see, we want it only big enough to be able to handle reasonable overcurrents, like when a motor starts up, and still be small enough to melt quickly when an obvious fault occurs. It's a balancing act, and there's no quick answer. Start smaller, and expect that the fuse could be undersized and may blow. If it blows, then go up one more step in current rating. Remember too, that a fuse will not blow immediately with a moderate overcurrent. It takes some time for the conductor inside the fuse to heat up and melt. There are also special commonly available fuses, called slow-blow fuses that are designed to endure higher overcurrents for longer periods of time before they finally open up, but they're not commonly used in automotive applications.

Fuses should normally be placed as close to the source of power as is practical. That way, most of the wiring in the circuit is protected. For instance, if I ran a wire from the battery to the trailer connector, and put the fuse right behind the trailer connector, all of the wire ahead of the fuse would be unprotected. If a short to ground occurred anywhere on that wire, well, you know what's gonna happen. #ad
It's logical to put the fuse as close to the source as is possible. I know, I'm starting to get redundant. redundant. I'm nearly finished!

Chris' question 8. One fuse for each load is sufficient. We have discussed sizing of fuses above, so the correct size of fuse can be estimated by either physically measuring the operating current of our load, or by using the manufacturer's advertised current draw. The manufacturer's rating is probably close enough for us. As an example, my Carter P4594 pump draws 4. 5 Amps, according to the Summit catalog. I would probably fuse it at 8 amps, and it would be OK. If I were concerned about the current-handling capabilities of the wiring, I might fuse it at 6 Amps, if that value of fuse is available. See, there's lotsa room to experiment here. Just don't get caught out in the toolies without spare fusies!!

Chris, yes, in your question 9, if you had a smaller fuse between the relay and the pump, it would have blown first, and would have prevented the 30 amp fuse between the battery and the relay from blowing. I see that the current rating of the BG220 pump is 8 Amps. A 12 amp fuse is adequate for this application. I'd carry a 15 Amp fuse in the truck just in case it isn't. #ad


Bob, if your oil filter survives that diesel fuel without dissolving, and it seems like it does, then I guess it's all right! Another great idea!

There. I hope I've answered most of the questions. Comments and further questions are welcome. Thanks for your time and consideration.

Tom

[This message has been edited by TommyTurbosaurus (edited 05-28-2001). ]

[This message has been edited by TommyTurbosaurus (edited 05-28-2001). ]

[This message has been edited by TommyTurbosaurus (edited 05-28-2001). ]

[This message has been edited by TommyTurbosaurus (edited 05-28-2001). ]