I am looking for a relay that has 2 inputs and one output for switched power. I would like to wire the lift pump so that if necessary (like to prime after a fuel filter change or whatever) I can switch power on from inside the cab, but also use the current power line as the other side of powering the relay. That way would allow me to turn the power on inside the cab without bumping the starter and also let the ECM keep the relay turned on when the truck is running. This will also assure me I can keep 12+ volts or more going to the lift pump.
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someone help me correctly identify a relay I'm looking for (lift pump related)
- Thread starter Jeepmullet
- Start Date
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This would be easy if someone would just tell me it would be okay to splice 2 wires together and just use a standard relay similar to one for wiring auxillary lighting. My concern there was the small amount of current that may run back to the ECM if the engine was off and I turned the lift pump on via the in-cab switch that was spliced in with the same switch wire from the ECM.
I don't think you'd have any problems doing this. I don't know about your ecm seeing an error and throwing a code, but why would it???
Wait a minute..... if you're using the ecm wire to run the relay... Is that what you're doing?
If so, I don't know how it'd react being back-fed while truck is off. I have no clue... it shouldn't do anything, but who knows.
Now if you ran the relay off of something else that was live, only when your key was on, such as a cigarette lighter circuit, then I'm sure you'd be fine.
Then you could tap off this hot, run it to your in cab switch, then back to the relay and turn your pump on to prime whatever, whenever you needed.
Wait a minute..... if you're using the ecm wire to run the relay... Is that what you're doing?
If so, I don't know how it'd react being back-fed while truck is off. I have no clue... it shouldn't do anything, but who knows.
Now if you ran the relay off of something else that was live, only when your key was on, such as a cigarette lighter circuit, then I'm sure you'd be fine.
Then you could tap off this hot, run it to your in cab switch, then back to the relay and turn your pump on to prime whatever, whenever you needed.
use a pair of diodes to isolate the controls from each other. . then it will be ok. the relay coil doesn't draw too much current so a pair of diodes like this should do nicely
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nickleinonen said:
Okay, it's been a while since physics and calculus... This will keep me from sending current back to the ECM if I was using the other control wire?
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Jeepmullet said:
yep. . a diode is an electrical check valve. . power goes one way only [unless you feed enough juice to it to blow it through, but that can happen with pneumatic and hydraulic checks too]
ecm voltage--►|--\
... ... ... ... ... ... ... ... ..... |-- fuel pump relay coil terminal
manual sw----►|--/
-►|- = diode- voltage can only flow in the direction of the arrow. no voltage, no current
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I found one of those relays in a local radioshack. It was the 3A version. The max operating temperature was 75 degrees celsius. Would underhood temps be too high for too long to use those in the summer?
Why a relay? 
Why not a simple 2-way toggle switch, power comes in from battery source or ECM control, and the switch selects which source is applied to the LP?
It's called the KISS system... :-laf :-laf
Why not a simple 2-way toggle switch, power comes in from battery source or ECM control, and the switch selects which source is applied to the LP?
It's called the KISS system... :-laf :-laf
Gary - K7GLD said:Why a relay?
Why not a simple 2-way toggle switch, power comes in from battery source or ECM control, and the switch selects which source is applied to the LP?
It's called the KISS system... :-laf :-laf
hmm... that would work. I think I avoid directly wiring something like that because of an ''incident'' when I was 18 and thought I knew how to wire stuff. I ended up with a cab full of smoke and shocking the bejesus out of myself. I melted a handful of wires in the process
I have the habit of wiring things with wires that are 2-3 feet too long, in case my setup ever changes. This may be a good time to utilize that setup
Right now the relay is being triggered by the ECM power. The in cab switch accessory wire is not connected to anything.
As mentioned further above, you might get a LP error code when the ECM senses an open circuit when it gets switched out of the circuit - but...
Gary (and others) I'm struggling to find a three position toggle or rocker switch on this site. http://www.nkkswitches.com/products.asp The nomenclature isnt familiar to me.
This whole idea started out simple (I swear) when I was having intermittent lapses in voltage to the lift pump. Another member PM'd me that they had the same sort of issues and just wired a relay to make sure the pump was always getting 12 volts regardless of varying loads supplied by the ECM.
I thought I could have my cake and eat it too by adding an in cab switch to the relay so I could manually prime the fuel system if ever need be. (now here we are)
This whole idea started out simple (I swear) when I was having intermittent lapses in voltage to the lift pump. Another member PM'd me that they had the same sort of issues and just wired a relay to make sure the pump was always getting 12 volts regardless of varying loads supplied by the ECM.
I thought I could have my cake and eat it too by adding an in cab switch to the relay so I could manually prime the fuel system if ever need be. (now here we are)
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This, from Mouser will work nicely:
http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=623020
OR, either S302 from
http://www.nkkswitches.com/products.asp
will work - S302 is a on/on switch - no center "off" position.
S303 provides a center detent for an "off" position, if you want that feature. The mouser switch I pointed to also has both options available.
enjoy!
http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=623020
OR, either S302 from
http://www.nkkswitches.com/products.asp
will work - S302 is a on/on switch - no center "off" position.
S303 provides a center detent for an "off" position, if you want that feature. The mouser switch I pointed to also has both options available.
enjoy!
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ONE last word of caution before this dies - I have no schematic of the ECM, and no way to determine if what we would call the "ground side" of the stock LP power lead really IS a direct ground, or relies on other ECM internal circuitry for LP control - be a shame to fry an expensive ECM!
A good quality test meter should be used to establish direct continuity of the stock LP "ground lead" to the vehicle frame. And if that is inconclusive, or proves that both stock leads to the LP are above ground potential, a DPDT (Double Pole Double Throw) relay could be used instead of the SPDT one spec'd above, then BOTH old and new +- leads controlled by the added switch - not much more work or cost...
A good quality test meter should be used to establish direct continuity of the stock LP "ground lead" to the vehicle frame. And if that is inconclusive, or proves that both stock leads to the LP are above ground potential, a DPDT (Double Pole Double Throw) relay could be used instead of the SPDT one spec'd above, then BOTH old and new +- leads controlled by the added switch - not much more work or cost...
Gary - K7GLD said:
Gary, I have an expensive electrical tester but honestly dont know all the functions of it. There was a +12. 5 volt potential difference between the red and other wire.
I still cant figure out why my stock LP power wire (on the ECM side) only gets intermittent power. Right now I unhooked the stock power wire from the relay trigger and just hooked in the switch wire from my lighted switch in the cab.
Do you think I should just reroute the ground wire to a frame ground by the lift pump?
Just in case, I have a spare ECM I carry with me. I dont have quite as many spare parts with me as you do, but I have a LP, ECM, wiring, switches and relays as well as a multimeter and all the tools I've ever used on my truck.
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I have read all the posts and I understand what you want to do , I just can't quite get how you are trying to do it. I am interested because this is a project that I would like to do for my truck also. I will require a report from you.....
Let me take a stab at it what I understand. You want to tap into the wire from the ECM that controls the relay that turns on the LP? When you say "relay trigger" is this what you mean?
I had envisioned doing this kind of, but not quite the same as the way you plan to do it. Keep in mind that I am not 100% sure about all of this, because I have not done this project yet.
The LP can function at two speeds, as mine did before the Smarty. At engine start it ran at slow speed for a few seconds, then kicked to high speed. Because the pump only has two wires going to it I am going to say that this speed switching occurs at the pump relay. Lets say it requires 12v to run at 100% speed and 6v to run at 50% speed. The relay most likely has 2 ways to power up the pump. One way has a resistor (6v - 50%) and the second way has no resistor (12v - 100%). The ECM chooses which circuit to power up in the relay and power is applied to pump. The pump itself does not care.
On to the meat and potatoes.
The pump relay will act as an isolator between the pump and the ECM. By this, I mean that there are two sperate electrical circuits. Power for one circuit is through the ECM to the relay coil to ground. Power for the other is from the battery (via 12v bus) through the relay contacts to the power side of the pump to ground. They are not electrically connected directly. Think of your RV battery isolator here.
Keeping this in mind, to power up the pump all you need is connect power downstream of the relay. Electrically the pump does not care where its power comes from. Most likely the relay does not switch open the ground wire, but rather leaves it connected at all times to the battery ground bus. Only the 12v side is switched open and closed. But without experimentation I don't know for sure. This connection would be a likely spot to install the diodes as described above. With the diodes you will only apply power to the pump and will decrease the likelyhood of damaging the contacts in the pump relay.
To make this work most simply requires that you fuse a power circuit through the switch in your dash to the pump. Has long as the switch is closed power is is connected to the pump everything will work fine. Either the dash switch or the ECM relay can start the pump, but both need to be off / open to stop it.
A glitch could be that the pump itself pulls more power than a dash mounted switch would really like to carry. If this is the case, then this is why you would need a second relay. The dash mounted switch controls the second relay which controls the pump. In that case the dash switch and the second relay would both need fuses on the 12v supply wire.
So the circuits would be:
Stock
ECM ---> Relay coil ----> Ground
Battery Bus ---> Relay contacts ---> Pump ---> Ground
Dash Switch
Battery Bus ---> Fuse B ---> Dash Switch ---> Pump ---> Ground
-or-
Battery Bus ---> Fuse A ---> Dash Switch ---> 2nd Relay coil ---> Ground
Battery Bus ---> Fuse B ---> 2nd Relay contacts ---> Pump ---> Ground
My plan is that I eventually will just disregard the power from the ECM relay (probably just pull it out). I plan to install a mechanical 12 valve LP and only use an electric LP for priming and back up. The electrical pump will be mounted at the fuel tank.
Sorry to have such a long post, hope this helps you out.
Jim
Let me take a stab at it what I understand. You want to tap into the wire from the ECM that controls the relay that turns on the LP? When you say "relay trigger" is this what you mean?
I had envisioned doing this kind of, but not quite the same as the way you plan to do it. Keep in mind that I am not 100% sure about all of this, because I have not done this project yet.
The LP can function at two speeds, as mine did before the Smarty. At engine start it ran at slow speed for a few seconds, then kicked to high speed. Because the pump only has two wires going to it I am going to say that this speed switching occurs at the pump relay. Lets say it requires 12v to run at 100% speed and 6v to run at 50% speed. The relay most likely has 2 ways to power up the pump. One way has a resistor (6v - 50%) and the second way has no resistor (12v - 100%). The ECM chooses which circuit to power up in the relay and power is applied to pump. The pump itself does not care.
On to the meat and potatoes.
The pump relay will act as an isolator between the pump and the ECM. By this, I mean that there are two sperate electrical circuits. Power for one circuit is through the ECM to the relay coil to ground. Power for the other is from the battery (via 12v bus) through the relay contacts to the power side of the pump to ground. They are not electrically connected directly. Think of your RV battery isolator here.
Keeping this in mind, to power up the pump all you need is connect power downstream of the relay. Electrically the pump does not care where its power comes from. Most likely the relay does not switch open the ground wire, but rather leaves it connected at all times to the battery ground bus. Only the 12v side is switched open and closed. But without experimentation I don't know for sure. This connection would be a likely spot to install the diodes as described above. With the diodes you will only apply power to the pump and will decrease the likelyhood of damaging the contacts in the pump relay.
To make this work most simply requires that you fuse a power circuit through the switch in your dash to the pump. Has long as the switch is closed power is is connected to the pump everything will work fine. Either the dash switch or the ECM relay can start the pump, but both need to be off / open to stop it.
A glitch could be that the pump itself pulls more power than a dash mounted switch would really like to carry. If this is the case, then this is why you would need a second relay. The dash mounted switch controls the second relay which controls the pump. In that case the dash switch and the second relay would both need fuses on the 12v supply wire.
So the circuits would be:
Stock
ECM ---> Relay coil ----> Ground
Battery Bus ---> Relay contacts ---> Pump ---> Ground
Dash Switch
Battery Bus ---> Fuse B ---> Dash Switch ---> Pump ---> Ground
-or-
Battery Bus ---> Fuse A ---> Dash Switch ---> 2nd Relay coil ---> Ground
Battery Bus ---> Fuse B ---> 2nd Relay contacts ---> Pump ---> Ground
My plan is that I eventually will just disregard the power from the ECM relay (probably just pull it out). I plan to install a mechanical 12 valve LP and only use an electric LP for priming and back up. The electrical pump will be mounted at the fuel tank.
Sorry to have such a long post, hope this helps you out.
Jim
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Whoa Jim that's way more complicated than I ended up with. Here's what I'm doing now(and I verified it with an electrical tester)
All the dash switch does is tell the relay to turn on. When the relay is turned on by the in cab switch, it supplies up to 30 amps of current to the LP. I've scrapped the idea of using both the dash switch and ECM to turn the relay on. The relay only needs around 200mA of current from the in dash switch to turn on and supply power to the LP. I'm using a relay I bought for $4 at oreillys. It is intended to be used for off road lighting and has two places that are powered when the relay turns on. I'm only using one. Right now I have completely disregarded the power coming from the ECM, it's not even hooked up to the relay. When I first did the hookup, I was using the ECM power to turn the relay on (instead of the dash switch) but it was only supplying power intermittently and the relay kept switching on and off. This was only to turn on the relay, the relay's 12volt power supply to send to the pump was coming from the battery, and fused at 30 amps.
If you want to use a switch plus the ECM the easiest idea I've found to use is those diodes. Gary's idea seems like it will work fine also. The reason I have just the switch powering the relay now is I couldnt find any 6amp diodes at our local radio shack, and the next biggest ones I could find had a max operating temperature of 75degrees.
All the dash switch does is tell the relay to turn on. When the relay is turned on by the in cab switch, it supplies up to 30 amps of current to the LP. I've scrapped the idea of using both the dash switch and ECM to turn the relay on. The relay only needs around 200mA of current from the in dash switch to turn on and supply power to the LP. I'm using a relay I bought for $4 at oreillys. It is intended to be used for off road lighting and has two places that are powered when the relay turns on. I'm only using one. Right now I have completely disregarded the power coming from the ECM, it's not even hooked up to the relay. When I first did the hookup, I was using the ECM power to turn the relay on (instead of the dash switch) but it was only supplying power intermittently and the relay kept switching on and off. This was only to turn on the relay, the relay's 12volt power supply to send to the pump was coming from the battery, and fused at 30 amps.
If you want to use a switch plus the ECM the easiest idea I've found to use is those diodes. Gary's idea seems like it will work fine also. The reason I have just the switch powering the relay now is I couldnt find any 6amp diodes at our local radio shack, and the next biggest ones I could find had a max operating temperature of 75degrees.
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Good deal and thanks JeepMullet. No quite so complicated, just too many words I guess. You did it exactly how I intend to do mine.
As far as your intermittent problem with power from the ECM have you looked in replacing that relay in the fuse and relay box or cleaning the knife contacts etc... ?
Jim
As far as your intermittent problem with power from the ECM have you looked in replacing that relay in the fuse and relay box or cleaning the knife contacts etc... ?
Jim
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