Not-so-tiny tachometer?

[RBurleson]

Is there no easily available tachometer that is actually decent-sized?



From searching the board, I see plenty of references to the digital Tinytach, 2" Isspro tach, 2. 75" Auto Meter tach, and discontinued Mopar tach (ranked in order of ascending size, and probably price)... but what I'd like is a big-honking 5" guage -- preferably one without the boy-racer Honda-style Parr-lamp shift light.



Plug-n-play is an obvious plus, since I do have a 93 truck with the under-dash harness... . but I never really was that good at coloring inside the lines, so...



I'm willing to get creative -- here's a VDO tach from Summit Racing:

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http://store.summitracing.com/partdetail.asp?part=2077



I don't need all the snazzy do-dads, but this tach does report compatibility with 2, 4, 5, 6, 8, 10, 12 cylinder vehicles... that should net me a large needle-sweep, if I'm willing to disregard the numbers. (I can't for the life of me remember which way the math goes right now, but halving the 12 cylinder revs would result in a 4500rpm peak... or tripling the 2 cylinder revs would result in a 3000 rpm peak - anyone recollect which way the math works?). I'm betting that I could disassemble and reassemble this guage and modify or replace the face numbers...



Randii

 

[RBurleson]

I did some more thinkin' on this... . and consulted a buddy. (Thanks, Dan!)



If I understand correctly, 92-and-up Cummins get tach signal from the engine speed sensor on the outside of the harmonic balancer. This feeds to the engine control module that then sends the signal to a 4-wire harness under the dash. I presume this uses a permanent magnet and Hall's effect to induce one pulse per revolution.



Many standard tachs drive off a coil sensor, and count pulses. Each turn of the crank on a conventional engine produces 6 coil incidents -- and most aftermarket tachs have a switch to divvy this up by 4, 6, or 8 (corresponding to the number of cylinders). Problem with the above plan is that Cummins engines have no coil. Oops... .



To use some other tachs, I might choose to use multiple magnets, depending on their signal threshhold. Multiple magnets, along with the correct setting on the tach, could get the reading I want. For example, 3 magnets evenly spaced around the flywheel/pulley would produce 3 pulses per revolution... if I set the tach to expect 1 pulse per rev, the tach would indicate 9000rpm at an actual 3000 rpm. More magnets also give potentially better resolution and less fluttering of the tach needle.



Does anyone know what the average pulse measures out to be for a CTD?



Randii

 

[boatpuller]

I purchased a VDO Tach a couple of years ago, this is what the tech from VDO told me about the signal at that time:



This is the info I have on the signal coming from pin #43:



V6, V8 and diesel 100HZ = 3000 RPM (same as 4 cylinder gas ignition)

V10 125 HZ = 3000 RPM (same as 5 cylinder gas ignition)



John A. Stover

Technical Service Advisor

VDO North America



I could not get the tach to work as there is no signal coming out of my ECM, I wasn't going to buy a new ECM just to get my tach to work, one of these days I will wire it into my alternator, just hasn't seemed that important yet.



You have part of the system correct per my understanding but the pulse is not a hall effect it is an approximately 5 volt signal that is generated by the ECM,signal is zero pulses when engine is not rotating but 100cycles per second at 3000 RPM and a linear taper in between.



Hope this helps,

Caleb

 

[power grizz]

tach

i thought autometer made a diesel tach for pulling tractors!



i priced one and it was $300, went to 5000 rpm and had memory and a shift lite too.



but couldnt get the funds for one.



have one planned for the new truck.



would have the gassers thinking what the hell seeing a diesel tach read 5000 rpm and a redline at 4300 rpm.

 

[Diesel Freak]

Tomorrow I will ask my Dad where he got his Tach for his '90. It takes a signal off the alternator.

 

[JLEONARD]

I bought an ISSPRO tach for my 91. It gets its signal from the alternator. I removed the cover off the back of the alternator and hooked a wire into one of the rectifiers and into the tach. Easy stuff. pm me if you need more help with that set-up.

 

[RBurleson]

Exxxxxxxxxxxcellent, Caleb! hanks!



This is the info I have on the signal coming from pin #43:

V6, V8 and diesel 100HZ = 3000 RPM (same as 4 cylinder gas ignition)

V10 125 HZ = 3000 RPM (same as 5 cylinder gas ignition)

This really opens up the tach choices 00 just about EVERY tach supports four-cylinders.



You have part of the system correct per my understanding but the pulse is not a hall effect it is an approximately 5 volt signal that is generated by the ECM,signal is zero pulses when engine is not rotating but 100cycles per second at 3000 RPM and a linear taper in between.

Good to know -- my guess is that the ECM receives a hall effect signal and sends the 5-volt signal.



You've given me numbers that I can take directly to a manufacturer tech support line to get the tach I need -- THANKS!



Randii

 

[racing]

Autometer makes a diesel tach that is 5" tach it is part # 6806 it has a 6,000 max rpm on it but IF it is alternator driven you can just adjust it that 6,000 is 3,000 so on. It says that it is "With gear probe trigger for diesel engine tractor pullers" it does not say anyting more about it or if it is also appliable to the alternator pulse. I would think that this would be alot easer then trying to get something that is not even designed to for a diesel to work with one just my 2 cents though.

 

[RBurleson]

The 5" Autometer that racing and power grizz are talking about is part #6806 'with gear probe trigger for diesel engine tractor pullers' (about $250-300).

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There's more details about midway down this page (looks like a rebadged Autometer): http://www.dalhems.com/electronics/autoprocomp.html

Looks like this 'gear probe' senses off the flywheel gear teeth, similar to a Hall effect pickup, except instead of counting magnetic fly-by, it counts the tooth fly-by (or more specifically, the frequency of the resulting waveform).



There's also the 2 5/8" Autometer diesel gage -- part #2888, this one actuates off the alternator (about $100):

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There's more details about midway down this page: http://www.performancenmore.com/autometer.htm

Autometer has installation instructions up here: http://www.autometer.com/hp/instruction_download/instructions/148i.pdf



You can't link directly there (frustrating!) but if you go here: http://www.autometer.com/hp/index.html and click TECH TIPS/FAQ then scroll down and click SPECIAL TACH CALIBRATION, you'l see special applications, one of which is quoted below:

ALL Dodge V-10 motors and Vipers ... need to be set for 2. 5 and 5 pulse per rev, and also 5-volt square wave

(the bold is mine)

Leveraging boatpuller's response and extrapolating just a little, the Dodge V-10 motors ALSO have an ECM-out-to-tach lead... if Autometer can customize a tach for the V-10 feed, it should be simple enough for them to do it for the CTD feed (better still if you could get one of their simple tachs without having to pay for the shift light and memory features). I'll have to borrow an oscilloscope to check the waveform, then call Autometer's tech line (815-895-8141) -- it sure would be nice if we CTD folks could source a tach from them that actuated off the under-dash harness.



Doesn't hurt to ask.



Why go through all this trouble? It might be a bit more inconvenient to draw off the stock harness, but that has GOT to be the best, most reliable signal... Alternator belts can slip or break, and magnetic pickups have to be calibrated.



Picky, ain't I?



Randii

 

[RBurleson]

Might as well stuff in a few more links...

Here's a place that does Stewart Warner tachs:

http://www.lspeedo.com/sw6.htm



Also, there are Gaffrig guages (marine stuff):

http://www.vikingmarina.com/Gaffrig/pages/Monsters.html



... and lastly, there's some cool info on different types of tachs here:

http://www.tflx.com/dslhome.html

 

[Diesel Freak]

My Dad is using the 3 3/8 inch diameter 0-4000 RPM Tachometer/hourmeter made by Datcon that takes the signal off the alternator.

 

[boatpuller]

What the information that I posted and what was explained to me by the Tech person at VDO is that any tach that will work on a four cylinder gas engines ignition will be compatible with the signal off of pin #43 of ECM which is the tach lead on our harness under the dash. A gas engines tachometer counts every firing of the coil. If you take the 100hz=3000 RPMs you can see how this works:



3000 RPM divided by 60 to get Revolutions per second=50



On a four cylinder 4-stroke engine 2cylinders fire every revolution so you have to multiply the 50RPS by 2 to get the number of signal cycles per second.



So what this means is that any Tachometer that will work on a 4cylinder gas engine with a standard points ignition will work on our trucks.



I can't prove this because I have no signal coming out of my ECM but this is what the VDO tech told me, therefore there are many choices of large tachometers you can get.



Sorry to ramble, hope this helps,



Caleb

 

[RBurleson]

I have a long history of beating dead horses.



So what this means is that any Tachometer that will work on a 4cylinder gas engine with a standard points ignition will work on our trucks.

This sounds right based on what the fella on the Autometer and VDO tech line told me -- the rub, as far as I can tell, is with the 5-volt square wave (12V is more standard).



I'm gonna crawl under the dash with an oscilloscope to verify that this is the signal we have at the 43 pin... once I get a peek at the waveform, I should be able to see whether I need a tach signal conditioner (to quell needle flutters)... . can't imagine I'd need a tach signal amplifier - haven't seen one of those in YEARS. FWIW, this link has both: http://www.holley.com/HiOctn/ProdLine/Products/IS/ISA/870-400.html



I'm expecting to see a 5V signal two times per engine revolution. We'll see... .



Randii

 

[boatpuller]

Randi,

The 5 volts should be comparable to a point type ignition, the tachometer comes off the side of the coil, the coil is only being fed 12 volts at startup in most vehicles with a ballast resistor lowering the voltage when in the run mode, I believe that it lowers the voltage to around 6volts which is very close to the 5 that comes from a correctly operating ECM.

Therefore you are correct in asssuming that any 4cylinder point type tach should be able to work.





Good Luck,

Caleb