NEED tach signal on Duramax - please help!!

dbm76 · October 28, 2002

Greetings to all, hate for my first post here to be begging for information, but I have the privilege of working with a brand new 2002 2500HD Duramax/Allison for some aftermarket R&D work. I am preparing to get into some dyno testing but I am having a terrible time finding an RPM signal that I can use - I don't have an optical pickup and can't find anywhere convenient to use for a magnetic pickup.

If anyone has had the opportunity to dyno test one of these beasts, do you know where you found a tach signal? I've heard to look in the wire bundles on the passenger side near the firewall, but I hate to just go attacking wires to try to find a signal. All my dyno experience has been on gas engines so this is new territory for me.

Thanks in advance for any information!!

SoMnDMAX · October 28, 2002

I've dyno'd my truck a couple of times now. Using the Mustang dyno, the only option was to use the optical style pickup. As far as I know, there isn't any place to get an analog tach signal. You're SOL.

382 HP to the wheels... A friend of mine is putting 430+ to the ground on straight #2 fuel...

dbm76 · October 28, 2002

I've dyno'd my truck a couple of times now. Using the Mustang dyno, the only option was to use the optical style pickup. As far as I know, there isn't any place to get an analog tach signal. You're SOL.

382 HP to the wheels... A friend of mine is putting 430+ to the ground on straight #2 fuel...

OK, thanks... I'm actually pretty sure there IS a pickup because I've heard from a couple of other owners that their dyno operators did find a wire, but they didn't remember the exact location. I'll also be doing some on-road datalogging which will REQUIRE an electronic tach signal so I hope there's some more info out there. Anybody have an owners' manual or anything?

Thanks again for any and all info...

SoMnDMAX · October 29, 2002

I'll look thru my service manuals to see if I can find anything.....

Maybe there is something available...

SoMnDMAX · October 29, 2002

Ok.. Here's the info I could find, direct from the GM service manuals.

The pins/connectors listed below are found on the ECM, which is located next to the battery on the drivers side. The TCM is bolted to the back side of the fan shroud, so make sure you've got the right controller.

Pin #76, connector C2 is a Engine Speed Signal, looks to be an input to the ECM, or an output the the injection controller. The manual is not too clear as to where or what this wire does. The wire is light blue in color.

Pin #10, connector C2 is also an engine speed signal, and according the the manual, is the output signal to the instrument cluster. The wire is white in color. I would suspect this is the one you would want, but I do not know what kind of data it's giving. Is it a usable signal? Who knows.

Connector C2 on the ECM is the one with the red shield on it. This connector is also refered to as the white connector. If unsure, remove the ECM, and look down into the aluminum casting. There is a C1 and a C2 cast in there, which also corresponds with the connector number.

Hope this helps you out.

****Disclaimer****

This info is accurate to the best of my knowledge. I will not be held responsible for any damages incurred. Use this info at your own risk.

dbm76 · October 29, 2002

Wow - thank you SO much, I took the truck into the local GM dealership looking for a service manual today and they said it would be at least a week to get one, and Helms said the same thing. I really appreciate you finding this for me; I've got access to a portable oscilloscope so I can check to see exactly what signal those lines are carrying before I do anything too drastic! I've also got a couple of electrical engineers familiar with the CAN bus so they can look over my shoulder and take the blame if I screw anything up.

I've only got this truck for 3 weeks for development testing so I can't afford to wait a week to get a manual!

Thanks again!

SoMnDMAX · October 29, 2002

Sounds like you've got everything under control. :thumb:

If you've got access to the O-scope AND a couple of EE's, you'll get something figured out.

Good Luck with your testing!!

Cyclic · October 31, 2002

If the tach signal to the gauges is the same as a normal gas motor( which I suspect it is) the signal should be a 4 to 5 volt square wave, at 1/2 rate, like a four cylinder engine.