My great wife got me a ProForm shift light as a present and tonight I finally got around to installing it. I have the delteq system so I decided to grab the tach signal off the ECM. Now the problem is that the Y-body 94/95 did not wire the ECM tach signal the same as the F-body did. On a hunch I decided the signal was there just unused. The first thing out of the box I had to deal with was the fact there is not even a pin in A13. So I pulled the connector apart and extracted pin A14 (Air pump enable) since I no longer have an air pump. Then I cut the wire and inserted the pin into A13. Next I had to find a place to mount the darn thing that did not interfere with the gauges. After removing the bottom part of the mount that came with it I decided on the column. I mounted it hooked up power, ground and the A13 signal for the input and it worked first try. I will say that either my tach is off or the proform is not the most accurate. It is about 200 RPM different between my selection on the SL dial and the tach. Not a big deal I will double check it on the dyno next week and set the light because it does always light at the same RPM once set.
Here are a few pics

Bracket mounted


Mounted


Let there be light

 

Cool deal,

I also have about 21094234098 different imputs used that are not usually used..

The vettes have some handy inputs the Fbodys don't have and visa versa

 

I've often wondered about that, but never needed it yet.
So, I never researched it.

But, logic told me the ECM HAS to be generating an internal tach signal from the Opti-Spark signals -- just like the '96 ECM.
The ECM needs this information internally and doesn't get it from the tach filter.
But, I wasn't sure it was exposed through the external connector pin -- thanks for that information.

However, there is one question that comes to mind:
Since GM decided to generate the tach signal on the early ('92 to '95) LT1s for the tach and ASR from the ignition coil primary instead of using this, already existent, ECM signal, I wonder what the reason was?
Was it because it was not considered as accurate?
Was it because it did not have the power to drive external loads that may load down the signal and cause it to give inaccurate information to the ECM internal circuits?
Was it because, if an external short happened on it, it would damage the ECM, where this would not cause nearly the disaster if the tach filter output was shorted?
I would be tempted to put an high input impedance (close to that pin output) amplifier/driver circuit on it to use it to eliminate problems.

But, you have given me information that I consider important -- thanks.


Tom Piper

 

Alvin,

Are these the same ECM number?
If these ECMs are the same part number and the pins are used externally by the F-body or Y-body, then it must be OK to use them externally without the possibility of damage.

Is there any documentation on these pins for the F-Body and Y-Body?


Thanks,


Tom Piper

 

Tom,
Just to clarify the 92/93 ECM is different then the 94/95 ECM. The reason I was pretty sure the 94/95 had the external tach signal is that I have done some work with 94/95 F-body and they use that signal out of the ECM. When I did my LT1 swap into my 84 I used a Y-body LT1 harness and an F-body ECM and it worked fine. As for the ASR I am not sure. Maybe it was a left over from the 92/93 on the engineering side of the house. Does the 96 use the ECM output for the ASR? The reason I ask is that it is possible the ASR wants an analog signal, just a stab in the dark. I don’t think signal strength is the problem there the F-body uses it to drive the Tach and I have to think the isolation is pretty high. Speaking with the guys at Delteq the first thing they suggested was the ECM tach output but then I told them it was not wired on the Y-body and that is when they said they would send me the new harness with a diagram of what pin to hook up. I may try to contact Pro Form today and see what sort of signal isolation they have as I want to hook up a FJO progressive nitrous controller as well. I contacted FJO and they have over 100K ohms of isolation so the pull would in the micro amps. As for it interfering with the internal ECM circuits I think this should also be nearly impossible as I think it has to have some sort of output driver that would provide complete isolation. Now without the circuit details this is all speculation but I believe it is a pretty logical approach.

 

Ok, thanks for the clarification.
I am actually thinking more about the '92 and '93 ECM -- a different animal.
But, logic tells me the '92 and '93 ECM have to generate an internal tach signal from the Opti-Spark signals for ECM use. But, from what you say, it may not be exposed externally.

And, your logic sounds correct, if the F-Body uses the '94 and '95 tach signal generated inside the ECM for external loads, then it is probably isolated.

If you have the Delteq setup, the Northstar Coil pak has an external pin that supplies the composite tach signal from the four coils -- on the Northstar unit, it is beside the Ground and Power terminals.
Delteq takes this (already filtered) composite tach output and exposes it through their "Opti-box" connector B pin D. On '92 to '95 cars, this signal is then fed on through the "coil jumper connector" (this plugs onto the end of the original harness that normall plugged directly into the original coil), and this feeds the tach filter -- the tach filter feeds the tach and ASR with a square wave.

On '96 Y-Bodies, the tach signal from the Northstar and Delteq are not needed because the dedicated ECM tach output feeds everything.
However, these Northstar and Delteg tach signals would still be available for you to use, if you decided to tap into them.

You can download the Delteq manuals, with schematics and wiring, from here:
http://www.delteq.com/support_manuals.htm

Tom Piper

 

Tom,
I know you have read Steve's post about problems using the old tach wire with the Delteq in place. I was thinking it may because the tach filter was by-passed and the signal may be a little dirty. I have spoke with Auto Meter and they said that a 10K 1/4 watt resistor will probably fix the problem. Now with that said Delteq stated there is another tach signal on the Northstar Coil pack that is not used by the Delteq system and is not wired, one of the unused pins, and that may be a cleaner signal to use.

 

I actually have a listing of the "unused" Northstar pins that Pete sent me some time ago.
When I get home tonght, I am going to attempt to find this "unused" tach pin on the Northstar.

Tom Piper