I'm in the middle of a project to replace a carb w/a TBI, harness and ECM, which happens to be an 8746. Can anyone produce a digram/JPG what ever, showing which pin on the ECM does what?? Thanks, this is quite urgent! Thank you. -Tom

http://www.diy-efi.org/gmecm/ecm_info/1228746/

Bottom of page.

RBob.

HEI Help now
OK, Thanks for the link. That was very cool and helpful.

Now, I am trying to get this TBI to work, and we have no injector fire. What we did was to connect Pin #B5 to the "tach" output on the old school HEI mechanical distibutor. Will that worK?

For sensors we are using a MAP, TPS, Coolant, IAT, and a pick up from that tach out put on the dist.

For out puts, we just want to use the injectors. We want to use the old HEI dist, and just leave all the ESC stuff disconnected.
We want to keep it as basic and rudimentary as possible.

What are we missing here?? Thanks again!

-Tom

TTT Thanks!

Need to ground ECM pin B3, ref low. The ECM pin B5 is the DRP (ref high) input. The ECM is expecting a square wave of about 5 volts. I don't know what the tach output of the HEI looks like.

To use the tach output it may be required to clip it to 5 volts. IIRC GM tach outputs are from the negative side of the coil which has a spike on it.

RBob.

Oh, man. This could get a lot more complicated than I wanted, real quick. So are you saying the old HEI module's output is different than a '92's? Do I need to run a '92ish distributor and the ESC?

Thanks
-Tom

Well, I don't believe that the older vac/centrif HEI's ever had to signal an ECM. So yes, they are different. The tach output was to drive a tach, not supply a DRP to the ECM.

The computor controlled distributors have an ignition module with 7 pins to the outside world.

It would be better to run a computor controlled distributor. Can still skip the ESC system (which is only the knock detect circuitry).

RBob.

Gotcha.

So theoretically, I should be able to slap an '83 up, large cap HEI module into the HEI dist. I have now, and grab a tach signal off that?

Thanks again for helping out here.

-Tom

Sorry, but this is a grey area for me.

Could I put, say, an '85 TPI module, into this old distributor, then use the pin on that module that the TPI would use for a tach signal for? Am I making sense?

-Tom

Yes, that just may work. Leave the bypass open so that the module fires at the DRP and use the distributor advance functions. Hmm, seems like it should do the trick.

Would be better of course to have the ECM also control the timing. . .

RBob.

Thank you very much! I agree that ESC is better, but this is an industrial application (ski lift APU) and mechanical timing will be sufficient. Thanks again.

-Tom

This project is going no where fast. There is no fuel injection occuring. Only injects fuel when "triggered" by the TPS. I don't know why.

I got another scan tool (long story and long time frame there) and I was going to hook up the scan too to see what I couls see, and there is no ALDL on this harness!? Does the ALDL come from the big firewall connector on the drivers side, or does it come from the ECM directly, straight across the dash?

I can't believe that this isn't more plug 'n play than it is. I also can't believe that I can't figure this out! Grrr.

The fuel triggered by the TPS is Acceleration Enrichment (AE). For normal fueling the ECM needs to get a Distributor Reference Pulse (DRP). Check the distributor 4 pin wiring to the ECM. Note that the two pin distributor connector to the coil provides power for the ignition module.

Not sure how the ALDL gets to the connector. Should be able to wire it right to the ECM. Only need ground and the ALDL serial line.

RBob.