OK so here is the deal. 89 Vette with a converted SD 7730 ECM. Sometimes the car won't start. It fires the injectors but not the plugs. I swapped the ECM and Memcal and it does the same thing. If I jiggle the ECM wires it will fire. It never stalls once running even if you jiggle the wires. Whatever circut has a loose connection, it only affects the ECM during start up.

Does anyone know what circut would do this? What circuit would keep the plugs from firing prior to start up but have no affect after start up?

Thanks

 

What kind off dizzy do you have on your Vette???

//RF

 

What do you mean by dizzy? I'm pushing 40 so I'm not up on the latest slang!

 

During crank the distributor is on it's own. No ECM control. To double check that the ECM isn't interfering, open the bypass (EST) connector. Then crank to start. If/when the engine starts, DO NOT drive it like this. The timing will not be correct.

During crank the ignition module sparks the plugs at the base timing setting. IOW, what the distributor is set to. If a light shows otherwise then something is not correct.

RBob.

P.S. dizzy == distributor

 

Thanks Rob. I was going to guess Dizzy equaled Distributor. Your theory makes sense about the distibutor. Maybe the wiggling of the ECM wires is not doing anything. The next time it does it, I'll disconnect the ESC bypass and see what the base timing says. I have a feeling it will not read becasue the plugs are not sparking. So what you are saying is that ECM ahd nothing to do with suppying spark during crank? What does the start up spark advance table do?

Thanks for the help.

 

What RBob means is that EST Bybass may be accidentally getting 5V, which will tell the ignition module not to fire : Once RPMs are above 400, the ECU will tell the ignition module to fire (with an EST pulse, supplied on EST wire, not bypass wire). Thus, if you disconnect the 5v EST Bypass, it should fire the spark plugs by using the ignition module's automatic mode - with no ECU control. It's sure that the ECU is recieving the signal from the module (via the REF wire), though, because the injectors fire.

Startup spark is used once the ECU assumes control at 400rpm. Cranking is completely controlled by the ignition module. The ignition module cannot handle being controlled at low RPM (they burn up for some strange reason), and the ECU would also have a problem with poor accuracy, and it's a limp home backup that can at least keep the car running.

 

Oh OK. So based on what I stated the ECM is receiving the dist ref signal because if not the injectors would not fire. If the EST bypass is producing 5V then the ignition module will not fire correct. So under normal operation, the EST bypass has zero volts until it fires and reaches 400 RPM at which point the ECM sends a 5V signal to the ignition module letting the ECM take over.

So the ECM would have to malfuntion in order to keep 5V on to the EST wire OR the ignition module is bad. Am I on track here?

 

Yes.

Also, the HEI control switch is comparing the ~5v signal (ESC from the ECM) to ground.
A loose or dirty ground connection could also cause an intermittent condition where the
gate looses track of its ground-reference, and doesn't know which signal to use.

Might be worthwhile to check all the wiring connections and/or try a new HEI module.

See the HEI diagrams here:
https://www.thirdgen.org/forums/dfi-...tml?highlight=

HTH

 

It turned out that my MSD7 ignition was the problem. I disconnected it and now it runs great. I don't knwo if it is bad or I had a loose connection. The car runs great without it so I'm all go now!

 

That'll do it. Buddy of mine had the same issue with his MSD (6A?). About 30% of the time the car wouldn't start. Bypass the MSD and it ran without a hitch. Unit was back to MSD a few times with NPF (no problem found). After the last return trip it just stayed off the vehicle.

RBob.