mddaniel

iTrader: (1)

I am still trying to solve an engine cranks, no spark issue on 92 TPI camaro. I posted in engine section but would like to rule out ECM problem or if i need to test further, but not sure how to proceed.
Problem is no pulse to coil which no spark to distributor. Have 12 volts power to coil from ignition.
I switched 7730 ECM with another junkyard but had to move the AUJU memcal and prom from my ECM to new since not from GM TPI V8 so not sure if memcal is problem. I did take PROM out and save to TunerPro RT and compare to BIN from year ago and the compare tool showed no differences.
I did datalog a few seconds of cranking and I did get data from sensors but 0 from BPW. Also, all codes checked out as OK. I don't have datalog info with me at work, but could add later if needed. I did record a pulse reading but number was high (1000+) but could be nano seconds or other small factions of second but not listed what measurement. I did verify at ECM that it was receiving pulse but could not find a pulse leaving ECM, but not sure if ECM send pulse back under 400 RPMs or only takes control above 400 RPMs which brings me to my question in Title.
This weekend I ohmed each injector and each was 14.8. Using test probe i do have power and ground at injector wire but no apparent pulse. I do not have a noid light so why I checked what I could with DMM.
I did have Ignition module checked at parts store and ohmed pickup coil. I did get another distributor at junkyard with tested IGM.

Here's my understanding, that the Distributor shaft rotates when engine turning and pickup coil sends voltage to IGM and IGM translates to digital pulse signal that ECM can read and/or sends pulse to ignition coil to fire spark to Spark plugs. I believe the ECM takes control of timing of pulse above 400 RPMs through control wire. But not sure if under 400 RPMs or at startup that the IGM sends pulse to both ECM and coil or if the IGM only sends pulse to coil after receiving pulse from ECM.
Thanks for any information you can provide.

bigal55

iTrader: (1)

Does your distributor have a good ground? The ICM needs to be grounded through the distributor to the engine. Some members use a ring terminal under the mounting screw for the ICM and run a wire from there to a good ground spot. If you changed distributors and used a gasket under the lock down collar, you may not have a good ground. You can run the wire from the ring terminal at the ICM mounting point to the engine block itself to be sure of a good ground. HTH!

mddaniel

iTrader: (1)

Thanks for the reply, I will try that when i get home tonight. I have checked many of the grounds for ECM, injectors, etc. and all had connections to back of heads but i did not consider a direct one for the distributor.

mddaniel

iTrader: (1)

Okay, a ground wire from ICM to body did not make any difference. Any other suggestions?
Thanks!

RBob

iTrader: (1)

The two wire connector setup between the coil and ICM may be bad. The wire tends to break inside of the insulation where it exits the coil end connector. The pink wire is +12 volts to the ICM with the white wire being grounded by the ICM to charge the coil.

To eliminate the ECM as the issue unplug the 4-wire connector from the ICM/distributor. The distributor should still fire the coil/plugs while cranking. Note that the engine won't start as the ECM won't fire the injectors.

RBob.

mddaniel

iTrader: (1)

Thank you for replying! We did continuity check on the two wires between coil and ICM and both had continuity. I did follow Chilton's testing tree for no start condition and one of the steps is to unplug 4 connector from distributor and did not see spark out of coil, but didn't realize that took the ECM out.
I think i see a flaw in my testing procedures to test for pulse, since I don't have a way to pierce wire to test while connected i removed 2 wire from coil and attached test light to white wire (pulse from ICM) but didn't supply power to pink wire to power ICM then. I was following the pulse and assumed power to ICM was from ECM. I feel a bit stupid. I knew I had to be missing something.
Is it possible that ECM and Coil both failed at same time? I originally swapped coil with friend's 91 camaro with no change so i put mine back on after ohm testing with DMM and had no obvious problems but then later we got different ECM and distributor from junkyard. Now that i have some datalog info and removed 4 wire we eliminated ECM as problem. I have been using a tested good ICM and pickup coil so I am back to ruling out ignition coil. I guess i will find out if the Parts Store can test coil or not.

ASE doc

Aside from KV testing on a working coil, the only test for an ignition coil is the resistance test that you can do yourself. Rather than try to write out the procedure, I would urge to to look for the information on line. Someone should have posted this info. Roughly, the secondary should run about 27Kohms to either of the primary connections. The primary coil should read .5-1 ohm. There should be no continuity from any of the 3 connections to the coil frame.

A quick test of the coil is to apply 12 volts to the B+ and tap the B-to ground. You should get a spark from the secondary when doing this.

I have seen coils test okay for resistance while having an internal leak from the secondary coil to the frame. Sometimes you can see this, a hairline fracture in the epoxy with a dark blue trace around it. A coil with this type of failure will resistance test okay but may not produce spark. It may still support a low KV spark, like 10KV, but not the 40KV GM specifies, and not enough to fire the plugs though the secondary circuit.

mddaniel

iTrader: (1)

I have tested the ignition coil by DMM and resistance within normal range and tested by 12 volts to B+ and B- to ground and did get brief spark.
I connected volt meter to power wire of ignition coil and measured 9-10 volts while cranking. From research it appears to be enough volts for ignition coil to fire, but is that enough volts for ICM to switch coil on and off? If not, then how do I find the voltage drop? No fuses have ever blown to point to a single circuit. Is it possible the starter is drawing too many amps than normal?

bigal55

iTrader: (1)

Have you checked the pick-up coil? Have you seen the ChevyThunder.com website? There is an HEI/EST troubleshooting section that is very comprehensive with a good explanation of how the system works, check it out.

ASE doc

10.5 volts at the battery is typical while cranking. You can expect to see 9-10 volts on the ignition circuit at that time. That will not cause an otherwise healthy ICM not to operate.

I agree will bigal55, check the pick up coil. It is often overlooked and the ICM replaced with no good result. The coil should read 500-1500 ohms at room temperature and there should be no continuity to ground from the coil. Check pick up coil leads for broken insulation or intermittent open circuit.

mddaniel

iTrader: (1)

I did test the original pickup coil and the replacement one from Junkyard and both were around 978 ohms and no continuity to ground on either lead. When i datalogged with TunerPro RT, i did get reading for pulse.
I did also see the ChevyThunder.com troubleshooting for HEI but toward end requires Noid Light which I don't currently have but may have to purchase soon. I have ohm tested injectors and everyone was 14.8 ohm. I do have power at injectors with key on but have not been able to test for pulse while cranking. I do have continuity from ECM on both leads to injectors on both sides.

ASE doc

All HEI tests can be done with a DVOM and 12 volt test light, except for the module itself, which can be tested by most parts stores. Just because it tests okay cold, doesn't mean it won't fail when hot.

mddaniel

iTrader: (1)

Finally fixed my problem so to update for future reference. I replaced the 4 wires from Ignition module under distributor cap to ECM. I don't know which one was problem since ran 4 new wires and spliced all at once. Even though we tested continuity on all 4 wires, i believe one must have had scrap or scuff that caused the Ignition Module to think that the ECM had taken control of pulse to coil so never could get spark out of coil on startup.

soarestransam

Ok so i know its been a while since i posted was wondering what year shouldni get a knock sensor from pulled mine out to see how it is and found out its a original so 25+ year sensor could be the reason why its backing out timing im wondering what year should i should get one from should i get one from a 91-92 car since that was setup for speed density

RBob

iTrader: (1)

The stock replacement for your '88 will work fine.

RBob.