johnnyzvette

Is there a way to check if my ECM is bad? I'm getting a code 43. I have a 92 auto and I've lost a lot of power. I'm getting some backfire/hesitation/stalling/rough idle...etc. I've checked the knock sensors and I'm getting 1.72 volts with the ignition ON. FSM says if it's below 4 volts I have an open, short to ground, or faulty ECM. Any input would be appreciated. I just would like to know my ECM is bad before I have to replace it....thanks guys! I miss driving my vette....

geogolf

sounds like optispark how many miles

Casethecorvetteman

Johnnyzvette, id imagine youve been over a few other items as well as checked that code?
Have you checked the connections at the ECM to make sure they are all nice and tight and clean?
Have you checked all the ground points to make sure they are tight and clean?
Have you checked the wiring from the knock sensors to make sure its all in good condition and has no damage?
Have you checked to be certain that no spark plug leads are anywhere near any knock sensor or related wiring? (this can cause EMI)
Have you checked the spark plug leads are good and none have burnt through, none are arcing on anything, and none have damaged boots at either end? (especially check the coil wire where it goes behind the water pump)

The GM service manual i have here is not for a 92 so i cant be sure about the diagnostic procedure it states in there, but it is abit different in the book i have.

If you really want to be certain the ECM is good or bad, find someone else that has a known good working 92, and swap the ECM from your car into theirs and vise versa. If it sets a DTC 43 in another 92 with your ECM you can be pretty sure its related to the ECM.

Id love to know what facts you base that on.... 150 different things could cause "some backfire/hesitation/stalling/rough idle...etc". Its not out of the question that it very well could be the distributor, but it seems youve instantly jumped to the "lets blame the Optispark distributor" form of diagnosis. DTC 43 was all that is mensioned, and its totally unrelated to the distributor. Mileage has nothing to do with the ditributor and when it will fail.

scranage

The 92/93 ECMs are very hard to come by unfortunately. Some who have gone with rebuilds have reported getting bad replacements ECM's after long waits, so as Case says, try to find someone with a running 92/93 that you can swap with. If the doner car has a different trans, swap the MEMCALs too.

You can also try swapping the ECM from a 93 F-body LT1, but you will need to swap the MEMCAL regardless of the transmission type from an F-Body, the knock sensor circut is different. If you can't find one locally, PM me. I have a spare known good ECM on the shelf I can loan you.

johnnyzvette

Case, thanks for your reply...I will check all the grounds and connections more thoroughly. I did find a spark plug wire that had contacted the exhaust manifold, but I did an ohm's check on it and it reads about 5200 ohm's...I believe that's ok. I didn't find any other ones with problems. Spark plugs and wires were done about a year ago. I checked fuel pressure at the shrader valve, getting 47 psi. I will try to find someone who can loan me an ECM.

Casethecorvetteman

That plug lead could be damaged though mate, the ohm test may be fine, but did you test it to see if it arcs to anything? You could be lucky and it might be fine, but the only way to know for sure is to be thorough and check every aspect of things that are capable of causing the issue youre having.

Being thorough takes alot longer, but it has a much better sucsess rate, and is usually alot cheaper too

Let us know what you find on the way

johnnyzvette

scranage was kind enough to loan me a spare ECM for my car....I will keep updating as I find out more....thanks for the help guys!

Casethecorvetteman

What a bloody good bloke he is!!

Fingers crossed you find your ECM is still good Johnny, youll know soon mate!!

Ill have a scotch or 2 for you both later on tonight

johnnyzvette

Well, back to the drawing board! I installed the ecm Scranage was nice enough to loan me and no help....same problem.
I will keep searching.

Casethecorvetteman

Well at least youre now aware that your ECM is not the issue, so that is one me key item youve ruled out, and one step closer to being fixed

scranage

If you say the voltage you measure in your first post is with both knock sensors connected, it sounds like the knock sensor bias is right where it is supposed to be. It should be around 5 volts with both disconnected. You should also check the voltage with each disconnected, one at a time. Each with the other disconnected should read 2.2-4.1 volts, and each should read 3300-4500 ohms out of circuit. It is also possible the knock filter circuit is bad, that is in your MEMCAL, which you transferred to my loaner ECM. I could loan you a spare MEMCAL, but I'm not sure I still have a stock 92 Auto program to load in it. I'd need to look around some.

Are you sure you don't have a real source of knock??

johnnyzvette

I was getting 1.72 volts coming out of the connectors with both disconnected from the knock sensors, and about 3500 ohms at the knock sensors out of the circuit.

I'm not sure about a real source of knock...there is a slight ticking sound coming from somewhere but I can't pinpoint it. I'm not sure that would be enough for the knock sensors to pick up, though.

scranage

Disconnected, that knock sensor bias should be 5VDC. Since you have eliminated the ECM, I'd say you must either have a wiring problem between the ECM and knock sensors, or a bad knock filter in your MEMCAL. Look for the broadcast code on your MEMCAL, or on top of the ECM next to the service number (Should be something like "BDZL"). If you can find a copy of that bin on the net and email it to me, I can program a good MEMCAL and shoot you a loaner to try.

johnnyzvette

Sounds like a plan to me! I will do that as soon as I get back from a mandatory week vacation in Florida....yeah I know...the wife says we have to go. Just wanted to let you know why I won't be responding for about a week. Thanks again for your help Steve.

johnnyzvette

I just wanted everyone on the forum to know, Scranage is a great member....He's been helping me with my problem tremendously. He loaned me a spare ECM and now is going to loan me a memcal that I'm sure he had to dig around his garage to find. All this without even knowing me! It's nice to know the forum has such great people.

scranage

No problem John. I just dropped it off at the post offce, hope this gets to the bottom of the problem.

johnnyzvette

Thanks Steve, I hope that's it as well.....I'd like to get back in the vette before summer is over.

johnnyzvette

Problem solved! I must of had a bad connection somewhere....I think I took every electrical connector on the car off and re-connected. This morning I was going to do a compression check on the engine and as I was looking at the manual to label all the wires I noticed the #6 and #8 spark plug wires were crossed. I must have crossed them while I was trouble shooting earlier. Anyway, the problem started before I touched the wires, but I must have fixed it and didn't know it because the crossed wires were making my car run like crap! Thanks for all the help guys! Thanks Steve for all your extra help and loaning me parts.I'm back in business!!!

scranage

Great news, but it still doesn't explain the bogus voltage reading on the knock sensor circuit. Did you get to the bottom of that?

johnnyzvette

That's true...I was just so excited the car started running good again.Just for fun, I installed the memcal you sent( I like the way the car drives with your memcal)....I will check the knock sensor voltage with that memcal and see if I get different readings.By the way, did you get the ECM I sent back?