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93 ECM trouble

5K views 6 replies 2 participants last post by  kwacasey 
#1 ·
1993 Z28, Recently had a tune and updated to long tubes and heated O2s. Car sat for a few months due to some other maintenance items/lack of time. Went to start it and it stumbled and would not stay running without pedaling. Note that i have owned the car for over 5 years, has always started first try no excessive cranking so i assume that my (what looks to be original) ignition components have finally gave up the ghost. I planned to replace all of this anyways so i ordered and installed a fuel pump, a new opti, coil, and eventually ICM.

Long Story Short (lol), I eventually troubleshot my issue to what i believe was the ECM so i decided to pull to ensure that the moates connector did not come loose. I was surprised to find that I had a leak on the access panel of the ECM. This is based on the corrosion found when opening the unit and standing water found inside (yikes).

Pulled the ECM apart and cleaned it up with some 97% alcohol. Luckily the conformal coating seemed all intact, however any exposed chassis metal was corroded, cleaned this up with a wire brush and reassembled the unit. I am still finding a troublecode 16, which is low-res issue based on my documentation. Checked the connector and found what seems to be the correct supply/signal per shbbox.com. Ended up pulling out the O-scope and capturing both low and high res (A/B on the connector) and they seem to be OK. Posted some pics for reference, let me know if these look wrong.

low-res
https://www.dropbox.com/s/z1ynzue5ui1ldpe/2016-11-27%2018.37.50.jpg?dl=0
high-res
https://www.dropbox.com/s/2jj0puzc110yblc/2016-11-27%2018.39.13.jpg?dl=0
(lmk if the links do not work)

I was able to track these all the way back to the ECM. However inside the ECM i have no schematics so i cannot troubleshoot the ECM/EPROM to find out if one or both must be replaced. Sent Solomon an email to see if he could share any info on where i might look on the EPROM to see if it is working.

Any help you guys could lend would be appreciated, ultimately if i cannot fix myself I may consider using one of the repair services such as SIA. Experiences with this would be great as well.

Thanks
 
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#2 · (Edited)
On the high res signal, you need to change the speed so each individual pulse wave form can be checked.

CORRECTION- On the low res pulse They should have different wave lengths with the same wave height. Just like your pic shows. These tell computer what cylinder is going to fire. So Disregard the original info after this.

On you low res pulse, that's not right. With engine running or cranking at a fixed rpm, all pulses should be the same pulse width. The 1st, 3rd & 5th pulse look good The rest of them are too short a time period. You have a low res problem there.

Since the pulse originates at the opti and goes to ecm, your problem should be in opti itself. But I would try a new opti wire pigtail, just to make sure. Before buying a new opti.

If you find you need a new ecm, good luck finding one. They are in very short supply as 93 was the only year that type of ecm was used.
 
#3 ·
On the high res signal, you need to change the speed so each individual pulse wave form can be checked.

On you low res pulse, that's not right. With engine running or cranking at a fixed rpm, all pulses should be the same pulse width. The 1st, 3rd & 5th pulse look good The rest of them are too short a time period. You have a low res problem there.

Since the pulse originates at the opti and goes to ecm, your problem should be in opti itself. But I would try a new opti wire pigtail, just to make sure. Before buying a new opti.

If you find you need a new ecm, good luck finding one. They are in very short supply as 93 was the only year that type of ecm was used.

Is it possible that i have my high/low res pulse labeling incorrect? Note that i labeled high res as the pulses that are all of the same duration. My labeled low resolution is based on the varying pulses that indicate position. Here is the reference material that i used to determine this:

Opti-Spark Oscilloscope Pattern - CorvetteForum - Chevrolet Corvette Forum Discussion

https://www.dropbox.com/s/rlzcysncb30t34p/ccrp_0801_10_z-gen_ii_chevy_small_block_engine-.jpg?dl=0

I removed the opti and checked by hand and with a drill, found that both produced patterns similar to what is found above.

Additionally i should add that i am finding that my fuel pump will not go through a prime cycle now. It seems that it should be turning on for some number of seconds then turning off until the car is cranked over. I checked several different ways.

Let me know your thoughts
 
#4 · (Edited)
Your low and high res pulses are labeled correctly.
It is I that made the mistake of forgetting the low res pulses are of different duration. So they will have different pulse widths on the scope.

Sorry if my incorrect diagnosis has caused you any problems. It's been several years since I've looked at the low res pulse on a scope.

Try your fuel pump prime connector. See if pump powers up with 12 volts directly to pump. If no, wiring to pump or pump is bad.

If pump runs using prime connector, try another fp relay. It's behind left front kick panel, mounted to foot rest bracket.

Also make sure you have key on power at relay terminal C1. It has pink wire coming from fuse #5 (ecm fuse) of ip fuse box.

Also make sure that in the 2 seconds after turning key on, that you have 12 volts on terminal C2 connector. It's fed by a dark green/white wire. If no 12 volts when tested for the 1st 2 seconds, then check connector C220. Its a 10 cavity connector behind right front kick panel.

If no power there, check dark green/white wire at ecm itself. It's ecm connector D, cavity #7. (brown connector.

If no 12volts there, check for codes dtc 16 (opti low res pulse missing) is the only code that stops fuel pump from working.

Report your findings.

This is a extreme long shot. If fool proof coil connectors break and they are installed backwards, this will keep fuel pump from running and no injector pulse when engine cranks.

My 90 f-body did this to me a couple of years ago. A piece had broken out of connectors that allowed me to install them backwards.
 
#5 ·
thanks for the confirmation on the pulse info, that is truly helpful.

as for the fuel pump, it comes on just never goes off. I confirmed this at the relay behind the kick panel by monitoring the relay drive. I will check again as a stuck relay makes sense here.

Still looking for ECU but would feel better if i could prove the signal is getting to the correct place on the ECU itself. I wish I could find schematic or block diagram for this.

Thanks again for all of your help and I will make sure I post my progress/results for others reference.
 
#6 · (Edited)
We have the 93 service manual in the 4th gen section. Unfortunately, it is for the firebird, so there may be some differences in your camaro. But it will still help you.

Go to 4th gen sticky threads and look for the service manual thread. Use the link to get the manual. In 94 GM started including both camaro & firebird in the service manual.

You can get most of the 93 camaro wiring diagrams at the link I will provide. When you get there hit the technical info tab and it will have a link to wiring diagrams. www.bbbind.com

Not completely sure if fuel pump coil, which is fed by ecm, is 12 volt from ecm, or 5 volt from ecm. Most relay coils get their ground from ecm, or pcm in newer models. This one gets power from ecm, so there's a chance it could be 5 volts instead of 12. So test voltage on the dark green/white wire.
 
#7 ·
turns out it was the PCM, replaced it and it fired up. Thanks
 
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