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Discussion Starter · #1 · (Edited)
I just got my '84 back from the paint shop (after almost 2 years) and the check engine light is on all the time. I bought an old style GM code reader and it's not throwing any codes, not even the "12" code which I guess means the system is working. I replaced the PCB last year and all the lights and gauges work so I don't thing that's the problem and it runs great, just like before. I thought it might be a bad code reader so I jumped the 2 correct pins with a paper clip and still nothing. Any ideas?
 

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It should always have code 12, that's the code it's working
If I remember correctly
 

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It should always have code 12, that's the code it's working
If I remember correctly
Correct. Check the two fuses for the ECM first. If they are good, then is may be a bad ECM. If the paint / body shop did a lot of welding, could have fried something internally.
 

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Discussion Starter · #4 ·
Correct. Check the two fuses for the ECM first. If they are good, then is may be a bad ECM. If the paint / body shop did a lot of welding, could have fried something internally.
I see 1 fuse in the fuse panel marked ECM, where would the other one be?
 

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One line is 12v Battery power ( 10 amp) the fuse you found. The other one is 12V Ign Power, powered through the Gauges fuse (10 amp). I'm assuming the gauges work, so if both lines have power, then it's likely the ECM is bad.
 

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The 2 ecm fuses on my 87 are on the right side of the block. One is the very top right, the other just above the blinker.
 

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perhaps your ecm was disconnected or is loose. Your car will run without it connected . it will run like crap but will run
 

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When does it run great though? You can have a motor purr at idle, but as soon as you apply a load, and ecm/dizzy advances the timing, if that's not being done somewhere fully, the motor bogs.

If the cat is clogged almost all the way, there's enough to let the little pressure from idle rpms to escape, but higher rpms create toouch backpressure and the motor bogs.

If the coil or cap is bad, a motor can purr at idle, but once the timing advances and demands more, motor can bog.

If your secondary butterfly isn't opening but the plate is, you'll dump fuel when stepping on the pedal and not enough air and the motor bogs.

Your car can run great at idle, even start right up, but trying to get beyond that is the issue. Is it a motor bog or e-brake stuck on or?
 

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Discussion Starter · #12 ·
It runs just like it always has. You can stomp on it and it takes right off, no issues. It idles fine. If it wasn't for the CEL, you'd notice no difference.
 

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And you are sure it's the CEL that's actually lighting up and not the yellow Choke light that's right under it and looks the same? I know I'm having low oil pressure issues and if the guage hits close to 0 even for a second, the Choke light will come on temporarily (its linked to the oil pressure sensor) and shortly after that the CEL will come on and stay on until I shut the motor off, after which it goes off and no codes.
 

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Discussion Starter · #14 ·
Nope, not the choke light. It will come on when you turn on the key but as soon as it starts running it goes out.
 

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hmmmm, have you pulled the dash out since the CEL came on? maybe you have a bad light bulb or possibly something else in the back that has grounded the circuit to full power? we did have a thread recently about a member whos turn signal bulbs would all flash when used. seems it may have been caused by a grounded failed filament inside a bulb.
 

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Discussion Starter · #16 ·
I replaced the PCB about 6 months ago but the issue didn't show up until it came back from paint. Tonight I pulled both fuses to the ECM and left them out for 10 minutes while I pulled the ECM and checked all the connections and everything looked good. Replaced the fuses and the light was still on. Since all the gauges and lighting work I can't imagine it being a ground issue.
 

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Its possible your code reader isn't working. You can read codes without it by grounding the test terminal at the data link connector and watching the check engine lamp flash. Use a paper clip to short terminals A and B together. They are on the top row of the data link connector and the first 2 terminals from the right. Terminal A has a black wire, and terminal B has a white wire with a black tracer. When shorted together, the check engine lamp should start to flash on and off. Count the flashes to get codes. For example; on, off, pause, on, off, on, off, pause, would indicate 12. If your lamp doesn't flash, you have a bad ECM.
 

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Look at the first attachment in VinnyD's post. It has a picture of the data link and describes the same procedure.
 

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Discussion Starter · #19 ·
I've tried the paper clip technique and it won't even give me a code that way, just a constant light on.
 

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Did you follow the trouble shooting flow charts that I posted?
They are taken from a 1984 Factory service manual.
 
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