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Discussion Starter · #1 ·
Hello all. I'm trying to get my '87 Shelby Charger back on the road. Since I last dealt with it, the system that displays fault codes via the Power Loss lamp has stopped working.

The lamp works. The first thing I did was remove the instrument cluster, clean each contact and replace each bulb. I cleaned the contacts on both the red and blue connectors on the logic module. I saw no burns on the circuit board.

Here's what happens. With the ignition On, the Power Loss lamp stays off. It comes on while the starter cranks then goes off once the engine starts. I tested it further by unplugging the engine temp sensor. The computer started the fan and the Power Loss lamp came On.

When I cycle the ignition to request codes, nothing happens, not even code 55. The computer is linked to the instrument panel as shown when I unplugged the temp sensor, but the OBD fault code system seems dead.

Does anyone know what causes this and what a fix might be?

Thank you.
 

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Since we know the bulb is OK and the LM is capable of turning on the power loss lamp the issue has to be that the LM is not seeing the request for code retrieval.

Do you have access to a scanner so you can see if the scanner is able to communicate with the LM?
Verify there is no damage/corrosion/looseness at the ignition switch connector and LM connectors.
 

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Discussion Starter · #3 ·
Since we know the bulb is OK and the LM is capable of turning on the power loss lamp the issue has to be that the LM is not seeing the request for code retrieval.

Do you have access to a scanner so you can see if the scanner is able to communicate with the LM?
Verify there is no damage/corrosion/looseness at the ignition switch connector and LM connectors.

I'll check ignition switch connectors next time I'm working on it.

I have some memory issues after an injury. Ignition on and off 4 times to start the request, correct? I think that's what I used to do.

Thank you.
 

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3 times, key, on/off, on/off, on.

 

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Discussion Starter · #5 ·
Problem Corrected.

If anyone else encounters a similar problem: I checked and cleaned all under dash plugs, ignition included. There was no change. I decided to pull the ECM to clean it. I opened it up mainly because it is easier to get to the plug prongs with the cover off. I cleaned the thing inside and out. I used electrical contact cleaner, a small brush and cotton pipe cleaners to clean the contacts on the male and female ends of the plugs. I decided, since the battery was out to open and clean all the wiring harness plugs nearby as well as two I found on the right side of the engine compartment. On reassembly, the power loss light came on and on request gave codes. It seems the problem was a communication issue between the LM and the ECM or one of the other harness plugs.

I still have a high idle problem (another post). I got code 12 and 55, so the high idle isn't tripping the computer. At least the OBD system is working again.

Thanks for the input.
 
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