Check Engine Light Steady On

[mamaya3367]

Hello Everyone,

I have a check engine light that is always on.  Here is what I've done so far.  My suspicion I have a short somewhere but I can't find it.  Any help would be appreciated. Thanks

1. Disconnect the battery to see if it will reset.  No change
2.  Tried to ground the diagnostic connector to see if the light would flash.  Although I'm not sure I had the correct pin out. No change
3.  Disconnected ECM connectors.  No change
4.  Checked the brown/black wire shown on some diagrams to see if it was touching ground.  I did not.
5.  Used a friend's diagnostic tool and it could not connect to read any codes.  I visited a few auto zones and no one had an OBDI adaptor.

[bd]

Is this on your '86 K10 with the 5.7L engine?

Post a full-frame pic of the VECI label located on the air cleaner or radiator support, so that it is clearly legible and a large pic of the engine compartment with the air cleaner removed, please.  I would like to see the emission control system configuration as it currently exists.

[mamaya3367]

Thanks bd for the response.  See the attached pictures.  I hope the resolution is ok

[bd]

According to the VECI label you have a California emission certified truck with Computer Command Control.

Start with the diagnostic procedures attached, below.

Enter Diagnostic Mode by jumping the ALCL connector cavity A to cavity B with the ignition switched ON engine not running.  If the diagnostic system is working correctly, code 12 will flash 3 times, followed by 3 flashes of any stored codes in numerical succession starting with the lowest code stored in memory.  FYI - code 12 is never stored in memory; it represents no tachometer signal from the engine in real time and serves as a diagnostic system self-check.  Code 12 will flash as 'blink' (short pause) 'blink-blink' (long pause) 'blink' (short pause) 'blink-blink' (long pause), a total of three times before moving to the next higher code in memory.  EDIT:  When the last of the codes stored in memory 'flash' the cycle will start over with code 12.  In other words, if there are no codes in memory, code 12 will flash indefinitely.

[mamaya3367]

Hello bd,

Thank you for the diagrams.  I was able to find the issue.  In chart A6, pin C on the ECM side had no power.   This was due to a bad connection.  Everything is all good now.  No more steady check engine light!

Mamaya3367

[bd]

Well done!  Thanks for the feedback.   

[1987ChevyR10Man]

I had the exact opposite problem on my '87 R10 2-Wheel Drive truck, no Check Engine light... but the engine would hesitate badly and miss on acceleration. The analyzer said I had MAP sensor and ECM issues, with a possible failed ECM. I found out that there was no bulb in the instrument cluster for the Check Engine light, someone had swapped it to the 4-Wheel Drive spot which is not used on this truck. First thing I did was disconnect the battery and then check and carefully clean the wiring harness connections from the firewall back to the ECM. I found no obvious corrosion, but at the ECM the harness connector pins and PROM chip pins looked dull. Making sure I photographed the orientation of the chips (will cause damage if plugged in upside down, etc.) I carefully cleaned all pins and put it back together. The codes never came back and the engine runs like new. 

[bd]

The ECM and sensors run on a 5-volt buss.  It doesn't take much resistance to cause a problem.  A modest rinse of harness connections with electrical contact cleaner such as CRC QD or DeoxIT can eliminate all sorts of ghostly misery.