Guys, i posted something similar to this a few days ago.
The service light will not work(for diagonostic mode) or turn off at all. I checked all of the connections and even tried different memcal/prom combos. When the car is running , the service light is on constat. When trying to go to diagnostic mode, the light stays on constant still. Will not flash at all or do anything.
Bad ECM right?
I'm going to replace it soon, just want to know if there is anything i can check to make sure?

Brendan

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Maybe the SES light output from the computer is shorted to ground. Or the computer is shot. Try cutting the ses light output wire and run it to a small 12v bulb with the other side to + and see if it works. Otherwise bad memcal or ecm.

 

sounds like the computer or it may be the guage cluster itself. On the back it will have a plastic/copper/plastic sheet. This sheet contacts all lights and guages in the dash. IF your car still runs fine it may be the contacts on this sheet have shorted or have become corroded. But usually when this happens the light stops working.

 

Isn't there a code for a bad ECM?

 

The earlier year F-body models used a light driver module to switch the current in the SES or check engine light. I'm not sure in what year GM changed over to the ECM driving the light directly. If you have one, the light driver module may be the problem, as well.

Tim

 

Everybody that replied, thanks.

I think i've been able to find out whats going on. TPI383 from the boards here sent me a nice diagram to work with to find out the problem.
Turns out that while having the ignition on the light was on, while having the car on, the light was on. While having the ignition on and computer unplugged the light was off. So the only thing that could possbily be wrong would be the computer. If the light was still on while the computer was unplugged with the ignition on then the SES light wire would have been short to ground.

Turns out that my luck is showing a bad computer.

Brendan