Haynes manual 24071
I have a problem with my 1998 Chevy Blazer. The fuel pump is running constantly... The relay is energized at all times even with the key in the off position. The book is not as clear as I would like it to be. On page 12-26 of HAYNES MANUAL 24071 it shows "IGNITION / FUEL PUMP RELAY CONTROL" in a box with dashed lines around it. Next to that it says "POWERTRAIN CONTROL MODULE (PCM)". Either this is the key switch or a contact in the PCM. There is not a "legend" in the book that clarifies it. but it would seem that this is the PCM and should only be energized with the key in the on position. This indicates that the PCM is likely to be bad. I will then need to know which connector and pin is the output such that I will be able to trace the circuit back to the bad component on the board. I have a spare PCM with the same components to use as replacement parts.
In cases where the problem is traced to the PCM, the PCM is replaced and not serviced. In your case, one thing you could do is plug in your replacement PCM (without starting your vehicle) and see if the fuel pump relay is energized at that point. Let us know what happens.