85 TPI Fuel Pump Woes

[Les-10]

Having a fit of a time trying to repair my club members '85 TPI fuel problem. Pump stopped working so we installed a new relay (before taking time to test the original) to no avail. We since tested the original relay (good), so now we've got 2 good ones. Using a 'Haynes' we started trouble shooting & have learned a lot but still have not solved problem. FP fuse checked out OK but there is no power on that switched circuit. Ran 12 volts to fuse which runs pump & allows engine to run 'hot-wired.' Did power checks at relay connector & found everything checks out except there is no power coming from the ECU on circuit 465 (green/black) to cycle the relay. Removed the ECU connectors & found that it's getting 12 volts on 3 of the 4 designated pins with only exception being circuit 458 (red wire) & it's not hot because circuit 465 when switched is not cycling the relay. It seems to me that the computer is bad but our GM trained confidant says if the computer was bad, that it would not run the engines fuel injection controls, etc. Of course, the GM guy won't come & help with the trouble shoot. Question is--Is something prohibiting the computer from sending the voltage to the relay or is the computer causing the problem? Sorry to be lengthy, Thanks in advance for your help.

[c4c5specialist]

HI there,
First, your oil pressure switch would send power to the fuel pump even if the PCM was totally dead. So, that already eliminates your PCM as the main culprit.
Next, monitor your fuel pump fuse with a test light and see if it goes on after a 5 seconds of cranking the engine.
If it doesnt, that tells me that you have a blown fusible link at the battery junction block or the oil pressure switch is no good.
The FIRST thing to do is to validate all the fusible link.
Allthebest, c4c5

[Les-10]

Originally Posted by c4c5specialist

HI there,
First, your oil pressure switch would send power to the fuel pump even if the PCM was totally dead. So, that already eliminates your PCM as the main culprit.
Next, monitor your fuel pump fuse with a test light and see if it goes on after a 5 seconds of cranking the engine.
If it doesnt, that tells me that you have a blown fusible link at the battery junction block or the oil pressure switch is no good.
The FIRST thing to do is to validate all the fusible link.
Allthebest, c4c5

Greetings!!
Can't Thank you enough for your on spot diagnosis & advice. Investigated oil pressure switch & was excited to see which circuits went to it. As is often the case, a previous procedure caused the trouble. Before the fuel pump problem, we had changed out the valve guide seals & apparently inadvertently loosed the oil pressure switch connector from the unit's seating surface. Without your response, we never would have thought about the oil pressure switch. After approx 6 to 8 hours of hunting around in the dark, you got us on track. Running great. Again, Many Thanxxx!!!!!!!!!!!!l