1987 Chevy 1/2 ton 4x4 , 305 tbi. Feul pump went, replaced it. Now I can hear it , no feul coming out of injectors . I’m only getting 6volts at tank . I changed they relay and OPS. No change . Gonna pull the tank again and start tracing it back from there . Any suggestions? Thanks .
The fact that you are seeing (reduced) voltage at the pump would point to a problem in the power supply circuit - rather than in the control circuit. There are two paths that can supply voltage from the fuse block to the pump (through the OPS or the FPR).
The FPR is controlled by the ECM - which needs to see a distributor reference pulse before it will pull the FPR in.
On the other hand, the OPS is hydraulically operated and entirely independent of the ECM. It provides an auxiliary power supply to the pump in case of an FPR failure. The only requirement to close the contacts is that engine oil pressure be at least 4 psi. That 4 psi is easily developed when cranking the engine.
So, for the problem to exist in the control circuitry, it would mean that both the relay and the switch would have to have failed. You'd have to be really unlucky for that to be the case. On top of that , even after both the OPS & FPR were renewed, an undervoltage condition at the pump still exists.
I didn’t check the voltage at test port at relay, I did put 12 volts from battery there just to hear the pump run . But I will check the voltage there .
If you get power at the pump - and it runs - when 12V is supplied through the test port, you can forget about checking everything between the FPR/OPS and pump (TAN/WHT 120). And checking for voltage at the test port won't show anything. The only time that terminal is hot is if the FPR has failed and isn't pulling in (contacts open through FPR and closed to test port) and the engine is being cranked (to develop 4 psi) and current is flowing through the OPS.
I would concentrate on the line sides of the OPS/FPR. That is the ORN 440 circuit. It runs from the fuse block to the OPS/FPR and is fed from the 10A ECM B fuse. The voltage drop is most likely happening in that part of the circuit.
Description of fuel control circuit and diagnostic chart:
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Normal - current flow both the relay and OPS:
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Failed FPR - oil pressure established by cranking (the only time test port will show voltage):
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More detailed dwg:
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