Fuel Pump Circuit Diagnostics
 

I'm working on an '02 blazer, 4 door, 4x4.

I have a no-start condition and verified that it's fuel related. Key "ON", pump does not prime and no fuel pressure at the shrader valve.

Fuses checked out OK. I pulled the relay and there's 12 volts at both the circuit and power terminals. I swapped the relay with a known good relay for the horn and the truck still wouldn't start so I know the relay is good.

I then checked the electrial connection at the pump (gray wire) and do not read any voltage.

I inspected the wiring harness and it seems to be wrapped up pretty good and I have not disturbed it for any other maintenance or repairs, so I don't see how there could be a bad wire in there. The only exposed section is near the fuel filter and from what I could see, there was no damaged wires.

Before I begin having to poke through the insulation on the gray wire throughout this harness, has anyone experienced something similar or can provide some guidance on anything I may have overlooked here?

On your 2002 Blazer: When the ignition is initially turned to the RUN position, the PCM activates the fuel pump relay by applying battery voltage through the dark green/white stripe wire for 2 to 3 seconds. If the PCM does not receive data from the crankshaft position sensor during this time, it will deactivate the fuel pump relay. While the relay is activated, battery voltage from ECM B 20 amp fuse is applied to the grey wire that runs directly to the fuel pump. Next to the fuel pump relay, in the under hood fuse panel, there is a vacant single female terminal, this terminal is connected to the grey wire and can be used to bypass the relay. Apply fused battery voltage to the terminal and the pump should run. Don't "poke through the insulation on the gray wire" as it will allow moisture to contact the copper wire causing corrosion very rapidly.

The fuel pump relay should be activated continuously when the ignition is in the START position.

Ignition ON, engine OFF, fuel pump running, pressure must be 60psi to 66psi and must remain above 55psi for 10 minutes after the fuel pump shuts off.

Thanks for the reply, but I don't understand how this solves the problem. Unless you're suggesting that the crankshaft position sensor may be the source of my trouble?

If I bypass the relay just to run the pump and obtain fuel system pressure, I don't see where that leaves me, as there would still be an electrical fault somewhere.

Post #2 describes how the circuit works. The first step to diagnose your problem is to check the relay operation.

Do you have battery voltage on the single terminal next to the fuel pump relay for 2 to 3 seconds when the ignition is initially turned to the RUN position?

Yes, there is voltage at the test terminal for 2-3 seconds when ignition is turned to "Run."

I connected the jumper wire from the postive terminal on the battery to the test terminal and was now able to get 12 volts at the fuel pump.

I don't understand this...if the relay is at fault, how come when I swapped the fuel pump relay with a known good relay, the problem wasn't corrected?

Possibly a poor connection somewhere... we'll get to that later, if necessary.

You have battery voltage for 2 to 3 seconds at the pump, which means all wiring and circuits from the PCM to the pump is OK. Next step is to check the ground at the pump module connector. Disconnect the module electrical connector. There are 2 black wires in the module connector, both black wires must show less than 5 ohms to ground.

Grounds are fine...half an ohm and a tenth of an ohm respectively.

What next? Based on all of the diagnostic work I've done, it would lead me to believe that the poor connection exists within the fuse box itself.

The ground resistance that you measured is good from the connector to ground.


The pump prime terminal shows battery voltage when your ignition is turned to RUN. Which means the relay is supplying voltage to the terminal. You have battery voltage at the pump when the pump prime terminal is jumpered, which means the connection in the fuse panel from the pump prime terminal to pump feed is good, it also means the problem is NOT in the fuse panel. You have battery voltage and ground at the pump connector and the pump is not running, which means the pump is faulty.

If that is the case and the circuit is completely functional, why do I need to bypass the relay in the first place? I am not understanding that. How can the circuit be considered OK if we have to force battery voltage to the pump via the test terminal?

If when bypassing the relay, the pump module connector has battery voltage, it prooves the circuits are complete from the fuse panel to the pump module. It also makes it easier for one person to diagnose.

Testing for voltage at the pump prime terminal on initial ignition, tests several things at one time, ie ignition switch, PCM, fuel pump relay, fuses and wiring connecting all that stuff together, etc etc.