Hey guys...my truck completely stalled out last week and have been diagnosing since. Here is waht I have, hope someone can help.

1) Turn key ON and I get NO fuel pump running but I hear the FP Relay click. Turn key fully and the starter truns over but does not want to start.
2) Meaured pressure at the fuel rail = 40psi.
3) Jumped the FP Relay at the DLC and I can hear the high pressure pump turning.
4) Haynes manual said that if the pump does not turn when the key is on but if it does turn on when jumped, then the problem is between the EEC and FP Relay.

Any ideas? Can the FP Relay still click and not run the pump? Is there a way to test relays? Could not find a way according to the book.

 

Yes, you can test a relay. The FP relay has four wires running to it. A RED, TAN/LT.GREEN, YELLOW, and BROWN. The RED is the power to the coil inside the relay and the TAN/LT.GREEN is the negative but does NOT run back to a frame ground it runs to pin 22 of the ECM. Its still the negative side of the circuit for the FP relay. Since you are hearing the FP relay click when you turn on the key, you can be fairly certain this much of the relay is in working order. If it didn't click there would be a problem with the RED or TAN/LT.GREEN wiring.

The switched contacts in the FP relay are the YELLOW (power feed) and the BROWN (output to the FP) wires. The YELLOW wire should have battery voltage at all times even with the key off. The BROWN wire should only get power when the key is on. If the YELLOW wire does NOT have power at all times, follow it back and check the fusible link that feeds it to make certain it isn't blown. (If the fusible link is blown neither the FP relay nor the EEC relay are working. Furthermore the ECM is not getting "Keep Alive" power hence, the no-start condition.)

The BROWN wire feeds the fuel pump but not before it runs through the inertia switch. If you have power on both the YELLOW and BROWN wires with the key on but no FP running, make sure the inertia switch is not tripped and that the wiring through it is intact.

 

Thanks for the detail Greystreak! Another question...I think I have located the Fp Relay by the master cylinder on the firewall...correct? Is the EEC relay located inside the cab, driverside by the gas pedal on an 86? Stupid manuals are not very clear regarding location.

Ohhh BTW...am I just checking voltage for the relays? And should they be 12v?

 

Yes, battery voltage... take a reading from the battery first so you know what you should be getting. A little bit of a drop from battery voltage isn't bad but if it is significantly lower (more than a volt or two) you will want to check electrical connections for corrosion and security.

As for the exact location of the FP and EEC relays, I'm not 100% certain. My knowledge comes from reading diagrams more than actual hands-on with the older models. I do know that they used to be sort of color-coded in that one was typically a green-bodied unit and the other was brown. Not sure why, they are identical in their operation just slightly different in the current carrying capacity but that's good enough reason I suppose.

 

Thanks bud!