1978 fuel pump relay

[Zrod]

hi all, are there 2 different fuel pump relays for the 78 280z. the parts store stocks one that is different than the one in my vehicle. the one that's in my car now is twice the size than what the parts store showed me. the reason i'm replacing the relay is it looks like the original, and also because i had the rear of the car on jack stands doing brake work and when i finished the job and let the car back down on level ground i could not start the engine. i traced it back to the relay took it apart and the contacts were not touching, don't know what i did but got it to work, i could hear the fuel pump operating, car started . took it for a test run ran good till had to make a sharp u turn after that it seemed like i lost fuel pressure and barely made it home. when i got back messed with the relay again and got it started. any help with this would be appreciated. thank you.

[lmcguffin]

The 78 has two relays in the fuel pump circuit; one is the smaller "fuel pump relay" and switches the 12V to the pump, and the other is the larger "fuel pump control relay". The fuel pump control relay is a unique device that has two coils with an interlock contact along with a form C contact (common/NC/NO). The relay from the parts house could be the fuel pump relay, which is a standard single pole normally-open relay.

The fuel pump circuit has interlocks in the alternator (L terminal), oil pressure sending unit, and ignition relay. The alternator L terminal is basically an oil pressure buildup timer; once the engine is running and oil pressure is satisfied then the alternator is also operating so the L terminal is disconnected and the oil pressure switch along with ignition keeps the fuel pump running. If the alternator fails (ie: belt slips or breaks) the fuel pump will keep running as long as there is oil pressure. Lose oil pressure or ignition and the fuel pump stops. As you can see there are lots of places where a flaky connection can cause intermittant fuel pump operation.

Here is something to try: Disconnect the small wire from the starter solenoid so that the starter will not operate. Have someone hold the ignition switch in the start position. The fuel pump will run. Try tapping on the relays and wiggling the connectors at the relays and the fuel pump to see if the pump will drop out.

Then once the engine is running check the connection at the oil pressure sending unit. There are two wires there: One is for the oil pressure gauge (yellow/blk stripe) and the other is the oil pressure switch for the fuel pump (don't know the color). The oil pressure switch is common to ground at 0 psi and opens on increasing pressure, so if this circuit gets grounded then the fuel pump will drop out. So that could be a flaky oil pressure switch, an abraded wire, etc. There are also connections at the ignition relay that could be flaky as well. You really need the factory service manual to trace all this out. I would attach it but I bought it on cd and it's a protected file. I think you can download it tho from some other websites. Good luck.

[Zrod]

thank you for your time, I'll try these remedies.

[lmcguffin]

You're welcome. I replaced all the engine bay fusible links and relays in my son's 78 with modern fuse boxes and relay boxes that I gleaned from various Maximas at the junk yards, so I got to be quite intimate with all that circuitry.

Check the mechanical side as well. I believe there is a filter screen between the gas tank and fuel pump or in the pump inlet. You said you had the car on jackstands; might have dislodged and sucked some trash into the pump suction which is causing it to starve.