help me with fuel pump hot wire confusion

[JDEstill]

Got a problem with my fuel pump hot wire setup - it doesn't work! It used to work fine, now it doesn't, no changes to the system, it just stopped working one day. Symptoms:

1. fuel pump does run if I disconnect the two power lines from the relay and hook them directly to each other. This is how I am getting around right now - I pop the hood, connect the two wires, pump runs, jump in the car and start it. Shut it off I have to do the reverse. Functional, but a PITA.

2. Hook up my voltmeter to the trigger wire. Car off, key off, I see ~0.2V. Key on it jumps to ~12V for ~2 secs, just like it should, then back to 0.2V. So I'm thinking the trigger wire is ok.

3. Ground for the trigger wire - multimeter says resistance is nil between wire at the relay and ground. Trigger ground seems good.

4. If I take the 12V power lead (normally used to power the pump) and touch it to the trigger wire post on the relay, the relay clicks loudly. So I think the relay works. Tried swapping relays, no difference.

5. So as far as I know, everything works. But when I have it hooked up as it should be, and turn the key on, the pump doesn't run.

Help!

John

 

[ChrisCairns]

Seems like you've covered all the bases John, circuit wise.

And that would only leave that both relays are bad. Radio shack sells a relay that will work (at least as a test) for $6.49.

RadioShack.com - Cables, Parts & Connectors: Component parts: Relays & transformers: 12VDC/30A SPST Automotive Relay

 

[mikestertwo]

2. Hook up my voltmeter to the trigger wire. Car off, key off, I see ~0.2V. Key on it jumps to ~12V for ~2 secs, just like it should, then back to 0.2V. So I'm thinking the trigger wire is ok.
John

Check the voltage on the trigger wire when it is connected to the relay. Could be a high resistance connection to the power source.

 

[JDEstill]

Thanks for the part no. Chris, I'll go pick one up, though I think the relay I've been using for the checks is brand new (it's been a good while since I started troubleshooting this, but I seem to remember it being a brand new one I had sitting around).

As for the high resistance on the trigger wire, I was wondering about something along those lines. It doesn't take much power on the trigger side of the relay, right? Could the trigger wire be showing the right voltage when I check it off the relay, but still not provide enough power to flip the relay when it is hooked up? If so, what would be the culprit?

John

 

[mikestertwo]

As for the high resistance on the trigger wire, I was wondering about something along those lines. It doesn't take much power on the trigger side of the relay, right? Could the trigger wire be showing the right voltage when I check it off the relay, but still not provide enough power to flip the relay when it is hooked up? If so, what would be the culprit?
John

Yes, it could show the right voltage when not connected to the relay. A meter draws very little current so it could read correct even though there is a connection problem. What's the culprit? Could be anything. Corroded connection somewhere. Try running a wire straight from the +12 on the battery to the trigger post on the relay and see what happens.

 

[JDEstill]

just a final update for future searchers...

turned out to be the fuel pump relay on the pass side fender. I don't know why I saw the trigger wire voltage jump and then drop when the key was turned on, maybe the FP relay was failed enough to let a bit of signal through, but couldn't pass the amps to close the hot wire relay when it came time? (The FP relay has to be working for the trigger wire to work.) Doesn't make much sense to me, but I diagnosed it by removing the FP relay and jumpering across the two power terminals. When I did that the hot wire setup all worked like it was supposed to. So ordered a new FP relay and the sun started shining again.

John

 

[Boost231]

glad to hear you figured it out. always nice when its a cheap fix