How to test a working - but spotty - electrical circuit?

[mike123]

I was diagnosing a no-start issue and had a jumper in the RPM relay slot from brown to white-purple. I believe the fuel pump should run all the time with that jumper, but it was not.

I put my foot in the spare tire well very lightly to start to climb in and take a closer look at the pump, and the fiberglass flexed a bit and the pump started running. As a result, I think there is a connection or wire near there that is not great.

Since the pump started, I have been unable to wiggle any wires or connectors to break the circuit. As a result, I don't know how to diagnose or fix the problem... other than maybe replacing all wiring from the RPM relay socket (which appears fine) to the fuel pump, to the inertia switch and to ground.

Does anyone have any suggestions on what to do? Thank you!


* as a side note, I noticed my inertia switch has an extra ground wire (black-blue) that was added by previous owner. The pump still works from the stock black ground if I disconnect the extra black-blue. I don't think stepping in the spare tire well lightly would have wiggled something all the way at the inertia switch, but who knows. When I wiggle the gray plug on the inertia switch I do hear the pump's hum change tone a bit, but I don't know that that is indicative of a bad connector or if it's normal.

71674

[DMC-Ron]

I was diagnosing a no-start issue and had a jumper in the RPM relay slot from brown to white-purple. I believe the fuel pump should run all the time with that jumper, but it was not.

I put my foot in the spare tire well very lightly to start to climb in and take a closer look at the pump, and the fiberglass flexed a bit and the pump started running. As a result, I think there is a connection or wire near there that is not great.

Since the pump started, I have been unable to wiggle any wires or connectors to break the circuit. As a result, I don't know how to diagnose or fix the problem... other than maybe replacing all wiring from the RPM relay socket (which appears fine) to the fuel pump, to the inertia switch and to ground.

Does anyone have any suggestions on what to do? Thank you!


* as a side note, I noticed my inertia switch has an extra ground wire (black-blue) that was added by previous owner. The pump still works from the stock black ground if I disconnect the extra black-blue. I don't think stepping in the spare tire well lightly would have wiggled something all the way at the inertia switch, but who knows. When I wiggle the gray plug on the inertia switch I do hear the pump's hum change tone a bit, but I don't know that that is indicative of a bad connector or if it's normal.

71674

Mike,
The closest wire (from your camera's perspective) on the inertia switch wire connector would normally be the black ground wire. Perhaps the PO added the white/blue to pick up the ground for something else?
71675

It would seem that stepping in the spare tire well might compress or flex the fuel pump connector, so it might be worth taking the cover off and looking at the fuel pump.

Ron

[mike123]

The extra ground is spliced at the fuel pump with the normal black ground, and travels a foot or two to the firewall where it connects to a ground point. I suspect the PO had similar fuel pump powering issues to me, and thought that ground might be the cause so he added a "backup" ground.

I did check the connectors to the fuel pump and they look like they are brand new (picture below). The fuel pump was installed a few months ago, and I think those connectors probably came with it in the kit. I didn't do the install so I am not 100% sure. I did check the power/ground connector though and it is totally clean and gives power to the pump about 1/4" before the pins are pushed all the way in the connector.

My thought is that if the new connector was soldered or butt-connected to the original wiring, that could be the point of discontinuity. If that's not it, I am worried that the issue might be along the white-purple wire or the ground wire, both of which are in a big taped bundle under the spare tire well.

71676

[todd1561]

I don't have one of those new all in one pumps but I do remember at least when they first came out the new connectors weren't great and I think it was even recommended to zip tie to two halves together to maintain proper contact. Maybe do a little more investigation with those and check the pins, maybe you're making marginal contact.

[mike123]

Thanks for that info, I didn't know that. The connectors do seem to be OK, there is at least 1/4" of contact between one piece and the other, and they fit together very tightly. The clips seem to work well too, to ensure the connector doesn't slide apart.

I am really at a loss for what to do about this. I will keep asking around and see what others might suggest.

[David T]

Thanks for that info, I didn't know that. The connectors do seem to be OK, there is at least 1/4" of contact between one piece and the other, and they fit together very tightly. The clips seem to work well too, to ensure the connector doesn't slide apart.

I am really at a loss for what to do about this. I will keep asking around and see what others might suggest.

If you had an effect on the circuit you are close to the problem. Examine all of the connections, plugs, repairs, etc that are nearby. Undo them and redo them. Especially grounds. Clean them up and put them back tightly.

[mike123]

I did the checking and cleaning on the Inertia switch contacts / connectors (couldn't find the ground point for it's ground wire; I think I need the car on a lift to find/access it). Tested the contacts going to the fuel pump and saw 12.39 volts and I get 12.40 volts at the battery posts, so that seems to be a good sign.

It seems plausible that stepping in the tire well moved the tightly bundled wires and the disconnect point was at the inertia switch rather than right under my foot. Next time the car is on a lift, I should have better access to the ground bolt as well as the fuel pump connection points (not the harness but the butt connectors that connect the harness to the existing wiring).

[Bitsyncmaster]

I did the checking and cleaning on the Inertia switch contacts / connectors (couldn't find the ground point for it's ground wire; I think I need the car on a lift to find/access it). Tested the contacts going to the fuel pump and saw 12.39 volts and I get 12.40 volts at the battery posts, so that seems to be a good sign.

It seems plausible that stepping in the tire well moved the tightly bundled wires and the disconnect point was at the inertia switch rather than right under my foot. Next time the car is on a lift, I should have better access to the ground bolt as well as the fuel pump connection points (not the harness but the butt connectors that connect the harness to the existing wiring).

Were you testing the fuel pump contacts with the pump running? If not that test will not find a bad connection. I know the wiring would have dropped more than 0.01 volts if the pump was running.

[mike123]

Were you testing the fuel pump contacts with the pump running? If not that test will not find a bad connection. I know the wiring would have dropped more than 0.01 volts if the pump was running.

Well shucks, OK. I disconnected the bottom harness in my picture and measured voltage across the left side contacts with the RPM relay jumpered. I thought this would have been the correct way to test the voltage going to and from the fuel pump but I am pretty new to all of this and it sounds like that is incorrect.

The only other thing I can think of, might be to measure the resistance from the ground side of the connector, to the actual ground bolt. I'm actually not sure how to test with the pump running, unless I strip some wires.

[Bitsyncmaster]

Well shucks, OK. I disconnected the bottom harness in my picture and measured voltage across the left side contacts with the RPM relay jumpered. I thought this would have been the correct way to test the voltage going to and from the fuel pump but I am pretty new to all of this and it sounds like that is incorrect.

The only other thing I can think of, might be to measure the resistance from the ground side of the connector, to the actual ground bolt. I'm actually not sure how to test with the pump running, unless I strip some wires.

If your connectors allow it, meter probes can get into the wire side of connectors.