Mechanics

ELECTRICAL STUMPER

1989 Ford Probe • 172,000 miles

2.2L, LX, auto trans.

Problem originated with a 'no start' condition. Cranked fine. However, no spark with an inline light at spark plug. I investigated distributor cap, rotor, coil, and everything seemed fine. However, no voltage at coil primary. I then disconnected the wiring harness at the fuel pump, and then had spark. My thought was that the fuel pump was defective and somehow drawing down the coil voltage. I replaced the fuel pump, and sure enough, the old one showed no resistance between + and -. But the problem wasn't fixed with the new pump. The fuel pump relay is sending only 5 to 6 volts to the pump. When the harness at the fuel pump is disconnected, the voltage level rises to 10 volts... But not immediately. In fact, sometimes, I read 5 to 6 volts at the fuel pump relay socket, with the relay removed. Therefore, my question is: where does the supply voltage at the fuel pump relay come from? It must not be directly from the battery. Or if it does, I must have a bad connection somewhere. I've bypassed the inertia switch, so that can't be it.

Also, my new fuel pump started with about 20 ohms between + and -, but now I see its down to about 7. I wonder if my repeated attemps of sending low voltage to the pump is damaging it.

Any help would be appreciated greatly. Thanks.
Avatar
Mlc1830
April 27, 2013.



Update from original post: I connected my lawnmower battery through a switch to run the fuel pump. Everything worked fine. I was able to drive 4 miles from the parking lot to home. So the problem definately resides with the power supply to the fuel pump and fuel pump relay.

Also, disregard my thoughts on the fuel pump resistance. Evidently, a DC motor reads similar to an AC motor - almost a short. It read 1 ohm when I got home, and everything was still working. I bet the decline from 20 had to do with the brushes wearing in. Now makes me wonder if I replaced a perfectly good pump.


Tiny
Mlc1830
Apr 28, 2013.
Check your black red wire going to fuel pump relay to see how much volts you have. You rproblem may be you removing inertia switch and I don't know why you did that as it s' a safety device. Then check to see how much power to pump. Also check yoru ground to the relay as that may be the problem as well.


Hmac300
Apr 28, 2013.
Thanks for the repsonse. Good idea to check the ground wire. I will do that today. Do you know if that ground is supposed to be the same as the Chassis ground (- battery lead)?

The supply voltage at the fuel pump relay varies from 5 to 10 volts with or without the relay plugged in. I'm unsure of the wire colors, as I have yet to disassemble the termination block. I'm afraid that might be the next step. I would like to find where that source voltage comes from first, however, and avoid tearing into that block.

I temporarily jumped the two leads at the inerita switch, just in case the switch has bad contacts under load.


Tiny
Mlc1830
Apr 28, 2013.
I forgot to send the pic the blkred comes from the ignition switch through the starter interlock switch so you will have to go back from there. Black is ground, lt grn goes to fuel pump tester plug and air tempsensor and whtyellow goes to inertia switch then to pump. But you may have a bad ignition switch lead or something not making good connection there.


Hmac300
Apr 28, 2013.
Resolution: As hmac300 notes, the supply for the fuel pump relay comes from the ignition switch. It also goes through the 15 Amp 'Engine' fuse. The ignition switch tested good with very little resistance through the contacts. The 15 Amp fuse tested good as well. However, by jiggling the fuse in the holder, I could start the car. And more jiggling could kill it, and cause the voltage to drop to 5-6 volts. Looks like the culprit was a loose connection within the interior fuse block. I didn't attempt to tear that apart, and rather bypassed the fuse block. I found a neat device that holds an automotive fuse with two flying leads. Perfect for this application.


Tiny
Mlc1830
Apr 29, 2013.