Fuel Pump Doesn't Run

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Jan 25, 2020 | 08:08 PM

Tom_Co

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Fuel Pump Doesn't Run

1999 Expedition XLT 225000 miles.

Initial indications that fuel pump bad. Replaced and did not change issue.

Had to bypass power and connect the pump to my Aux power connector via the DeAccel switch connector
to use car for work.

Need to find real issue:
Fuse Good
Relay Good
power to pump looks good for a few minutes.

Power to energize relay from PCM Power Relay good but relay opens. It appears that the PCM module Gnd path is not present.

What does the PCM Module do to the relay ctrl? Can I bypass it since the PCM Power relay will turn it on and off.

Tom Coughlin

Jan 26, 2020 | 07:50 AM

Spaznaut

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From: NE Tennessee

You could energize it with a hot wire from a run circuit. Is that what you already doing or just a hot at all times? With the cpu controlling the injectors I don't think you could flood the engine if you accidently have it in the run position without the engine running. A hidden toggle switch on that circuit would make a nice anti-theft device.

Jan 26, 2020 | 08:04 AM

projectSHO89

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From: St Louis

If the PCM Power Relay opens, the PCM shuts off and so does the fuel pump and everything else.

You're focusing too far "downstream". You need to focus on the PCM and its power. If the PCM doesn't work, you have a dead vehicle.

Jan 26, 2020 | 10:23 AM

Tom_Co

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The PCM Power Control Relay is not the issue. It is working fine. The rest of the car works, The Fuel Pump Relay energizes for a short time then de-energizes. It appears that the PCM CNTRL line to relay opens shutting off pump.

I've checked for shorts, gnds and cold joints but nothing. Have no errors but it appears as if the PCM is shutting down the pump. If he PCM s doing this I don't know why.

Tom

Jan 26, 2020 | 10:23 AM

alloro

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From: 0,0,1

Quote:

Originally Posted by Tom_Co

What does the PCM Module do to the relay ctrl? Can I bypass it since the PCM Power relay will turn it on and off.

Here are the diagrams for the PCM and Fuel Pump relays, see if they help you.

Jan 26, 2020 | 12:53 PM

projectSHO89

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From: St Louis

Quote:

The Fuel Pump Relay energizes for a short time then de-energizes. It appears that the PCM CNTRL line to relay opens shutting off pump.

It's supposed to do that...until or unless the PCM sees an accelerating crank sensor signal that indicates the engine is firing. The PCM then turns the pump back on via the relay.

That initial pump priming cycle is only 2-3 seconds in duration.

Jan 26, 2020 | 02:56 PM

Tom_Co

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Thank you for the schematics.

The crank sensor is something I'll look at but I have no codes present. It does come back on but the voltage at the relay coil gnd side doesn't go to zero to energize coil it floats at 6 volts so the relay doesn't re-energize.

For the moment I have added a permanent Gnd to the coil so when the key is ON the pump runs. When I have a chance I'll look at the crank signal.

Question: If the Crankshaft Position sensor were bad would the truck still run?

Tom