Low Voltage to Fuel Pump

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Mar 13, 2022 | 12:55 AM

Angel Garcia

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Joined: Mar 2022

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Low Voltage to Fuel Pump

Got a 1992 F150 with low voltage to the pump. Was running fine until it rained and ran fine after that but a day later, the truck would only crank but wouldn’t start. I did a pressure test at the injector railing and had nothing, I also didn’t hear the fuel pump come on either. I have done voltage test to different locations as instructed by my teacher and I’m getting 12.6 volts to the fuse, and pin 85 on the relay. Im also getting 12.6 volts at pin 30. It’s at pin 87 is where I’m getting a weird reading. When I turn the key to run, it reads 12.6 for half a second then drops down to 8.5 after that. My teacher said it could be a bad fuse box. I wanted to have another opinion on it.

Mar 13, 2022 | 01:19 PM

rla2005

Fleet Owner

Joined: Oct 2005

Posts: 20,786

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From: Kentucky

The fuel pump relay is energized for 1-2 seconds when you cycle the key from the Off to Run position. After that time period the relay is dropped out. Cranking the engine over will produce pulses from the PIP sensor which "tells" the computer to energize the fuel pump relay, pulse the injectors and fire the coil.

The ~8 VDC you are measuring is expected when the supply voltage is removed and the circuit is open. This is VERY normal operation.

First suggestion is check the inertia switch behind the passenger side kick panel to see if it is tripped off.

Also start reading this thread with very similar issues as you have stated: https://www.ford-trucks.com/forums/1...fuel-pump.html