So how much voltage does the fuel pump get? - Diesel Forum - TheDieselStop.com
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99 & up 7.3L Power Stroke Engine and Drivetrain Discussion of the 99 & up 7.3L Power Stroke diesel engine and drivetrain in the 1999-Up Super Duty trucks and Excursions. No gas engine discussion allowed except on transmissions and drivetrain that pertain to all models. Please confine discussion of topics in this forum to those items that are specific to the 7.3L Power Stroke engine.

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Old 07-19-2008, 02:32 PM   #1 (permalink)
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So how much voltage does the fuel pump get?

I've been having engine issues. Included in that is my fuel pumps not working. I'd been messing around with vegetable oil stuff, but as part of the troubleshooting and to just make her run right, I've returned her to stock layout.

The problem I'm having is that the fuel pump is only getting 6.5 V. I have the batteries on a trickle charger now, but they tested fine, and the fuel pumps (both that were in the vehicle, and my spare) run perfectly when shorted directly to the battery.

As I remember, the PCM delivers power to the fuel pump based on various factors. But it should always be 12 V if it's anything. My inclination is to think this means I have a PCM problem and need to try replacing it. But I wanted to ask here as a sanity checl Any opinions?
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Old 07-19-2008, 03:36 PM   #2 (permalink)
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If you're measuring the voltage at the disconnected FP connector, you should see 12 V (actually battery voltage minus just a touch). If you're measuring it at the same point with the connector connected and the fuel pump running, you'll see a bit less - because the amperage thru the circuit will cause the wiring and components between the battery and measurement point to resist that current flow. Hard to say what that would normally be - I'd guess no more than 1-2 volts.

The PCM only activates the FP relay (the voltage doesn't go thru the PCM), so I doubt your PCM is at fault. You might consider replacing the FP relay, located either under the radio on 01 and older or in the fuse/relay panel on 02 and later truck.
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Old 07-19-2008, 03:38 PM   #3 (permalink)
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Exactly what I needed to know. Thank you very much.
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2002 F-250 black super cab 7.3 L PSD auto trans. 4X4 Lariat with a little lift and mud tires.

Veggie oil system removed while I rebuild and redesign it.
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Old 07-20-2008, 07:48 AM   #4 (permalink)
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with a voltage drop under load like that i'd be looking for corroded/loose connecters also...make sure the wires don't pull out of any connectors especially under the truck where road salt can get to them...
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Old 07-20-2008, 01:32 PM   #5 (permalink)
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It was the fuse. The fuel pump fuse was completely blown (eg tested it with my multimeter and got nothing). Even with it pulled, I was still seeing 6.5-6.6 v to the fuel pump. That doesn't make me think blown fuse. But any way, I replaced it, and it worked fine.
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Veggie oil system removed while I rebuild and redesign it.
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Old 07-20-2008, 04:09 PM   #6 (permalink)
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Interesting

I just checked the wiring diagram, and there's a "fuel pump monitor" circuit that goes from downstream of the fuel pump relay to pin 40 on the PCM. It's there so the PCM knows if the fuel pump is running (or supposed to be running) by reading the voltage. Your 6.5 V was probably feedback from that PCM circuit.

Glad you got it fixed.
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First truck -- 1929 Model A Ford pickup, restored from ground up. Wish I still had it!
99.5 F250 PSD Supercab LB 4x4, ZF-6 w/SB Con OFE, 3.73LS, Boost & pyro gauges, Swamps S175/146 injectors, DP 80 HP Econo PCM (classic version), AIS, coolant filter w/"hokum" bracket, regulated return, heated mirror mod, lighted cupholder, Marinco heater plug-in.

Hard or no-start? Check Here
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