Fuel pump relay and only 3.5 volts from PCM and at injectors?

BLAKE E CANTRELL

MEMBER
1994 FORD F-150
5.0L
V8
4WD
AUTOMATIC
200,000 MILES

Hello, the fuel pump relay is not clicking in. I have right at 12 volts coming from fuel pump fuse but I'm only getting 3.5 volts from the PCM. The PCM is new. I can jump the relay and the fuel pump will run but still no start. I checked voltage at the injectors and I'm only getting 3.5 volts. Also, the PCM relay does click in, both relays are new.

5 Replies

KEN L

MASTER CERTIFIED MECHANIC
42,940 POSTS

Did you install a new PCM power relay? Here are the wiring diagrams for the PCM and a guide to help check for power and grounds. I would start by unplugging the injectors because you may have one that is shorted which will cause the PCM to dump voltage.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

Check out the images (below). Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.

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Saturday, August 26th, 2023 AT 12:06 PM

BLAKE E CANTRELL

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3 POSTS

Thank you for the reply, it has new PCM and fuel pump relays. I will start testing the power and grounds tomorrow and let you know what I find.

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Saturday, August 26th, 2023 AT 4:49 PM

KEN L

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Sounds good, please let me know.

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Sunday, August 27th, 2023 AT 9:51 AM

BLAKE E CANTRELL

MEMBER
3 POSTS

Update: I had to pull the wiring harness and repair a few wires that had been chewed on or something. Now the wiring harness is back in and the truck will not start when you turn the key over. I can start the truck by using a screwdriver on the starter solenoid and the truck will start. I just had the Mas air hooked up without the lower box and filter on and the truck idled around 1,000 RPMs. But when I put the truck into any gear it will jump hard or jerk hard and shut off. I put the air box back on to see if the Mas air had something to do with it but no luck. Now with the air box on it will idle fine for a few minutes then started to idle up and down and then shut off.

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Thursday, September 14th, 2023 AT 3:52 PM

KEN L

MASTER CERTIFIED MECHANIC
42,940 POSTS

So, with the wiring harness repair is the first problem fixed? If so, let's start a new thread for each of the new problems, please post your new question here, you must be logged in.

https://www.2carpros.com/questions/new

Cheers, Ken