1993 150 351w truck in signature block. Rear fuel pump installed went to start truck for the first time, no go. Took tank back down to check wire connections because I didn't hear the pump. The fuel gauge works now which is awesome though. I checked voltage at wires to pump and I am only getting 6.7v.

Battery is good, power at fuse is 12v. Is there something else that could be doing this? What should I check? Thank you very much I appreciate any help. About time I drive this truck instead of working on it!

 

Power is applied to the fuel pump for only a few seconds when you cycle the key from Off to Run that is the typical cause of the phantom voltage you measured.

To keep power applied jumper Pin 2 & 6 on the self test plug under the hood then turn the key to the Run position. That will keep the fuel pump relay energized. If you still have no voltage then you know it's an issue from the relay back to the fuse link.

Jumper:

courtesy of subford

 

Ok that makes sense. Ill do that and check voltage again and actually it turns out I got 6.7v at the yellow/lt. blue (sender) which I think is good?. I didn't get any voltage at the brown/white wire. Unless I got the wrong hot for pump....I don't know.
If what you said to try doesn't work Ill supply 12v to pump to at least make sure I got a good pump.
Will report back. Thank you!

 

K my official findings. I jumped test pin 2&6 like you said. Got 6.8v to pump and 1.8v to sender... More worried about pump but if image it's the same cause for both.
What now? Thank you

 

I read about an inertia switch and that it is located under dash on passenger side...looked for a bit, finally removing kick panel. Saw an electrical component with a red button like thing on top and a three wire plug going into it. Checked voltage at dark green/white wire and got 12v. tried raising red button but wouldn't say up so I slid a flat head screw driver in there, went back and checked voltage to pump and got 12v.
Is that component the inertia switch? How do I remedy the "red button" not staying up?
One more question. the new wire harness for the new pump has different color wires. I figured out the sender quick. the remaining two wires (on pump side, new wires) are green and black. I ASSUME the black is supply hot and green is ground...is that right?
Thank you very much for the help. Hope to get this truck on the road sometime this year

 

My Ford EVTM shows these wire colors and functions:

Rear Tank
Brown/White = Fuel Pump Supply Voltage
Orange = Fuel Pump Ground
Yellow/LightBlue = Sender Supply
Black = Sender Ground

Front Tank
Red = Fuel Pump Supply
Orange = Fuel Pump Ground
Dark Blue/Yellow = Sender Supply
Black = Sender Ground

Very similar to this diagram from a 1991

 

Yeah I got the truck side, stock wires figured out. Just trying to confirm wire color/function on replacement wire harness. It came with instructions (the new pump) but just said connect wires before installing tank. jee thanks!!!
Anyways...I appreciate it though. I actually can use this to confirm I am testing the correct wire on inertia switch. Dark green/yellow.
So, if I can see the red on the button for the inertia switch, it is closed and should be sending all 12v back to pump. My inertia switch must be toast then if that is correct. I can see red and get 6.xV to pump. If I have the red button held up with the blade of a screw driver I get 12v to pump.
Fried inertia switch?
Man this truck is killing me with these parts!!

 

My experience has been the inertia switch should be in the down/latched position under normal working conditions. Once triggered it pops up which in turn removes power to the output pin. I have never seen one apply power when you physically pull it up.

 

The wire (GY/O) that you are getting 12V on when you hold the button up goes to the dash cluster and not the fuel pump.
The 12V is coming from the Dash Cluster Bulb to the inertia switch. This wire is grounded through the fuel pumps when the inertia switch is tripped.
The R/Y wire goes to the Fuel Pumps.
The DG/Y wire feeds the inertia switch.

When holding the button up in the tripped position and you are getting 12 VDC on thr R/Y wire means that you have an open going to and through the fuel pumps. You could have a bad dash selector switch or bad fuel pumps or a bad ground for the fuel pumps.



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Copy that. I'll check voltage on that R/Y going to fuel pump.
Crazy thing is I let the red button back down and still got 12vdc to the pump. So. I guess I reset the inertia switch? I'll check voltages and report back.
For now my fuel pump comes on for a few seconds for prime when I turn the key.
Just got odometer working and it reads 406+k ... might be why this truck has been an exercise in troubleshooting and repair

 

That is what it supposed to do.

 

Yes sir copy that. That is oneOf the few things about these trucks I know with certainty
Going out later to work on it. Will report
Thank you!!

 

Ok, got 12vdc in R/Y wire with red button down and none when it is in the "up" position. So, that is good from reading your post.
Same with voltage going to pump.
Got tank installed, waiting for leak sealant to set before adding gas and giving her a shot!
I hope this fixes it! I know it will get fuel now so should fix my lean on right bank code. Start praying fella's!
Thanks for all the help