I just purchased a 90 f150 but there is a problem somewhere in the fuel delivery system. I assume its a fuel pump problem but I want to check all stops before I replace it. My question lies with the selector switch. With the switch set on the front tank, and in the KOEO position, my test light shows power on the red wire that leads to the front pump but not on the dark brown/yellow that leads to/form the same pump nor on the yellow/white that leads to the gauge. Would this indicate a switch problem or pump problem? I have the same results with the switch set on the rear tank with the exception that neither wire leading to or form the rear pump shows any power. It was assumed that the rear pump never worked to begin with, or so I was told. I only have access to a test light so I can't tell the voltage or the such and I'm not very electronically inclined to understand what I should be checking for and how with a voltage meter anyway.
I have no yet tried grounding pin#6 on the pump relay yet. As for that I have another question. With grounding that pin, doesn't that really just bypass the pcm relay in turn checking if that relay works or not?
i believe that when you first turn the key on you should here it prime. then the computer should turn the pump off. the only time the pump will keep running is when the engine is running or being started. a sure fire way to see if the pump is not working is to put 12v on the connector on the pump and see if you here it running or not
Grounding pin #6 in the diagnostic connector will force the relay to energize and send 12V to one of the pumps through the dash switch. Without doing this, you have 2 seconds to check for voltage at the pump before the computer switches off the relay.
I understand that its most likely the pump but all I really want to know it how to check the selector switch.
The test light is the best way to check the fuel circuits.
Do not test these wires, Yellow with a white stripe, Yellow with a light blue stripe, Dark Blue with a yellow stripe as they are for the fuel gauge only and have nothing to do with the fuel pumps. They get there power from the fuel gauge and they will have a lower voltage on them.
The power wires for your fuel pumps is the red wire with a yellow stripe and it comes from the Fuel Pump Relay by way of the Inertia Switch. This wire should have power on it with pin #6 of the self-test connector grounded and the key on.
If you select the front tank then the Red wire should light your test light.
If you select the rear tank then the Brown wire with a white stripe should light your test light.
If you have power on the Red wire with a yellow stripe and do not have power on the selected tank wire (Brown/white or Red) then you have a bad selector switch.
Thanks a lot Bill. I assumed as much as what you said but I wanted a concrete answer from someone who knew.
I had the whole day to work on it today and I did get power from the rear tank wire (Br/W) when I tested it. Come to find out both pumps were bad. The front pump replacement went well and with a total of around $800 ($600 for the truck and $200 for the pump and other odds and ends) I now have a great running 90 F150. I still have a little more to put into it cosmetically as well as tires and new break pads, but over all the engine and transmission are up to par. Nearly 96k so I would hope not to have problems with those two if they were well kept.
Thanks again for the help and if I have any more problems I will be sure to post here again.
there was a recall on these fuel pumps, the owner i bought my 90 from didnt take care and at 110,000 miles they both went. i replaced both of them but made the mistake of not replacing the regulator, about 5 or 6 thousand miles it failed. i should of known because in the recall they replaced the fuel pumps and the regulator, they didnt say there was anything wrong with the regulator but it was part of the replacement. im guessing that the new pumps caused problems with the regulator, anyways after the regulator was replaced everything worked great. i bought my new pumps on ebay, $186.00 for both to my door, that was over 50,000 miles ago and there still working.
I also have a 90 f150 with a bad selector switch. It is just an old beater and I couldn't find a replacement switch so I just spiced the red and red w/ yellow stipe wire to get the pump working again. Which wires need spliced in order to get the fuel guage working again? I only use the front tank as the pump is bad on the rear tank. Any help would be appreciated.