I am narrowing down the problem with my truck. I thought I ran out of gas the other day (unfortunetly I don't know where the gauge was)and that might have caused my elec. fuel pump to go bad. I put 3 gallons of gas in the truck and it would not start. I pulled the fuel tank today and found there to be over 10 gallons of gas in the tank so I guess I did not run out after all. How can I tell if it is the fuel pump, computer or something else? When I turn the key on I do not hear the electric sound any more which I thought was the fuel pump starting. Any ideas what to check and how to test it?

THANKS!

 

Update: ok weve determined that the fuel pump itself is fine...we hooked it up directly to the batter and it turns on...we then tested the wiring harness under the truck to see if power was coming through the wires...there is...the thing is we are using a test light and the light is very dim...when we plug the pump into the harness and turn the key it doesnt turn on...can anyone help!?!?

 

Inspect the wiring esp where it leads up to & connects to the relay . It may also be the relay itself . Check to see if you have power to the Inertia Switch , thats the last connection before power goes to the fuel pump .

 

Hi Paul... I am helping with the work on this truck and appreciate your help, THANK YOU!

We checked all the wiring and it all seems fine. I used a test light and voltmeter on the wiring harness plug where it connects to the fuel pump and there is power at that point. I even ran a jumper wire from the harness plug to the pump, there is still power on the line but the motor does not start running. Again if we connect the pump directly to the battery it starts immediately.

We checked and the Inertia Switch is not popped. I left the volt meter connected to the wiring harness plug where it connects to the fuel pump and disconnected the plug that goes into the Inertia Switch which immediately killed the voltage on the meter at the fuel pump. When I plugged the Inertia Switch back in the voltage went right back up.

Where is the relay you mentioned and how can I test it?

From what I understand he was driving along and the truck just shut off. It can never be just a fuse.......

Again THANK YOU for your help.

 

If you have 12V at the connecter that goes to the fuel pump then the pump should run . Youre getting 12V at that connection correct ? Is the pump laboring or making any odd noises when it runs ? It sounds as if the correct voltage isnt getting to the pump thru the wiring . Trace all the wiring from the pump connection forward , there are a few places where it can chafe thru the wire loom . The relay should be located under the hood , left side .... It should be located with other relays , I beleive in the area of the power distribution box , I honestly cant recall the exact location & I dont have a diagram for your truck .

 

Thanks again Paul. I will check the relay in the morning. I am getting a 12v reading at the connecter that goes to the fuel pump. That's what makes it so aggravating.. I would think if there is 12v there it should turn the pump just like it does when I connect it directly to the battery.

When I connect the pump directly to the battery it starts immediately and at a consistant speed. It does not make any odd noises.

Could it have anything to do with the computer? I don't think it would but at this point I am running out of ideas

 

How many miles on this truck & what engine ?

 

About 150,000 and a 5.0.

 

If you have voltage to the pump connector, it almost has to be a bad ground. Maybe try jumping from the ground in the connector to the frame.

1985 F-150, Ext. cab, 302 w/EFI, 4WD, 8 ft bed,

 

I thought about the bad ground possibility and tried jumping the ground to the frame and also touching the metal of the pump area that would touch the tank to the frame but still nothing.

Just as a side note the fuel gauge which is also part of that wiring harness works fine. There are four leads on the pump assembly black & red to the pump motor, yellow to the fuel gauge sending unit and the last lead has no wire and looks like it grounds to the mounting area.

 

Just a thought.........
Check the voltage on the pump power wire with the pump connected. There should be 12volts there with the pump running, as you are measuring the voltage before the load (pump motor).If less than 12 volts possibly a corroded connection, or other high resistance such as burnt relay contacts, in the power feed wire. If it is still 12volts could be bad ground.
You sometimes can measure 12 volts with a meter but as soon as you put a load on the circuit the voltage drops, as it is dropped on the high inline resistance.
Good luck.

Sparky

 

Thanks Sparky... I did use a test light on the pump while it was connected but did not try the volt meter, I will do it tonight

 

I'm just gonna jump in here and say the fuel pump relay should be located on the firewall to the right of the master cylinder.

John

 

Ok... I checked the voltage again tonight at the connector for the fuel pump, 12volts. Then I pluged the pump into the wiring harness and put the volt meter on the black and read leads on the fuel pump, 0 volts. Disconnected it and tested the harness again 12 volts at the plug. Plugged it in to the pump again 0 volts! I think I found the relay you are referring to, it located on the driver side fender just in front of master cylinder and has a cable going to the throttle, correct? The problem now is I don't know how to test it. Thanks again to everyone that is giving some direction to this problem. Should I just replace the fuel pump?

 

I don't have a wiring diagram, so I am unsure of wire colors, try disconnecting the relay and jumping 12volts direct to the wire, I am guessing it is a red one, that feeds the pump. If the pump runs it is good, as is its ground, and the problem is between the fuse and the relay. If it still doesn't run the problem is between the relay connector and the pump.
Good luck.

Sparky