I thought it was just the Airtex pumps failing because they are junk, but the truck ran for literally five minutes before the ford o.e. pump I got from a junkyard truck with a very clean tank quit working. I tested it before installing. The the pumps are failing electrically, but I can't figure out why. I've been all through the harness and found no loose/broken/grounded/etc wires. I've also tested the alternator and its not over voltage or anything. I don't get it.

 

Electric pump?

If you've got a multimeter, it's probably time to start checking voltages at the pump.

 

Solid 12 volts at the connection, already checked. That's why I'm stumped.

 

Is this a fuel injected 302 or a carbed 460 truck? I have forgotten if you have mentioned it in some other post. Both had in tank fuel pumps.

 

Carb 460, dual tanks with electric pumps

 

Do you still have the return system hooked up at the carb? It should have a thing up at the carb with a orifice and a return line. If this is not working, the pumps may be dead heading, and I am not sure they are designed for that.

 

Did you test the pump(s) after the failure?
With your meter set to low ohm scale you should read about 10 Ohms across the motor terminals.

It's possible to have a bad connection either on the power side or ground side that measures 12V but will not sustain voltage under load.

Problem could be anywhere in the circuit. Dash switch, tank selector, sender connector or anywhere in between.

Here is a diagram of your system (from '86 shop manual):
http://imageshack.us/a/img38/2729/460dualtanksel.jpg

 

Luthoro, these pumps are only supposed to have 12v during cranking, there is a resistor that feeds through the relay once the oil pressure comes up. There are more pages to that whole system, go to my website and under downloads is 86_7.5L_fuel_pumps, that is the whole thing. I have a good front pump from mine and possibly rear.

 

So could this be because I bypassed the oil pressure sensor? Power wasn't getting through it so I jumpered the connector with a very short piece of wire.

 

Bypassing the oil pressure switch will not bypass the resistor.

 

Okay I broke out the miltimeter to test resistance at the pumps...first I tested resistance at the OPS plug where I jumpered it and saw only 3 ohms...then the multimeter quit working before I tested at the tank plugs.

 

Here's a diagram.



What do you expect to see by checking the plug with a ohmmeter? You may have blew the fuse in the meter when you put it on the oil pressure plug.

 

Never connect an Ohm meter to a powered circuit!
To test the pump you need to disconnect the connector and measure at the pump terminals on the sender.

Here is the shop manual version of the relay circuit to go along with the other diagram I posted.
http://img441.imageshack.us/img441/1029/460pump.jpg

What did you jumper exactly the oil pressure switch or the oil pressure sender?

 

The resistor is only 0.75 Ohms so the voltage drop is less than a volt. Assuming a 10 Ohm load on the pump motor he should measure about 11.2 Volts if the pump is on. Of course if the pump is not in the circuit or if the pump winding is open, or the pumps ground is floating, then he would measure the full 12 volts or whatever the battery voltage is.

 

I wonder what the pump actually draws? Plugging a 10 ohm resistive reading into the formula might not give you the actual amp draw, since the motor is a inductive load, and would vary depending on the physical pump load on the motor.