Truck: 93 f-250 w/ dual tanks 5.8l auto tranny

The problem with this truck is that the rear fuel pump went out.... so we thought. we pulled the pump out of the truck and hooked up the pump directly to a 12 volt source, and worked fine. using a multimeter, we checked the selector switch and had the same output to the front pump as the rear. at the harness that the pump plugs into, we are getting 10 volts as soon as the key turns on, then drops to around 4 or 5 volts. so we are getting voltage to the connector piece, and we are able to used an external 12 volt power source to "hotwire" the fuel pump and works just fine. but when we hooked the pump up directly through the harness....... nothing. tomorrow the front tank is coming down, and we are going to see if we can hook that pump setup to the front pumps harness, and switch them out, and see what kind of results that will bring. basically we are sitting here scratching our heads trying to figure out what the problem is. anybody ever had a similar problem or have any ideas, it would be greatly appreciated and will give us more to check through while we are messing with it tomorrow.

the relay is clicking, so I am guessing that is working. if the relay was bad, would both pumps not be working?

We are lost, please help.


Joe

 

the relay can click even if you have points burnt off inside of the relay. you need to check it with a multimeter for continuity to fully prove if the relay is bad. here is a website that can explain how a relay works and how to test yours http://www.autoshop101.com/forms/hweb2.pdf

 

my truck would do the same thing, but it was an F150 4.9. i thought it was the pump, then the relay, and then i put a new fuse in and it fixed it all. replace the fuses first, saved me a lot of money and time.

 

My '91 F-150 has an intermittent rear tank fail. Front tank is always fine...but usually if rear is selected no pump buzz is heard. Sometimes while driving on front tank I can switch to rear and all is well.

 

I have seen this.. a partially siezed pump will run when connected to a battery because there is almost no limit to the current it can draw. That's not the case on the vehicle, the small gauge wire used will only pass so much current and it's not enough to start the pump. Motors generally draw the most current at startup.

 

I like Paul's idea, sounds very plausible to me. The one thing that puzzles me is that if the fuel pump requires more power to run I would think it would just draw more power. Of course, if it draws too much power it should blow a fuse or the wire would actually melt. I wonder if the relay some how only allows a certain amount of current to be drawn by the fuel pump. I bet if you pull the the front fuel pump and it works just fine connected to the rear wires you will have confirmed what Paul said.

 

There are a number of connections between the rear tank and the power source, these degrade(corrode) over time and this increases the resistance in the circuit. This can produce a situation where the circuit won't deliver enough juice to run the pump, but won't blow the fuse either.

 

If that's the case then rewiring that part of the circuit might be in order. An Ohm's resistance check might help verify that. I wonder what would be considered an exceptable reading?

 

we were trying to figure out what other readings we could do with a multimeter before taking stuff apart, but in the haynes manual, it is really not going into too much detail about the problem on hand. we didn't get a chance to pull the bed, and swap the pumps yet, but as soon as we do, I guess that will answer a few questions anyways.


joe