After several weeks of crawling under my motorhome built on a 91 Ford E-350 chassis with a 460 engine, and after reading all I can find on this forum about fuel problems, I hope that I have enough information that I can ask intelligent questions about my fuel system. I have been vapor locking. The in tank fuel pump does not run. At this point I have the fuel tank removed the fuel pump removed. I can make the fuel pump run by hooking it directly to a 1 amp trickle charger, but the pump has a loose terminal, and I need to wiggle the terminal around to make it run. Thus, I plan on replacing the pump.

However, I have questions about the voltage that I measure back at the rear fuel pump. When the ignition is first turned on the voltage goes to 12 volts. Then it almost immediately drops to 7.5 volts. Is that normal? If so, what determines that the voltage is supposed to drop? It can’t be fuel pressure since I have disconnected the front fuel pump. Does the voltage drop until the engine starts and then goes back to 12 volts?

I have checked the fuel pump relay and it appears to be like new. (Invoices from the former owner indicate that the relay has been replaced within the past 6 months.)

I am assuming that the one relay operates both of the fuel pumps. Is that correct? Thanks for your help.

 

not an expert on this but I believe voltage drop is due to excess resistance in pump meaning pump is not much good.

 

Thanks, but to clarify, I am testing the voltage with the front fuel pump disconected, and the in tank fuel pump removed. So I am just conecting the voltmeter to the positive wire that connects to the rear fuel pump and grounding the negative lead. I have made sure I have a good ground.

 

that sounds like you are losing the voltage before it gets to the pump then.

 

It sounds to me like you have a crack or split in the wire somewhere between the origin and the pump. Some advice on Ford in tank fuel pumps. Don't let them get below 1/2 tank. The pumps use the fuel as lubrication and if you are consistently letting your tanks run below 1/2 then your seals and parts in the pump will be more prone to drying out and failing. I learned this the hard way.