Jump to content

Archived

This topic is now archived and is closed to further replies.

Forever Tinkering

Voltage out of Fuel Pump Relay

Recommended Posts

Heard this the other day and I wanted to come to someone who knows fact from fiction - so here goes.I have an 02 2500 ram 5.9 w/ an in-tank lift pump the dodge guys put there under warranty. I installed an edge comp instead of my E-Z, then a fuel pressure gauge because I was afraid of my pump, then I replaced my lift pump with a Holley because of low pressure.For those who don't know, you don't have to crank the motor to start the pump, just "bump"the starter and it will run for 30 seconds. When I do this, I get about 16 lbs. of pressure. If I start the motor, it drops slowly to about 10. I was told by someone that when the motor starts, the relay lowers the power output to the pump so it doesn't put out as much pressure (as opposed to high starting pressure). I think it goes down because the VP44 is using some and recirculating the rest. which one of us is crazy? Does the factory add on pump relay have a constant voltage out or is it variable?

Share this post


Link to post
Share on other sites

The electronic control module (ECM) enables the lift pump by sending a signal directly to the lift pump. The ECM fast-duty cycles the lift pump on and off during cranking, and it enables the pump once the engine has started.Lift pump is turned on for approximately 5 seconds when the keyswitch is first turned on. With the hood raised, it is possible to hear the lift pump running when the keyswitch is turned on.The schematic of it shows constant 12VDC supply. It is not variable, but the ECM can turn it on and off. Pressure drop can be a number of things from the VP44 using it and the pump not being able to keep up, having a weak overflow valve, or having a clogged fuel filter. This is assuming your fuel pressure sender is post fuel filter.

Share this post


Link to post
Share on other sites



×