Fuel lift pump mystery
 

Three weeks ago my fuel lift pump (stock, original, 195K mi) decided it doesn't want to shut off anymore. Not only does it drain my batteries, but it's getting close to wearing out and burning my VP44. The shop couldn't find what's pushing electricity to the pump or why it won't turn off. Any ideas before I install a new pump to have the same thing happen? Thanks.

Any competent shop should be able to figure that out pretty easily. Lift pump is powered by the ecm. Either the wire to power the lift pump is shorted to power somewhere or the ecm is supplying power.

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Disconnect the ECM connector. If the pump is still running, the wires are shorted to power somewhere. If the pump stops, you will most likely need a new ECM as the driver is shorted to power in the ECM. The ECM uses 2 pins to power the the lift pump. Pin 15 and 35. They join at splice 150 and go strait to the lift pump at pin 1. Pin 2 goes to ground. It obviously has a good ground as the pump is running though. Good luck.

Hey, I'm new here and I don't want to be a crackpot...but I think the LP is powered via a relay. Dodges seem to have so many relays (only because I have one and used to pick those relays up by the fistfuls at a wreckin' yard; maybe all makes are like this) that I can't imagine that this circuit isn't fed by one. Maybe the relay is kaput. Stuck in the ON position. I'm not near my truck or the manual (I'm 2,000+ miles away in MA) so I can't tell you for sure, but check to see if there is a relay which supplies juice to the LP. If so, pull that relay and replace it with a new one (or at least one which is used for something you don't really need for the truck to run, for testing purposes) and see if that fixes it.

--HC

The LP is powered directly by the Cummins ECU:
http://img15.imageshack.us/img15/4223/liftpumppower.jpg

The Fuel Pump relay you are referring to supplied power to the injection pump.

http://img708.imageshack.us/img708/9...lpumprelay.jpg

A relay was my first thought as well but then I seemed to remember that it was a direct feed by a high side driver in the ECM. I went and looked at a wiring diagram to verify. I havnt figured out how to do a screen shot of my wiring diagram to post it however. (Thanks RAW for Posting it:c:)

RAW is right. The ECM directly powers the lift pump. Typically there is no other relay in the system unless there is a in-tank pump change there is a wiring change.

So, there is no separate fuel control module in the ECM, and the whole thing may have to be swapped? I was hoping for an easier (cheaper) fix... oh well. Thanks, all.

I suppose if you wanted you could wire up your own power source and install a manual switch or wire it off of the ignition switch. It it kinda ghetto but if you are just trying to get by until you get a new ecm, you could do that.

Okay, I've been following this thread and I had another crack-pot idea. It's been shown that the ECM provides power directly to the LP. It's been suggested to disconnect the relevant ECM connector to see if the LP runs in case the wire feeding the LP shorted to a hot wire which I assume you've done.

What about this: what if the problem is that the ECM is itself remaining energized? Some wire feeding it is staying energized indicating it needs to keep the LP (and some other circuits) hot. I put in a remote start in my 02 2500 (yes, it has a 5 speed, yes it's dangerous) and there are several circuits which get energized depending on whether the key is in the run, start, or accessory position (I learned this in the process of installing the remote start system, of course).

The theory here is that maybe one of those wires is remaining energized by the key switch and powering the ECM. Try turning the key to Accessory instead of just off (probably won't make a difference buy you gotta start somewhere so you might as well start cheap and easy...like my prom date). Next, pull the key switch and disconnect manually each wire which is normally energized by the positioning of the switch (that is, disconnect the various accessory circuit wires (A/C, Radio, etc.) all the way until only the ground and supply 12v are left (or just disconnect the 12v supply to the key switch). I don't recall how that set of wires attaches to the switch...it may be through one connector; if so, disconnect the whole thing. See if the LP shuts off.

I see the WTS light is driven by the ECM per the diagram from RAW; you might pay attention to whether or not the WTS light illuminates when you turn the key from Off to Run; if it doesn't turn on it's either too warm (like over 57 or 60 degrees...I forget exactly) or the ECM has been running pre-heat all the time 'cause it's been getting power like the key was just rolled forward.

I'm not sure I'm doing a good job of conveying what I mean precisely but the take-away here is that it might be something easy like a shot key switch energizing something it shouldn't be when in the Off position.

Or I could be full of bull butter. But it should be fairly easy and really cheap to test.

--HC