Archived Lift Pump Voltage

[DBar]

Hi All



98. 5 24V 3500 worked fine two nights ago. Next day when I started it, was running rough. Checked the Fuel Pressure Gauge and was reading Zero. Shut down truck and cycled key to listen for Lift Pump (Relocated to chassis with Vulcan's Kit last year). No go. Won't cycle at all for 3 secs after turning key on, or for 25 secs after bumping starter. Connected small 12 volt battery directly to lift pump on chassis and away it went pumping fuel as it should. Disconnected small battery after a few seconds after fuel was pushed to filter and truck started and ran OK - but lift pump still not running with vehicle supplied voltage. Checked voltage at pump and got 7 volts when key turned to on, but not to start. However this may have been intermittent, hard to tell as the meter probes were a little big for the pin holes.



Any ideas on what could be going? I've pretty well ruled out the lift pump, as both the new one and the old one (which I removed last year) work fine with the 12 v battery, but neither work off the vehicle harness.



Thanks for any info guys. I've read for a long while and have used a lot of the info from this site, but haven't had to write in before.

 

[Missouri Mule]

DBar, I take it your batteries are putting out at least 12. 5 to 14. 5 volts.

Seems that your hot wire to pump is shorting out somewhere if batteries are putting out enough volts.

Did you check your batteries?

 

[DBar]

Thanks for the reply, the batteries are good. They'll start the truck no problem, but the lift pump is not getting the juice. I charged the batteries last night anyway, but still no go at the lift pump. I ruled out the extension wire back along the chassis by connecting a working lift pump up to the lift pump wiring harness connector near the filter, but still no go. Could still be a short in the hot wire up to that connector though as you say. Does that wire connect directly to the ECM? Thanks for any suggestions.

 

[B.G. Smith]

I just looked at my 99 manual and it shows a fuel pump relay in the PDC, (may be for gas engine?). The PCM controls the pump but I don't think ot directly supplies the power but rather through a relay? bg

 

[Gary - K7GLD]

Sounds like a relay, wire harness or power plug to the LP issue...



These LP power plugs are pretty cheesy, and I have seen evidence of arcing burns on my own from past inspections - or is your LP one that uses the post-type connectoin?

 

[DBar]

Thanks guys, I checked the relay (Fuel System Relay) and swapped it out with the horn Relay (same # Relay), but no go. The horn works with either relay, the LP with neither one.



The plug connections look OK. I'm suspecting the wire that runs to the relay, or the PCM. Is there an electrical drawing in the Haynes Manual for this?

 

[Bob4x4]

The fuel pump supply voltage comes directly from the ecm. Pins 15 & 35 on the 01 drawing to a splice then to the pump. It is pulse width modulated so battery voltage is not always what you will see. It will be less cranking than running normally.



Bob

 

[DBar]

Here's the latest. I rigged a wire harness direct from the battery posts to the plug connector on the wire going to the lift pump. LP worked fine. Disconnected after 3 or 4 seconds to prevent over pressure at start. Truck started fine. I then reconnected to the battery post with truck running and was getting a good 15 psi fuel pressure (like normal). I then drove out the highway to the Cummins Dealer and had them take a look. Basically all they did was get the codes (which I had checked yesterday - none showing ECM failure or other short), then disconnect the homemade harness and connect the LP back the way it was and everything was working fine. They couldn't tell me what was wrong. Hard to swallow, but the truck is working fine right now.



I should mention that it was mild again today, the trouble occurred during colder weather. Anyway, thanks for the help guys, it was really appreciated. Wish I could give you more of an insight on what the trouble was. I'm thinking it may re-appear at the next cold snap... .