lift pump voltage problem

[HUGHY]

i have a 94 3/4 6.5 turbo,the pump is only getting 3 or4 volts. i have changed the ign. switch,oil pressure cut off switch,fuel pump relay,fuse on fire wall,checked fuses at fuse box,no luck.NEED HELP!

[MM9135]

If you read 3-4 volts with the lift pump unplugged you have an open circuit back some where in the line that feeds the pump. If the reading is across the pump while it is plugged in then you may have a bad pump. They do go bad.

To test take a car battery and two long wires and hot wire the pump and see if it runs. Watch your polarity. It should make a pretty loud thumping sound.

Remember that pump doesn't get power all the time. Bump the starter and it should get power for a few seconds and then stop. The computer controls the power. That is how my 6.5L 1996 does.

Good luck let me know what you find.

[StephenA]

In replacing the ignition switch, pump relay, etc, did you get proper voltages off each part when installed?
Stephen
P.S -can you give me the part number of the ignition switch...?

[rustypig]

1st time I've heard of this one...usually it's all or no voltage at all. The lift pump power on your rig runs through the oil pressure switch to the lift pump. I like many others here, installed a relay to take the load off of the OPS. If your getting voltage but not full voltage I'd have to say the OPS is working but that there may be a problem in the wiring between the OPS and lift pump...ie...high resistance. This is commonly caused by poor connection or corrosion.

If the lift pump check previously mentioned by others above check out ok, I'd have to vote for a new OPS and a relay to relieve the load. I can e-mail you a drawing I made after installing a the OPS relay if you end up needing it.

[moondoggie]

Good Day!

MM9135 said,

[MM9135]

Moondoogie that is the way the 96 and later works. I had a bad lift pump just a month ago and I called the local GM dealer and they let me talk to the mechanic and that was what he told me. Since replacing I proved it by bumping the starter and listening and you can hear it throb for a few seconds and then stop.

Since the one in question is a 94 it maybe different.

[StephenA]

Rustypig- can you email me the drawing of the OPS relay mod? My OPS causes my gauge to dance around a bit, and I've been stalling after startup, only to restart & run fine. I'm about to go through the line up from ignition switch to OPS to pump relay to pump, and so forth and I want to do any upgrading available in the process. Also, do you know of any upgrade for the ignition switch? Is it necessary or desireable? I saw JK's upgrade for headlights- what a relief- I thought I was going blind, come to find out the lights ARE really dim!
Stephen
stephendambrose@sensonics.org

[JohnC]

Just to clarify, 6.5's from '92 through '95 control the lift pump mechanically through the relay and oil pressure switch. '96 and up use PCM control.

[StephenA]

JohnC-or anybody who knows...: I have a 92; I've got no voltage to the lift pump at all. I replaced the pump relay & fuse, which were both good, I think. Tomorrow the OPS comes in. It still reads fine on the gauge except for dancing once in awhile. Is it possible for the OPS to read ok on the gauge & still not send enough voltage to trigger the pump relay into coming on? If the OPS replacement tomorrow doesn't do it, I'm thinking I may have some frayed wires; or would the ignition be the problem? Amazing, except for stalling once in awhile after startuo, it runs fine without the pump.
P.S. Does OPS replacement require draining the oil?
Thanks

[rustypig]

StephenA:

Check your e-mail...and oh yeah...you got it exactly right. Your oil pressure gauge can still function perfectly, however, the internal contact for the lift pump circuit can be fubar. These contacts/wire are SUPER fine...like amazingly fragile looking. Just another achilles heel from the general, but it can be fixed.

[StephenA]

Great drawings, Rustypig. Thanks a million. Will swap out the OPS today and do the mod. Also I'm replacing my defunct $400.00 stock fan clutch with a 97 clutch & Dmax 21" fan. I may devise a nylon set screw to engage the bimetal spring where it's set in the silicone to adjust it more counterclockwise, if the silicone will compress a bit- There's at least 1/4" space in the slot, and all it takes to get an extra 10 degrees is 1/16", supposedly. May wait & see how the clutch works first, though.
Thanks again,
Stephen

[jbplock]

Originally posted by JohnC:
Just to clarify, 6.5's from '92 through '95 control the lift pump mechanically through the relay and oil pressure switch. '96 and up use PCM control. JC, I thought the 96 & up 6.5 trucks (with OBD-II) also used the OPS in the PCM circuit to shut off power to the lift pump for safety (stop the fuel from pumping in the event of a crash). I remember my 98's lift pump would come on then shut off while I waited for the glow plug light to go off. I also used this feature to bleed the fuel filter by turning on the key with the shifter in drive. I assumed this was a PCM controlled OPS over-ride delay timed to insure the fuel system was primed at startup. Then, when the oil pressure comes up, the lift pump would start running again. Is this correct (??)… I’m curious since I have been thinking of adding a similar delay to the lift pump circuit I have added to my Duramax. (don’t know if you have followed any of the fuel filter threads on the Duramax forum but some of us have added AC (6.5 style) lift pumps as part of added filtration).
smile.gif

[HUGHY]

the last few days i have been real busy,no time to work on truck.looks like next week i will have some time.THANKS FOR THE REPLIES!

[rjwest]

1996. pcm turns on pump for about 10 secs than shuts off.

there is an " open " connector on the left side
wiring harness where hood hinge is, that can be
jumpered to 12volts to run lift pump continueslly,
this is easy way to check wiring to pump and pump... Also good for bleeding air after filter change.

The pcm will close relay again after it detects eng running.

ALSO, If relay fails, the oil pressure switch
is in parrellel with relay and will turn on lift pump....( fail save )

If you remove relay, eng oil pressure will
turn on lift pump.....

? low voltage? check voltmeter on battery.

next most likely relay contacts...

[Jim P]

Rustypig,

I would also greatly appreciate a drawing to add a relay to my lift pump circuit on my 95. Mine seems to work fine but I would like to keep it that way.

Jim P.

jpfarmer(at)raex.com

[ucdavis]

Rustypig,
I would also greatly appreciate your drawing to add a relay to the lift pump circuit. I just replaced the OPS (lift pump no workee) and realized there is no warning that this little devil isn't working (unless DTCs on a wearing-out IP count). I'm looking for a way to bullet-proof this circuit.
Thanks,

Mike@Fleming.cc

p.s. opened up the OPS & found teeny leetle burn spots on the contacts. "For want of a shoe..."

[tvtegtmeier]

Looks like I could use those OPS/Relay upgrade drawings.

My 94 GMC runs great, but stumbles at WOT or even cruise 'resume', I've already replaced the lift pump (I think it was bad too).

Are OPC failures sometimes intermittant - it doesn't always exibit the problem.

How hard are these to replace?

Thanks,

Tim

tim@ourfarmstead.com

[rjwest]

I added a Fuel pressure gage, Security is knowing
fuel is going to Inj Pump

" Tee " in Hose going into Inj Pump, Easy Install