oil pressure switch question
 

I realized that my lift pump wasnt working so I just switched my oil pressure switch and installed a new pmd with heat sync. Before I did this everything was working well. It would start right up and run down the road great but started to hesitate or surge this last week, that's why the replacing of those things. Now, after replacing them when I go to hook batteries up the lift pump starts running and wont shut off. So I think, ok I will start it to see what happens, but I cant cause my glow plug light comes on for about 2 seconds then goes off. Everything went off without a hitch doing the repairs but now I'm confused.

Did you plug the OPS back in all the way? And is it screwed done all the way also. It will also help if you tell us the year of your truck, because the lift pump works differently between some years.

Another question.... Why did you unhook the batteries? I did my OPS the other night, without unplugging the batteries, along with installing a new pmd. Are the terminals tight, and are all the wires connected?

Its a 1994 c/k 2500 TD. Yea I unhooked the batteries before I did anything and I'm assuming the OPS is in all the way and plugged in tight because the lift pump that wasnt working beforw is running strong now.....with or without a key in the ignition. I'm thinkin its some kind of a grounding issue that I cant figure out......yet!

lift pump running wild.
 

From what i been learning the lift pump runs off the fuel pump relay first then gets it power off the ops. Check your relay make sure it's not shorted for power all the time. Grounds also a good bet. Good Luck.

Thanks. I never even thought of anything else being wrong, I was too focused on what I could have done wrong. I will definately check into that tomorrow. Thanks again

If you bought an aftermarket OPS there is a good chance that it is junk-the aftermarket under engineered those switches by about a factor of 10.

I suspect that your new OPS has failed in the closed position sending power to the LP,The circuit feeding the OPS for LP operation is always hot-thats why the LP runs for a few seconds after shutdown while the oil pressure bleeds off.

AC Delco is the only switch that stands up in these trucks.

To let even them live the OPS relay mod is almost mandatory(google it).