95 lift pump not pumping while truck is running

[Jesse.r.thompson18]

Hey guys I am new to the site so please forgive me if this has already been answered. I have searched the other threads and could not find my situation.
I will start off by saying that my truck is running fine but feels like it has a little less power when going up hills or towing. So I thought I would check my lift pump.
I pulled the relay and jumped the appropriate terminals and the pump turned on.
I tested the relay and it was fine. I did not trust that so I put a new relay in just to be safe.
The relay will engage during cranking but once the truck is running the relay opens back up, which I am sure that is normal. I believe the LP is ran from the OPS while the truck is running. The weird thing is the LP does not pump while the truck is running. I opened the bleeder valve and a few seconds later the truck dies.
So while the truck was running I unplugged the LP to check the voltage at the plague feeding the pump, assuming I would see 0.0 volts indicating a bad OPS, but instead I had 14.4 volts when I measure across the pins. When i was measuring I put one probe on each pin and got 14.4v, I am not sure if I was testing it right.
Also being a 1995 I can take the truck out of park and put the selector in the D position then turn the key to crank and the truck will not turn over but the LP system kicks on and pumps.
Is it possible that I can get the proper voltage through the OPS but not enough amps to operate the LP.
I am at a loss because everything seems to check out, except the pump does not run while the truck is running.
Oh I forgot to say, I have a 1995, GMC Suburban with the 6.5 Turbo Diesel K1500.
Thanks guy.

[ronniejoe]

Hi, and welcome. I suppose that it is possible to get voltage under no load, but not with a load. I would replace the OPS. That is probably the problem.

[DmaxMaverick]

Welcome aboard!


......Is it possible that I can get the proper voltage through the OPS but not enough amps to operate the LP.....

Quite simply, yes. That's the most common way the OPS fails, before the contacts burn completely. At any rate, you should start there. If possible, install a low-amp relay engaged by the OPS to run the LP (DIY, if you're able, or buy a ready-made controller (http://www.kennedydiesel.com/categoryresults2.cfm?Category=1&SubCategory=158)). This will extend the life of the OPS and ensure the lift pump is getting full power all the time (which also extends the life of the lift pump).

[Jesse.r.thompson18]

Perfect I am going to swap out the OPS and see if that fixes it. I will update with findings.
Thanks again.

[arveetek]

(DIY, if you're able, or buy a ready-made controller (http://www.kennedydiesel.com/categoryresults2.cfm?Category=1&SubCategory=158)). ).

I have that Kennedy Diesel OPS harness, and absolutely love it. I installed it around 13 years ago, and have not had a single issue since (other than failing lift pumps). I also love being able to push a button on the controller to engage the lift pump.... it really speeds up fuel filter changes.

Casey

[Jesse.r.thompson18]

Quick update. I got the new OPS install (not a very convenient location) and the problem is fixed.
The only issue now is that I could not get my hands on an ACDelco in time for my trip to Nevada so I am going to have to swap it out again. Not a big deal because I would like to install the relay model anyway.
Thanks for the help.