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coxfmly
08-13-2011, 00:06
I have zero flow from the overflow fuel valve, truck quit. grounded lift pump fuse and the pump started. Primed system and it started but quit within a minute. could it be the OPS or PMD? lift pump, OPS and filter changed 7K ago. Have new DTECH but I havent installed it yet...thanks
Robyn
08-13-2011, 07:15
The OPS should be supplying power to the lift pump when the engine is running.

Some aftermarket OPS will not live very long. Be sure that you have line voltage to the lift pump when the engine is running. No voltage means bad OPS or some other fault in that circuit.

Now you said you "grounded" the fuse ?? actually, when you touch the center of the fuse to the stud in the eletrical box, you are powering the system. The stud is HOT.

Be sure the fuel pump relay is OK too.

If the pump will run when you touch the fuse to the stud the pump is OK as is the wiring. Good bet that its the OPS that quit.

Be sure the fuse is OK though

94 and 95 trucks power the lift pump with the entire voltage load being supplied through the OPS. These little switches will not always handle the task.

Missy
coxfmly
08-13-2011, 12:50
Hi Robyn,
The current OPS is a factory unit from GM, fortunately I have a spare if needed.

It looks like the lift pump is working fine since it fires up when I supply power to it. Is there a way to check the fuel pump relay before I change out the OPS? Would it be best just to replace the relay and see where i'm at then?

Have you ever used one of those extension hoses that lifts the OPS up to a more manageable height? The last time I changed it out it was a bit of a bear..

Thanks...
Robyn
08-13-2011, 19:06
The only time the relay works is when the key is in the CRANK position. During run the power is supplied via the OPS.

You can check to see if there is power to the Pump during crank times, if so the relay is fine.

The relay is out of the picture any time but during startup.

Have not used the hose thingy

Remove the glow relay and its an easy job then.

Missy
Burning Oil
08-13-2011, 19:50
The LP ext hose is great, but I am biased ;) www.pmdcable.com
HH
08-14-2011, 07:54
If you pull the plug off the OPS you can jump the terminals, gray and orange wires (IIRC). When I was trouble shooting I connected an LED to the lift pump circuit so I would know it was working. I would start with the OPS.
coxfmly
08-14-2011, 23:52
It seems the OPS was the problem, changed it out and the truck lives, thanks!

Since I have already gone thru 2 OPS's, 2 PMD's and 1 lift pump in the last year, is their any way to extend their life? I've already mounted a new DTECH PDM on the fwd skid plate, but I have no idea what I can do with the OPS and pump.

If I install an extension line on the OPS, will the extra length have any effect on the oil pressure? Does the heat and vibration of the engine shorten the life of the OPS or is it a design issue?

My biggest concern is trying to stop the truck when these things go south. I think my wife would have a difficult time if all systems suddenly failed.

Thanks for the help....
arveetek
08-15-2011, 06:19
Purchase Kennedy's OPS harness from www.kennedydiesel.com and be done with it. I installed the harness on my '95 back in late '05, and that was over 70K miles ago. Still working great.

The OPS harness removes the load from the switch by using a relay. It also has a handy button that you mount on the firewall that allows you to push and manually engage the lift pump....makes changing fuel filters a breeze!!

I have had to replace the lift pump since then, though. There doesn't seem to be much of a way to improve the LP life span itself...

Casey
Burning Oil
08-15-2011, 06:42
You can upgrade to my Walbro LP and LP bypass fitting kit.
http://www.pmdcable.com/index.php?main_page=index&cPath=6&zenid=mbjddhvh5b64vqlqd0afhq0jb4
Robyn
08-15-2011, 06:55
The problem is not the vibration ot heat but, instead the tiny contacts inside the switch.

The entire power demand of the lift pump is being handled by the OPS which is not up to the task.

As mentioned, go with a harness and relay and the issue will go away.

The OPS then operates a relay which draws little power and has a secondary set of contacts to handle the power to the lift pump.

Another thing that is seldom considered, that with the small contacts in the OPS, when they get a tad flakey the current flow to the LP starts to fall off too, resulting in failure of the LP.

Installing the relay system will help all the way around.

Good luck

Missy
coxfmly
08-15-2011, 18:43
just want to be sure...the "OPS harness" is the "lift pump control harness" on the Kennedy site? thanks for the help...
arveetek
08-15-2011, 20:45
just want to be sure...the "OPS harness" is the "lift pump control harness" on the Kennedy site? thanks for the help...

Yes. That's it.

Casey
JohnC
08-20-2011, 07:11
The factory relay, which energizes the lift pump during cranking, can still cause a failure as the lift pump power still passes through the normally closed contacts in the relay when it is not energized.