I checked the lift pump with a direct wire. It ran and seemed to put out an adequate amount of fuel.
I ran the vehicle with the tester on the lift pump plug and had 14 volts at the plug while running.
I reattached the plug and ran the vehicle, when I opened the drain line, nothing came out and eventually the Tahoe died.

Fresh fuel filter and the filter primed easily and quickley with the lift pump hard wired.

Where do I jumper this vehicle to run the lift pump?
I thought this was the vehicle with the little red wire by the fuse box. But there is none there. It does not have the same plug under the dash with the G pin.
There is a 2 wire plug that runs through the firewall with brown and green wires. It goes through where the fuse box is but is not in the loom. What is this male plug for?

Thanks

I do have a G pin. So I'm blind. What can I say?

I put power to the LP plug and had 7.49 volts to the G pin.
With the vehicle running I had 7.51 volts to the G pin.
With the G pin hot wired I had 0 volts to the LP plug.
With the vehicle in drive and the ignition turned to start the lift pump was not running bit I never checked for voltage at the LP plug.
With the LP hot wired and the Tahoe running, I opened the drain and had a steady flow.
It is my understanding that with the G pin hotwired you should have the same volts at the LP plug. Is this correct?

Pictures of the plug by the fuse box

check the oil pressure switch,it controls the power to the lp when engine running they seem to be a common problem.

check the oil pressure switch,it controls the power to the lp when engine running they seem to be a common problem.

I have one on hand. I'm just confused by what I am seeing is not what I am expecting to see.
I thought that if you jumpered the G pin the lift pump would run regardless of the condition of the OPC.
Tonight I was informed that on 1995's it can be the F pin. I will be trying that tomorrow.
A concern is that if the G pin is indeed the correct pin, what damage can be done by putting power to the F pin. Makes me a little hesitant.

If you disconnect the OPS socket, you can jump the pin on both end. The LP should run. You definitely have a bad OPS.

without the wiring schematic for your setup I can't even guess about it.
I do know though that if you ever put power to a ground circuit you could be in big trouble.and its very easy to do since the puter controls the ground and not the power to most every system.

If you disconnect the OPS socket, you can jump the pin on both end. The LP should run. You definitely have a bad OPS.

I'm confused : removing the OPS socket will let me jump the pin on both ends? I do not understand at all. I could jump the gray wire from the OPS there. That would tell me what I suspect.
It was also suggested that I check the fuel pump relay.

new question for you.
does the lift pump work when you key on engine off-during glow plug sequence?
if it doesn't pump for priming or running then the relay is a good bet.

The only time it has pumped was when I hot wired it.
With the vehicle in drive and the ignition turned to start the lift pump was not running but I never checked for voltage at the LP plug.

if the pump does not run with vehicle in park and key in run-engine off or after starting you still do not have a working lift pump then it is possable you have a bad lift pump relay or a bad power/ ground circuit that controls the coil side of the relay.

Best way to test relay and pump on a 94-95 (OBD-1) is to power the "F" pin on the OBD port under the dash. The "F" pin is on the top right of the plug. Just run a power wire to this pin and the pump should run if your relay is good. Great way to prime after a filter change... This will bypass the OPS. OPSs go bad all the time.... I have fuel pressure gauges on both trucks just for this reason. Pumps only run if oil pressure is present. Lift pump does not run on start up, only after the engine is running( oil pressure present ). Just my 2 cents....

I'm confused : removing the OPS socket will let me jump the pin on both ends? I do not understand at all. I could jump the gray wire from the OPS there. That would tell me what I suspect.
It was also suggested that I check the fuel pump relay.

Sorry, the OPS have a connector. I am not sure how to describe it but that connector one pin is 12V source and the other is to the LP. When you put a paper clip, the LP will run. At least that happens on my 95. Yours may be different. This is not the pin on the OPS but the pin on the connector.

Or if you just trust us, change the OPS and you will be good. Make sure it is ACDelco OPS. Then you can do the relay.

The OPS relay on OBD-I power the LP during crank but it is a rare chance that it is bad.

Best way to test relay and pump on a 94-95 (OBD-1) is to power the "F" pin on the OBD port under the dash. The "F" pin is on the top right of the plug. Just run a power wire to this pin and the pump should run if your relay is good. Great way to prime after a filter change... This will bypass the OPS. OPSs go bad all the time.... I have fuel pressure gauges on both trucks just for this reason. Pumps only run if oil pressure is present. Lift pump does not run on start up, only after the engine is running( oil pressure present ). Just my 2 cents....

Is the DLC connector shown in the lift pump sticky labeled backwards A-F?
In the picture the F area appears to be empty and is located top left.
I'll try to post a link the page if you are not familiar with it.

Correction from my earlier post..... I replied before I checked service manual...Relay does power pump while key is in start position. OPS is out of the circut while switch is in start, but once the key return to run, the OPS powers the pump. OPS needs 4 psi of oil pressure to close contacts. JMJnet is correct... Change your OPS and all should be well.... I have had good luck with Oreilly OPS # MPI-S4000. Made by TRW and at ~19 bucks its a good deal.

Clarification on my post is the relay mod:

http://mysite.verizon.net/vze54tx9/id13.html

If OPS change solves your problem, that means the LP relay is fine.

The relay mod is intended to relief the OPS as the amp carrier to the LP which is the part that is weak and broken all the time.

Yep, I am planning to get parts and do the relay at the same time. I'm planning to do the combo WTS and OPS relay mod. This will run the pump during WTS and will take the load off the OPS. On my 89 I used a toggle switch to operate the LP when I turned the key to on. And flipped the toggle off after the OPS took over.

A link to the lift pump thread with a picture of the OBD1 DLC connector.
http://www.dieselplace.com/forum/showthread.php?t=39350
The F pin is shown top Left. as apposed to top Right.

Post shows DLC port from a 98(OBD-2). 94 and 95s(OBD-1) have pin in location "F" that will power the fuel pump relay and run the pump if provide 12 volts to it. I use this to prime system after a filter change. I also used this on a trip when the OPS went south. Fuel pressure gauges are nice protection and great way to monitor lift pump operation.

Age sucks... "F"pin is top left on DLC. OBD-1 and OBD-2...I said top right..WRONG. No coffee this morning...

Age sucks... "F"pin is top left on DLC. OBD-1 and OBD-2...I said top right..WRONG. No coffee this morning...

At least it's Not supposed to drink coffe period. That really sucks.

105634

This is a double post but it applies to both threads.
Got the double relays done.
I came off the 3 wire connector below the master cylinder for the gray wire. Off the auxilary power post on the underhood fuse box for power. I had a heck of a time figuring out why it wouldn't work.
It turned out to be a looks like new on the outside, bad fuse holder and fuse. It would pass volts and pass a continuity test but wouldn't pass the amps. Near drove me crazy.
I'll try to post a picture or 2 tomorrow.
Thanks for the schematic.
Also the relay sockets I found turned the relays a 1/4 turn from the schematic. Not such a big deal but it made my eyes cross.
Thanks again for all the help
I never did change the OPS. the 14 volts it was passing that wouldn't run the LP, ran the relay just fine.

picturse. Not as clean as I would like but it works