Lift pump problem 95 Tahoe

[jrsavoie]

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.

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 but 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?

Where do I test for power off the OPC? Previously I always tested at the LP plug.

Where do you find the LP wires in the engine compartment?
It seems to me I found them near the fuse box on another vehicle. Seems they were kind of under the master cylinder heading down. Are there other places to look?

When you do the paper clip trick on the Data link where do the codes flash. I've never done it before and do not know where to be looking.

 

[buddy]

A few members with 95s have commented that its pin F on theirs for the LP, and it was in a 95 manual they posted. So you need to check on pin F or provide power there.

The LP wont run with Ignition ON, unless you modify the system. It only runs during crank and engine operation from the OPS. Sounds like your OPS and relay are dead. Good idea to do the OPS relay modification and get an ACDelco OPS. Or there is a blown fuse or cut wire. Test continuity from the relay socket to the LP harness.

The SES light flashes when you put paperclip across pins A and B and turn IGN to on.

 

[jrsavoie]

I also thought that the LP would run with the vehicle in gear and the ignition turned to start.

Where can you order the good relay sockets and relays? Any part numbers? I looked through the posts here and couldn't find the info. It seems to me that on one of the relay threads there was a post about not getting the cheapo Chinese relays and there were a couple others mentioned as being better than average.

I checked the local Napa & Auto Zone but they said they didn't have any.

I have an empty spot for a relay in my underhood fusebox. Would it be hard to plumb one in there? Or would it be a waste of time?

I've been going to order the relay for a while but I'm not sure what to get exactly.

This would be the first time I had an OPS go bad and still had 14 vols to the LP when running.

I'll try jumping the F plug tomorrow.

You say it sounds like my OPS and relay are dead. Where is my relay? I didn't know I had one.

All AC Delco for electrical components.

Thanks

 

[buddy]

It should on a 94, but apparantly in 95 it moved to pin F

 

[Acesneights1]

I don't remember but I thought was pin g as well but I bypassed mine in my 95 and wired it to the key.

 

[buddy]

Maybe you are missing the relay, that is what is supposed to run the LP when you turn the ignition to start. You put it in gear so it wouldnt actually crank and turned the key all the way forward as though you were trying to crank it. That puts power to the starter solenoid and the lift pump relay coil to start the lift pump, but you said you got no power to the lift pump.

You said it had 14V at the LP harness while running the truck was running but with no LP connected, but when you plugged LP in you had no fuel flow. That tells me the OPS cannot pass enough current to run the LP, but its making enough contact for there to be 14V potential there. With no current there isnt any voltage frop. So if you test pin F while its running with LP attached you will probably get 7.5V or less since the voltage supply post OPS will drop when it tries to pull current through the LP.

Pin F (assuming it is F in yours) is showing you the voltage through the OPS to the LP while the engine is running.

 

[jrsavoie]

I'll try pin F tomorrow.
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.

 

[jrsavoie]

Is there anything bad that can happen by putting power to pin F if it is the wrong pin?

 

[jrsavoie]

Where is the SES light when it comes on? Center? Upper Right? Lower left?
Even with reading glasses I have trouble seeing stuff close up. I don't know how long the display lasts but if it doesn't last long I may miss it looking for it.

 

[buddy]

The Service Engine Soon (aka check engine or MIL) light is on lower right. It should always come on with just ignition on. With the paperclip in it will constantly flash, each code 3 times then repeat.

 

[jrsavoie]

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.

 

[jrsavoie]

relay pictures.
It's sloppier than I like but it works.

 

[buddy]

Hmm I dont have the harness under the master cylinder. I wonder if that is 95 unique or Tahoe unique. In 94 our LP relay and fuse is on passenger side, and we dont have a fuse center under the hood.

Did you even add the coil voltage kickback protection, or did the relays come with some already?

 

[jrsavoie]

They said the kickback protection was already in there.

I tied into a 3 wire plug that has the grey and black wire running to the lift pump. It also has a green wire going with it. It is about a 6 place plug but only had 3 wires.
I tied in for the 1989 at the same place but I can't remember if there was a plug there or I just found the desired grey wire in the same general vicinity.

 

[JMJNet]

Great work!!! What is important that it works. Awesome.