problem after replacing fuel level sending unit

sLAsh

Original poster
Member
Jan 26, 2012
39
Sorry for the novel but thought it was best to put down what i have already tried

Background info
Replace the sending unit about a year and a half ago. The seal was rotted and a little torn up but I reused it anyways because there was no place I could get one on a Sunday and wife needed the Envoy. Got an evap code for a small leak ever since but wasn't too concerned since I knew what it was and would replace it when it was convenient.

New info
Long weekend in Canada so I figured it was a good time to replace since my wife had to work. Replace the seal and everything went fine other than me forgeting to put the rear brace in first and having to lower the tank back down partway so I could get the brace in. Buttoned it all up but when I turned the ignition to run I didn't hear the fuel pump. Checked for leaks anyways thinking it might just be quiet. Not sure how loud it is usually in the Envoy because my wife usually drives it. Was a bit concerned so I tried starting it while it was still on stands but would start. Thought it might be the pump having problems priming since there was only a gallon or two in the tank so I put a can of gas in. Still no start.
Pulled front lines to see if there was any fuel in them but they were dry. Dropped tank again thinking I may have pulled on the wiring and disconnected the fuel pump while dropping it partway to put in the rear brace but it was still attached.
Pulled the tank completely out thinking maybe I pulled a wire out of the connector but they were all fine. Turned on ignition and tested the 3 prong connector and it was giving me the readouts that are printed on the fuel pump (5v, -5v, ground) couldn't find any markings on the other tank connector but when I probed the wiring side with the four outputs I got only three readings(5v, -5v, ground) and nothing from the fourth one.
Came in the house to look at the manuals I downloaded from this site to my tablet (thanks again to the person who uploaded it and the person hosting it) Unfortunately I wasn't able to find the info I needed because I can't see the wiring diagrams clearly enough on my tablet and my laptop is in my car that the site took to work.
Can anyone tell me if I should be getting a reading from that fourth pin? I even considered putting 5v to the connections I know are right on the fuel pump just to rule it out as the problem but don't like the idea of sparks and fuel and that still leaves me with the question on what voltage I would apply to that fourth pin.
Any other ideas are welcome.
 

IllogicTC

Member
Dec 30, 2013
3,452
Are we talking the pin that feeds to an orange/black wire? That's the low reference for the fuel sending unit (the purple supplies the input to that circuit, pin A) Pin B should be getting battery voltage (gray wire) when priming or on and of course the solid black wire is the motor's ground.

The fuel pump relay is controlled via the PCM, you can verify up front whether or not the relay is working by swapping it with another of the same model number (fuel pump relay is in position 41).

Wire colors may be different if you have the 5.3. Can you complete your profile with year and engine information?
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Nothing in the vehicle should show you -5V. Very, very odd you could read that unless the black/low side probe of your meter is not on a good ground.

The connector is as follows:

Pin A - purple: sensor output. The PCM has a current source built in, so this should be a variable voltage as measured by your meter. The level sensor is a variable resistor, 250 Ohms full and 40 Ohms empty. I don't know the exact voltage levels that correspond to empty or full.

Pin D - Orange/black: Sensor ground

Pin C - Black: Pump ground.

Pin B - Gray: Pump hot. 12V when the pump should run. Fed by front relay #41 which in turn is fed by front fuse #10 and controlled by the PCM
 

IllogicTC

Member
Dec 30, 2013
3,452
The_Roadie said:
Pin D - Orange/black: Sensor ground
Unless V8, in which case the big change is Pin D also has a solid black wire, though all functionality remains the same in terms of what the pinouts do. All other wire colors are also consistent.
 

sLAsh

Original poster
Member
Jan 26, 2012
39
So it was a combination of stupidity and old age.
Stupidity because I didn't put the fuel back in that i had syphoned out and the angle I had the truck jacked up at didn't allow the 1/2 gallon I had left in the tank to get picked up.
Old age was the -5. It seems whenever I work under a car nowadays everything is at the wrong focal length. But we already knew roadie's numbers were right didn't we :wink:
I keep meaning to get a pair of reading glasses that are the right distance for working on a car but never get around to it.
Thanks again for the quick replies. Once I realized which pin was supposed to be getting power for the pump and that it would only run for a short while without the engine running I was able to get my son to turn the key while I was under the vehicle and eliminate it from my concerns
 

sLAsh

Original poster
Member
Jan 26, 2012
39
Yep all is well now. Car runs and check engine light is gone.
Three sets sounds like a great idea. You don't realize how often you move your head a few inches to get things in focus until you can't do it.
 

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