Possible short???

thefinchnest · June 24, 2014

I have spent a year undoing the damage of a 20 year old (previous owner) and have been diagnosing one particular problem since I bought this truck. It is a 2002 GMC Sonoma Crew Cab with a 4.3L engine

It involves the transmission indicator, the transmission, the fuel gauge and maybe more. So here is the typical symptoms my patient is presenting with:

Before the truck is even started Symptom 1: the transmission indicator shows the "P" aglow ever so slightly. (This can only been seen at night) As soon as the key is inserted, the glow goes out. Symptom 2: In some instances, the truck will lurch forward as it is started. Symptom 3: Shifting indicator is slow to respond after Neutral. Shift into Reverse or Neutral and it acts normally. Shift into Drive, 3rd, 2nd, or 1st and the transmission responds but the indicator takes about 5 seconds to respond and then "jumps" to the proper selection. Symptom 4: Whether driving or static, when the transmission is placed in the Neutral position, the fuel gauge will drop showing a level of about 1/8 tank less than what is actually in the tank. When the shifter is returned to Drive, 3rd, 2nd, or 1st, the fuel gauge returns to its proper reading. It is especially problematic when the tank is less than a quarter tank full because the gauge drops as I am driving swiftly until I stop, Park, turn off the engine, restart and the gauge indicates 1/4 full.

Upon changing the fuel pump, I did note the sending unit (electrical measuring prongs) seemingly burned down to stub which I believe was cause ERRATIC fuel gauge responses.

I changed the instrument cluster and the truck presented with the same symptoms.

Replaced the shifter spring (it was broken in half) in hopes that it was causing a faulty ground.

I've found no noticeable wear on the wiring harness, though with the hundreds of wires I could have missed something.

Any techies out there experience these symptoms or have some idea of a particular wire or component that I should be paying more attention to???

02suburbanZ71 · June 24, 2014

Did you even scan it for codes?

thefinchnest · June 25, 2014

Yes, and no codes relating to the transmission and any codes for the fuel and emissions system were cleared with a new fuel tank. (Too many rust holes)

gmtech4 · June 25, 2014

i was a tranny guy for a few yrs and all i can say is ,,,,wow

scan is a great idea ,,,,,have you thought of replacing the NSBU switch on the trans shifter shaft ?,

how about buying service information ,,say alldata for $27/yr

you got some nasty $%#% goin on and throwing more stuff at it and misdiagnoses will piss ME off reading it ,,,lol,,,,the NSBU switch cant hurt is my point in that it is a common failure

internet will only confuse you im afraid ...

please update this as you go cuz no one does lol

badsavana · June 27, 2014

I have to agree with govtech. Sounds like the previous owner may have forced on a new one in the wrong position. Although if it is in the wrong position usually it will cause a no start condition. I changed mine because the connector was corroded and when I tried to take it off the whole unit crumbled. Didn't help my problem. Just throwing that out there.

thefinchnest · June 30, 2014

Govtech4- I will look into this switch. Though I'm not sure how this involves the fuel gauge or how it will affect it. The fuel gauge and tranny indicator appear to be related. But yes I will keep you posted.

Badsavana- Though I'm not sure how the switch works, I am skeptical to think that the switch was forced on in the wrong position as I would think that the indicator would show the incorrect position at all times. The case here is that the indicator pauses when it gets to Neutral, and then after a few seconds it "moves" to the correct position.

In the meantime, I'll try to post a video of what I am encountering, if I figure out how.

Thanks for the input.

gmtech4 · June 30, 2014

Fuel gauge has nothing to do with transmission NSBU switch ,

The NSBU switch can be bear , sand the end of the shaft as it can get flared out from the nut , it's is oval (2 flat sides) so lining up the new switch very important .

Get a deep socket that just fits the shaft to help press on the switch be VERY carful , abit of lube never hurts as well ,

This may take care of one of your issues but not everything ,these switches don't like moisture , as said they are a common failure point

badsavana · June 30, 2014

Make sure that the tang that comes installed on the unit does not fall off before you have it installed. Also the trans needs to be in neuteral. As stated be very careful.