2000 GMC K3500 Gas Gauge Troubles

[adam6979]

Hello All, I just signed up to the forums but have been a long time stalker. Looking for some help and advise on fixing a fuel gauge problem. While this is a 2000 it is the OBS so I am struggling to find the right information.

Truck is a 2000 GMC K3500 with 1 tank, 5.7L eng and manual transmission.

The fuel gauge does not work. Here is what I have done so far, and to be honest I am at a loss.

Actual fuel level is probably about 7 gallons - I filled it and went about 225 miles.

The gas gauge will not move off of empty.

I replaced the pump & sender assembly with a new GM assembly. No change in the gauge.

I have a 1998 C3500 that I have been able to compare readings with and swap parts.

I pulled the cluster (IP) and replaced it with the C3500's and still no good, also the cluster from the K3500works fine in the C3500.

I tested the wires right at the sender to be sure the float was working at it is - shake the truck and I get a difference in ohms so it appears okay as expected from a new unit.

I wanted to be sure the purple wire from just beyond the sender was making it to the IP - I found that it is not connected. Surely a break in the wire somewhere. However I cut the purple wire off at the sender and at the cluster connector (IP) and connected a wire from the output of the sender to the connector on the IP and thegas gauge will now go up to about 1/2 tank beyond full.

When I go to the chassis side of the purple wire from the sender and test to ground I get no reading, however turn on the ignition switch and it shows it grounding.

When I put a wire direct from purple on sender to gauge and then a wire from the sender wire orange/black to ground the gauge goes to a full tank.

When reading the schematics it would appear that the chassis black wire should go directly to ground up under the hood on the thermostat housing. However if I test this I get no connection between the two. However I get no connection on the working C3500 either.

I should also say the fuel gauge is the only one not working so a ground of the cluster (IP) should not be a problem.

My next step is to start cutting open the harness and testing the purple wire back from the tank up until I lose the connection. However I am trying to avoid this and am at a loss why this would not be fixed by running a separate wire to the gauge as I have already tried.

I greatly appreciate any and all help and suggestions.

[ChumminsDQ]

Ok the big issue with the fuel senders in these trucks (I have a 99 K3500 that used to have the Vortec 350). Is it actually runs to the ECM then to the gauge cluster. Its a pain in the rear, in my case I did an engine swap to a diesel (which in OBS diesel trucks and pre OBDII gassers fuel level does not go through the ecm) I have to make a module to convert the input to get a fuel level reading.

 

So your best bet is to check pinning (voltage etc) in the harness to and from the ecm and ecm to/from gauge cluster. And check the relative pins in the ecm. Cause bypassing the ecm gets you the constant over full state but will "fluctuate" once the tank starts getting pretty low.

 

Its a real bugger. I feel for you.

[ChumminsDQ]

Oh wanted to add, the best place we found for the info you seek is in a 99-00 G series Van manuals. I had a 1998 C/K truck series manual that didnt help much either. Only reason we had van series is because we did a 5.3L swap into my dads G3500 after his 5.7 Vortec spun a main. And ran into this problem with it, so we knew what to expect with my current swap.