Fuel gauge problem but not gauge or sender

[Kerrington Ross]

So the fuel gauge in my new to me 04.5 has been out of commission for some time and shows full to 3/4 (which is the current reading). I checked the fuel sending unit and I got 110 ohms back which is about what I thought it should be and would change when I rocked the truck.

I did the cluster reset and the fuel gauge checks out. However I got a boat load of c codes back. 01 72, 0d 49, 26 72, 2F 72, 33 72, 35 72, 39 72, 36 72, 61 20, A7 15, AA 15, AC 05, 67 61.
So I grabbed a jumper, disconnected the fuel pump/sender. Check the gauge and it’s still showing 3/4 full, yes I did turn off and back on. Did the jumper and when I checked it showed full as suspected.

At this point I’m lost as hell where to go from this point but my brain is thinking PCM. Any help or advice would be welcome.

The only thing not stock is the smarty on it set to 1 and the remote start. Fuel gauge worked after remote start was installed. No codes are shown with the key trick or on the smarty.

 

[PBJ2500CTD]

You used to be able to put a variable resistor in place of the sending unit to test the gauge. If you know what the sending unit's variable resistance value is from high to low you can substitute fixed incremental resistor values in place of a variable and see if the gauge follows. I would guess it is the sending unit that is causing your issue. Assuming it is a simple circuit and not relying on the ECM/PCM or other circuits, troubleshooting should be quick and easy. If a jumper (short) causes the gauge to read full as stated in your description, and approximately 110 ohms displays ~3/4, try a 200 ohm, 300 ohm, then a 500 ohm. If the sending unit is linear you should see the gauge read around 1/2, 1/4, then close to empty. Again, this is assuming simple circuit and linear sending unit. At least it should move the reading down scale. Good luck with the testing.

 

[Ozymandias]

With that bunch of codes I'd first use a real Codereader that translates all those numbers in to something useful.
Maybe the problem is somewhere far away from the gauge or the sender.

And it is always a good idea to set any tuner back to stock and take it away from the vehicle for the time as a problem exists.
In case of the smarty you'll also get the latest factory updates to your ECM that way - if the smarty itself is on the latest Software Update.

 

[Kerrington Ross]

I haven’t done any extensive testing this morning YET. However I noticed that the fuel gauge won’t relax down to the pin when turned off or when the cluster self test is done. I don’t have resistors handy but I’m thinking I’ll be dropping the tank fully today. Could anyone advise what pin I need to check at the ECM aside from 26 on C1? Looking for the ground for the sender to complete the circuit

 

[cerberusiam]

If it won't go to 0 during a cluster test the cluster is bad. With all those codes you need a new cluster or some work on that one. The cluster is just displaying what the ECM puts on the data bus so they are not real gauges.

 

[PBJ2500CTD]

If it won't go to 0 during a cluster test the cluster is bad. With all those codes you need a new cluster or some work on that one. The cluster is just displaying what the ECM puts on the data bus so they are not real gauges.

^^^^^^^
What cerberusiam said...

I don't have a gen 3 wiring diagram so I cannot say which wire/connector pin to follow. Maybe someone else can assist with that part. Save the tank drop until you fully check the wiring and gauge function.

 

[Kerrington Ross]

I did save the tank drop. I traced the wires to the foot well. Ohms are registering 139 and return voltages has changed as well to 3.2V from 2.5V. ECM is sending 11.43V. I was trying to trace it back to the dash but couldn’t find which wires in the FSM and was also trying to test at the ECM but was cold and couldn’t see down there that well. So off to auto-parts.com and the internet to find a repair person for the cluster.