Help - Fuel gauge troubleshooting

[JO-0321]

Howdy...I know this has been covered before...but every situation is a little different. I'm the worst with electrical stuff...but I think I've got enough info here that someone should hopefully be able to point me in the right direction...Here's my situation:

Tank is nearly full...I just filled it. Gauge shows below EMPTY at all times. When the ignition is turned to RUN, there is zero activity on the needle...not even a little jump. When I disconnect/reconnect/ground wires at the gauge and/or sending unit, the gauge still shows zero activity.

@ GUAGE:
- Hot wire is showing 24.7 V when...

- grounded to body​

- grounded to the ground post on the gauge (which means gauge is properly grounded, right?)​

- Also shows 24.7 when grounded to the wire that goes between gauge and sending unit (I assume that's good...?)​

@ SENDING UNIT:
- Hot wire to sending unit is only reading 6.6 volts (Shouldn't this be 24V? If so...what could cause the lower voltage? Could a bad gauge cause a lower voltage?)
- When checking resistance on the sending unit leads, multimeter zeros out...I think that means that it's showing no resistance (Which it should be showing some resistance if the sending unit is good, right?)

The fact that the gauge is showing zero movement leads me to think the gauge is bad. The lack of resistance at the sending unit leads me to think that the sending unit might be bad (but the fairly full tank could play into that maybe?). I just have no idea...could they both be bad...seems unlikely...but who knows. And the 6.6V in the hot wire going to the sending unit has me completely puzzled.

Thinking I'll start with replacing the gauge...then the sending unit...does that sound reasonable, or am I missing anything prior to replacing these two items. If that doesn't solve the problem I have no idea where to go from there.

Thanks!

 

[Mullaney]

Howdy...I know this has been covered before...but every situation is a little different. I'm the worst with electrical stuff...but I think I've got enough info here that someone should hopefully be able to point me in the right direction...Here's my situation:

Tank is nearly full...I just filled it. Gauge shows below EMPTY at all times. When the ignition is turned to RUN, there is zero activity on the needle...not even a little jump. When I disconnect/reconnect/ground wires at the gauge and/or sending unit, the gauge still shows zero activity.

@ GUAGE:
- Hot wire is showing 24.7 V when...

- grounded to body​

- grounded to the ground post on the gauge (which means gauge is properly grounded, right?)​

- Also shows 24.7 when grounded to the wire that goes between gauge and sending unit (I assume that's good...?)​

@ SENDING UNIT:
- Hot wire to sending unit is only reading 6.6 volts (Shouldn't this be 24V? If so...what could cause the lower voltage? Could a bad gauge cause a lower voltage?)
- When checking resistance on the sending unit leads, multimeter zeros out...I think that means that it's showing no resistance (Which it should be showing some resistance if the sending unit is good, right?)

The fact that the gauge is showing zero movement leads me to think the gauge is bad. The lack of resistance at the sending unit leads me to think that the sending unit might be bad (but the fairly full tank could play into that maybe?). I just have no idea...could they both be bad...seems unlikely...but who knows. And the 6.6V in the hot wire going to the sending unit has me completely puzzled.

Thinking I'll start with replacing the gauge...then the sending unit...does that sound reasonable, or am I missing anything prior to replacing these two items. If that doesn't solve the problem I have no idea where to go from there.

Thanks!

.
You have good logic. The only thing you might do is to follow the tank sending unit wire to both ends. One should end up attached to the fuel tank and the other end should attach to the gauge. Disconnect on both ends and use a continuity meter and confirm that the wire is good.

That is the cheap way to start your troubleshooting. It is possible that the wire that sends the signal was cut or was chaffed - and replacing the sending unit and the gauge (before you test the wire) still might not solve your problem.

Also need to confirm that you have confirmed that you have good grounding.

.

 

[Mogman]

If you remove the sensor wire #28 from the gauge it should go to past full, if not you have a gauge issue, the voltage will vary on the sensor wire depending on how much fuel is on board but would never be 24V
Sensor wire to the gauge open = over full, shorted to ground= empty

 

[JO-0321]

The only thing you might do is to follow the tank sending unit wire to both ends. Disconnect on both ends and use a continuity meter and confirm that the wire is good.

Okay...this is a good idea...So, this wire would be the cold wire at both the gauge and the sending unit, correct?

Also need to confirm that you have confirmed that you have good grounding.

Okay...are you talking at the gauge, sending unit, or where? At the gauge, I can get 24.7V reading from the hot wire if I ground it to the gauge ground post...that should mean the gauge is grounded correctly, right? Where else would I check?

 

[JO-0321]

If you remove the sensor wire #28 from the gauge it should go to past full, if not you have a gauge issue...
Sensor wire to the gauge open = over full, shorted to ground= empty

Yes...that's right...I had forgotten that I did watch a YouTube video of some guy with a bad sending unit and when he disconnected the wires at the sending unit the gauge jumped to full...and when he grounded the wires it at the sending unit it went to empty.

...the voltage will vary on the sensor wire depending on how much fuel is on board but would never be 24V

To be clear, you're talking about the hot wire that supplies electricity to the sending unit, correct?



Sorry for my complete lack of electrical knowledge and terminology! Ha!

 

[Mullaney]

Okay...this is a good idea...So, this wire would be the cold wire at both the gauge and the sending unit, correct?

Okay...are you talking at the gauge, sending unit, or where? At the gauge, I can get 24.7V reading from the hot wire if I ground it to the gauge ground post...that should mean the gauge is grounded correctly, right? Where else would I check?

.
@Mogman did even better than I did. If you can find the wire tag (little aluminum band - with a number on it) you can test the gauge with his instructions. Still need to meter out the wire that runs from the gauge to the tank too.

 

[Mogman]

Yes...that's right...I had forgotten that I did watch a YouTube video of some guy with a bad sending unit and when he disconnected the wires at the sending unit the gauge jumped to full...and when he grounded the wires it at the sending unit it went to empty.

To be clear, you're talking about the hot wire that supplies electricity to the sending unit, correct?



Sorry for my complete lack of electrical knowledge and terminology! Ha!

No I am talking about wire #28 that goes from the gauge to the sending unit, you can test the gauge at the gauge, unplug #28 from the gauge if it does not go to full you have a gauge problem, if it does go to full then you have an issue with the wires to or the sending unit.

 

[Mogman]

Yes...that's right...I had forgotten that I did watch a YouTube video of some guy with a bad sending unit and when he disconnected the wires at the sending unit the gauge jumped to full...and when he grounded the wires it at the sending unit it went to empty.

To be clear, you're talking about the hot wire that supplies electricity to the sending unit, correct?



Sorry for my complete lack of electrical knowledge and terminology! Ha!

Well yes I suppose that statement could be considered correct, the wire going to the sending unit is not power it the sense the sending unit needs to be "powered" it is measuring the resistance to ground, the sending unit is just a variable resistor, the #28 wire would be the "hot" wire and 58 is ground.