Fuel gauge problem on my 84 C10

[Joe69C10]

Ok, I'm stuck. I've done some testing and here is where I am. (Single tank)

Gauge stays at 1/4 tank all the time. key on or off.

Tested sending unit with ohm meter and it seems to be functioning properly. I filled the tank and got 92 ohms. drove it all day and this morning and I am down to 60 ohms.

I cut the wire about mid chassis and when I did this it goes to full. When I ground the sender wire running to the gauge to goes to empty. This would mean the gauge is working right??

I even bought a new sending unit, and wired it up as a test dummy & moved the float. all the way down it actually moved the gauge from 1/4 to empty. But when moving the float up, the gauge wont go past 1/4.

Any ideas or suggestions?? I'm stumped.

Its weird, like the gauge worked when I ground and remove the sender wire. It moves the full sweep, but when I connect the sender with ohms that should move it to full it wont go past 1/4!

[willem445]

Originally Posted by Joe69C10

Ok, I'm stuck. I've done some testing and here is where I am. (Single tank)

Gauge stays at 1/4 tank all the time. key on or off.

Tested sending unit with ohm meter and it seems to be functioning properly. I filled the tank and got 92 ohms. drove it all day and this morning and I am down to 60 ohms.

I cut the wire about mid chassis and when I did this it goes to full. When I ground the sender wire running to the gauge to goes to empty. This would mean the gauge is working right??

I even bought a new sending unit, and wired it up as a test dummy & moved the float. all the way down it actually moved the gauge from 1/4 to empty. But when moving the float up, the gauge wont go past 1/4.

Any ideas or suggestions?? I'm stumped.

Its weird, like the gauge worked when I ground and remove the sender wire. It moves the full sweep, but when I connect the sender with ohms that should move it to full it wont go past 1/4!

Have you looked at the gauge at all? Could be something mechanically causing it to stick at 1/4 mark.

[Joe69C10]

Originally Posted by willem445

Have you looked at the gauge at all? Could be something mechanically causing it to stick at 1/4 mark.

Have not looked at the gauge yet or removed the dash. BUT, when I cut the sender wire and grounded to the chassis it went back and fourth full sweeps. So its definitely not stuck.

[Camacorn]

Originally Posted by Joe69C10

Have not looked at the gauge yet or removed the dash. BUT, when I cut the sender wire and grounded to the chassis it went back and fourth full sweeps. So its definitely not stuck.

Did you check all of the grounds? Something as simple as a poor ground can cause a mess of problems.

[Joe69C10]

Originally Posted by Camacorn

Did you check all of the grounds? Something as simple as a poor ground can cause a mess of problems.

The only ground I checked was the wire that comes off the sending unit. It was good. Is there another behind the dash for the gauge?

[Camacorn]

Originally Posted by Joe69C10

The only ground I checked was the wire that comes off the sending unit. It was good. Is there another behind the dash for the gauge?

Looking at my truck for ya really quickly.

Also, I just realized your profile says you're in Ocala. I live in Inverness haha.

Recognize this truck?

[chengny]

I cut the wire about mid chassis and when I did this it goes to full. When I ground the sender wire running to the gauge to goes to empty. This would mean the gauge is working right??

Yes, those two tests would eliminate the gauge as the problem - it's working properly (both mechanically and electrically).

And this would confirm that sender resistance and float position are tracking along with tank level:

Tested sending unit with ohm meter and it seems to be functioning properly. I filled the tank and got 92 ohms. drove it all day and this morning and I am down to 60 ohms.

Eliminate the sender/float and sender ground to frame as the cause.

Since cutting the sensing leg (which jacks the resistance up to infinity) - halfway down the run - results in an immediate sweep to FULL, it would be safe to assume that there is a ground fault somewhere in the aftermost section of the lead.

By that I mean, there is something going on between the point of the cut (where you noted proper gauge operation by opening the lead) and the sender (where you confirm 90 ohms with a full tank).

Like you state above, as the resistance to ground in the sensing leg increases, the needle on the gauge should move towards the "full" end. Conversely, a decrease in resistance to ground will cause the needle to deflect over towards "empty".

The ohmic value produced by the sender ranges from 0 (when the tank is empty) up to 92 IIRC (when the float is at the very top). So a dead short to ground will result in the needle pegging low - same as an empty tank.

I'm sure you can see where this is going:

Unsplice the cut you made in the sensing leg, disconnect the other end from the sender and then use your meter to check the resistance between the sensing leg (the rear part - between the cut point and the sender) and the frame. It should be infinity but I'll bet it rings out at about 20 -25 ohms.

If that proves true, either locate the ground fault and repair it or run a new lead from the cut to the sender.

If that does not turn out to be the case, IDK...haunted truck?

[Joe69C10]

Originally Posted by Camacorn

Looking at my truck for ya really quickly.

Also, I just realized your profile says you're in Ocala. I live in Inverness haha.

Recognize this truck?

Your right up the road! I have never seen the truck you posted though.

Originally Posted by chengny

I cut the wire about mid chassis and when I did this it goes to full. When I ground the sender wire running to the gauge to goes to empty. This would mean the gauge is working right??

Yes, those two tests would eliminate the gauge as the problem - it's working properly (both mechanically and electrically).

And this would confirm that sender resistance and float position are tracking along with tank level:

Tested sending unit with ohm meter and it seems to be functioning properly. I filled the tank and got 92 ohms. drove it all day and this morning and I am down to 60 ohms.

Eliminate the sender/float and sender ground to frame as the cause.

Since cutting the sensing leg (which jacks the resistance up to infinity) - halfway down the run - results in an immediate sweep to FULL, it would be safe to assume that there is a ground fault somewhere in the aftermost section of the lead.

By that I mean, there is something going on between the point of the cut (where you noted proper gauge operation by opening the lead) and the sender (where you confirm 90 ohms with a full tank).

Like you state above, as the resistance to ground in the sensing leg increases, the needle on the gauge should move towards the "full" end. Conversely, a decrease in resistance to ground will cause the needle to deflect over towards "empty".

The ohmic value produced by the sender ranges from 0 (when the tank is empty) up to 92 IIRC (when the float is at the very top). So a dead short to ground will result in the needle pegging low - same as an empty tank.

I'm sure you can see where this is going:

Unsplice the cut you made in the sensing leg, disconnect the other end from the sender and then use your meter to check the resistance between the sensing leg (the rear part - between the cut point and the sender) and the frame. It should be infinity but I'll bet it rings out at about 20 -25 ohms.

If that proves true, either locate the ground fault and repair it or run a new lead from the cut to the sender.

If that does not turn out to be the case, IDK...haunted truck?

Thanks! I will run this test. Before I read your post I went ahead and cleaned, and wire brushed all ground connections.

Also too my entire dash apart to gain access to the bulk head connector that plugs into the back of the gauges.

Been trying to search for a pin out showing me what number and colors go to what so I can test off the connector to make sure everything is ok.

Can you tell me which one should be the fuel sending unit wire?

[Joe69C10]

Ok, I used a continuity tester and a long jump wire to determine #7 is the fuel sender wire. I re-tested the wire where i cut it off the tank. Getting about 20ohms there, and the same reading at the connector behind gauges.

I'm going to put about 5 gallons of gas in it to see if the reading goes up.

[chengny]

I re-tested the wire where i cut it off the tank. Getting about 20ohms there, and the same reading at the connector behind gauges.

Is that 20 ohms between the sensing leg and the frame? Did you disconnect the sensing leg from the sender terminal when checking resistance as described above (and below)?

To repeat:

Unsplice the cut you made in the sensing leg, disconnect the other end from the sender and then use your meter to check the resistance between the sensing leg (the rear part - between the cut point and the sender) and the frame. It should be infinity but I'll bet it rings out at about 20 -25 ohms.

[Joe69C10]

Originally Posted by chengny

I re-tested the wire where i cut it off the tank. Getting about 20ohms there, and the same reading at the connector behind gauges.

Is that 20 ohms between the sensing leg and the frame? Did you disconnect the sensing leg from the sender terminal when checking resistance as described above (and below)?

To repeat:

Unsplice the cut you made in the sensing leg, disconnect the other end from the sender and then use your meter to check the resistance between the sensing leg (the rear part - between the cut point and the sender) and the frame. It should be infinity but I'll bet it rings out at about 20 -25 ohms.

I wasn't able to get to the sender to remove the wire where it connects to the sending unit. This is why I cut the wire mid way down the chassis to perform tests.

After a few more hours today I finally figured out what the problem was. Kind of crazy b/c when grounding the wire to the gauge and removing the gauge goes back and fourth which would usually make you rule out as bad gauge. After many tests, I decided to take the dash apart and install another fuel gauge. Well....this fixed it!

I appreciate all your help!