gas gauge reads full when running

[Drakeule]

My Roadmaster gauge slams to "full" upon energizing. I have checked and it is not a grounding problem. In fact, I dissconnected the gauge positive from the rear harness so that it didn't get input from the tank sender, and it still registered full (!)

 

I then checked what voltage it was getting from the gauge when energized, and it was approximately 3 volts.

 

My next step will be to get to the gauge rear connection themselves (no fun because I'll have to remove that 22 lb. radio to gain any appreciable access.)

 

My understanding is that gauges rarely, if ever, go bad. Hopefully, a connection; and let's hope not some "partial short" in the harness in the roof that is dragging down the voltage. 

 

Any input is appreciated.

 

Charlie Drake, Potomac, Md. 

[DonMicheletti]

Ground the gage lead at the tank connection. It should go to empty. If it still reads full, something is open between there and the gage.

If it does you have an open circuit between there and the tank. Most likely the tank isnt grounded or a bad fuel level sender

[Drakeule]

As I said before, I disconnected the hot lead from the tank sender and the fuel gauge STILL reads full; the sender and its ground were therefore out of the circuit. Yes, the tank is also fully grounded.

 

I realize that there must be a "partial" open circuit (more likely a drain) between the hot lead and the fuel gauge; it is reading approx. 3 V at that hot lead.It should read 6 V.

 

Has anybody ever found a fuel gauge that is truly bad?

[Bloo]

1 hour ago, Drakeule said:

I dissconnected the gauge positive from the rear harness so that it didn't get input from the tank sender, and it still registered full (!)

That is normal operation. Try grounding the tank wire. That should make it read empty.

 

15 minutes ago, Drakeule said:

I realize that there must be a "partial" open circuit (more likely a drain) between the hot lead and the fuel gauge; it is reading approx. 3 V at that hot lead.It should read 6 V.

Do you mean the wire inside the dash supplying the gauge? If so then yes that should be 6 volts. If you mean the sender wire, all bets are off. No telling what the voltage would be there.

 

Edited October 27, 2022 by Bloo (see edit history)

[Drakeule]

Bloo:

 

Gauge reads full whether its disconnected from the sender or not.

 

I will try grounding the tank sender wire at the sender itself; if the gauge then reads empty, would this confirm that the gauge is indeed OK, and that the problem is either in a connection at the back of the gauge OR in the wire leading from the gauge back to the sender?

 

Also, why the low voltage (3 V) at the end of the guage wire?

[Bloo]

There is no specification I know of for the voltage on the sender wire, but 3v doesn't really surprise me.

 

If you can ground the sender wire to a good ground and the gauge reads empty, and you disconnect the wire and it reads full, there is about a 99% chance nothing is wrong in the dash.

 

At that point, it is down to the sender or the tank ground. If the tank is well grounded (I like to run a ground wire right to one of the sender mounting screws), and the gauge still doesn't work, then that only leaves the sending unit.

 

It takes a good ground to bring a normal gauge to empty. Senders are 0 ohms (dead short) at empty and 30 ohms at full. That's why when you disconnect the wire the gauge pegs high. Infinity is a lot higher than 30.

 

Edited October 27, 2022 by Bloo (see edit history)

[DonMicheletti]

I guess I explained it poorly

[Gary W]

Here are some diagnostics I ran on my '37 without removing anything.  These tests can be done with everything still on the car.

 

 

[Larry Schramm]

Take the wire from the fuel sending gauge that goes to the sending unit and ground it at the fuel tank. If the gauge still reads full you probably have an open wire between the fuel tank sending unit and the gauge in the dash. 

 

If you still have the gauge going to full, go to the gauge and ground the wire at the gauge that goes to the fuel tank sending unit.  It should go to empty. If it goes to empty, fix the open wire/connection between the gauge and the fuel sending unit.

 

If it still goes to full, you have a bad gauge with an open inside of the gauge.

[Drakeule]

Larry/all:

 

I have just done this test. I grounded the gauge wire (at the bumper) and it still read "full." I will do it again at the sender, just to be sure. If I still get "full", I guess I'll have to drop that radio and perform what Larry just detailed about the gauge itself.

[DonMicheletti]

The bumper may not be a good enough ground. Try someplace on the body itself

[Drakeule]

I grounded at the bumper and at the ground from the sender body to the frame (which I installed). Still reads full. I think I am going to cut back on the LR wire loom and cut the gauge wire back and test there.

 

If there is a break, it might be there. I would feel pretty foolish if I dropped the radio, tested there, and found out that the wire in the trunk did indeed have a break!

[Larry Schramm]

20 hours ago, Drakeule said:

I grounded at the bumper and at the ground from the sender body to the frame (which I installed). Still reads full. I think I am going to cut back on the LR wire loom and cut the gauge wire back and test there.

 

If there is a break, it might be there. I would feel pretty foolish if I dropped the radio, tested there, and found out that the wire in the trunk did indeed have a break!

 

I would run a separate wire from the battery negative terminal back to the fuel tank sending wire. 

 

That would be a good ground.  Anything else is suspect. 

 

If the gauge still shows full, then take that wire and touch it to the back of the gauge where the tank unit wire is attached. If the gauge still shows full, then a bad gauge.

Edited October 28, 2022 by Larry Schramm (see edit history)