[samiralfey]

My gauge seems to be stuck 1/4th of a tank and never changes from that. I measured the resistance from the sender and it measured 12 Ohms but when I measured voltage coming from the harness behind the seat it measured zero.

Am I totally wrong here but should't there be 12 volts coming to the fuel sender unit and then depending on the resistance the voltage should/would drop?

If so, is the fuel sender unit behind a fuse? I tried to locate it in the fuse block but nothing related to it. Shared with another?

[webfoot]

Its just a long tan wire going back to the unit and a ground at the sender. Did you recently change the sending unit in the tank? Ohms range in new units don't match the new ones.
I'd check the ground is still good and the tan wire is still on the sender.

[samiralfey]

Originally Posted by webfoot

Its just a long tan wire going back to the unit and a ground at the sender. Did you recently change the sending unit in the tank? Ohms range in new units don't match the new ones.
I'd check the ground is still good and the tan wire is still on the sender.

No, didn't change the sending unit. Should drop the fuel tank to check it properly but just wondering about the no-voltage to the tan wire.

[webfoot]

Pretty sure it doesn't have voltage, its just a sensing wire.

[Bill Hanlon]

12 ohms is about right for a little over 1/3 of a tank of gas.

Short the wire to ground at the sending unit. The gauge should read empty.
Take the wire off of the sending unit. The gauge should read full.

If one or the other is not true, tell us what you see and we'll go to the next step.

[Greg Reid]

If you don't have voltage in the tan wire that is your problem. I'd start working from the trunk checking the tan wire everywhere I had access to it until I found where the voltage is being lost.
I don't know about your model or make but on my cars that wire comes through a rubber plug in the trunk floor near the latch, then under the carpet. Eventually it winds up at the gauge after passing through several connectors. You could be losing voltage at any one of those connectors.

[samiralfey]

Originally Posted by Bill Hanlon

12 ohms is about right for a little over 1/3 of a tank of gas.

Short the wire to ground at the sending unit. The gauge should read empty.
Take the wire off of the sending unit. The gauge should read full.

If one or the other is not true, tell us what you see and we'll go to the next step.

Did both and nothing on the gauge so the connection must be cut from somewhere. I checked the gauge with a 12V power source before putting it back in so that's probably not the problem.

[Bill Hanlon]

If the connection between the sending unit is cut the gauge would read full with the key on. The problem is in the gauge, power to the gauge or grounding of the gauge.

[pfilean]

Probably no power to the gauge. If the ground from the gauge were broken, I think there would still be power at some voltage to the sender.

[samiralfey]

Originally Posted by pfilean

Probably no power to the gauge. If the ground from the gauge were broken, I think there would still be power at some voltage to the sender.

Wiring diagram looks pretty simple, brown wire from ignition switch to gauge, from there tan wire to sender. Going to try and see if I can get access to the gauge without taking the dash too much apart. Probably can access it from underneath if I remove the AC-duct.

[Peter Serio]

Inside of that gauge there are 2 wire wound coils. So the circuit is: 12 volts + battery power thru a fuse (key on) body ground at the rear steel mounting panel behind the dash where the gauge mounts. Then 1 tan wire from the back of the gauge thru the body harness (including several plug-in connectors), on back to the trunk, underneath the car, to the sender. Finally there is a black wire on the sender that grounds to body ground underneath your trunk floor.


The tan wire does carry some voltage as it's exit path out of the rear plug on the back of your gauge but not 12 volts. You have 12 volts + in (key on + power) but the exit wire (tan) is resisted due to the coil winding inside of the dash unit. Tan wire (key on) about 3 volts or 3.5 volts maximum. The sender does not read strictly by voltage; it is a variable resistor that changes ohms due to fluid levels.


I would need to know the exact year and model of your car to quote specks on that. Is the fuel gauge 100% original to the car? Is it GM?

[samiralfey]

Originally Posted by Peter Serio

Inside of that gauge there are 2 wire wound coils. So the circuit is: 12 volts + battery power thru a fuse (key on) body ground at the rear steel mounting panel behind the dash where the gauge mounts. Then 1 tan wire from the back of the gauge thru the body harness (including several plug-in connectors), on back to the trunk, underneath the car, to the sender. Finally there is a black wire on the sender that grounds to body ground underneath your trunk floor.


The tan wire does carry some voltage as it's exit path out of the rear plug on the back of your gauge but not 12 volts. You have 12 volts + in (key on + power) but the exit wire (tan) is resisted due to the coil winding inside of the dash unit. Tan wire (key on) about 3 volts or 3.5 volts maximum. The sender does not read strictly by voltage; it is a variable resistor that changes ohms due to fluid levels.


I would need to know the exact year and model of your car to quote specks on that. Is the fuel gauge 100% original to the car? Is it GM?

Well, it's a '65 Bonneville 2dr and stock gauge, all original there. I actually have two and put this one because it was in better condition and as mentioned, I put voltage through both and on both the indicator moved back and forth.

I also measured voltage using a multimeter and 0-12v range from the last connector behind the rear seat but got nothing.

If I'm reading the wiring diagram correctly, the same brown on the ignition switch powers the fuse box so I assume the brown provides 12V to the gauge too so must the the tan wire going from the gauge.

[Peter Serio]

Could you post a photo, of the gauge? If it's still in the dash that's ok. There is a year break-point (on big cars) when Pontiac changed the gauge from zero to 31 ohms to zero to 90.

If I can see a photo of yours it will surely jog my memory.


Also keep in mind; trying to measure the tan wire for voltage, you may get nothing until you first un-plug that wire from the sender at the tank. It will not hurt the car to have it un-plugged for the time it takes to measure. Key on tan wire un-plugged you should see around 3 volts output from the rear of the gauge. Yes; Brown wire, rear of gauge plug = 12 volts battery + (key on).

[samiralfey]

Originally Posted by Peter Serio

Could you post a photo, of the gauge? If it's still in the dash that's ok. There is a year break-point (on big cars) when Pontiac changed the gauge from zero to 31 ohms to zero to 90.

If I can see a photo of yours it will surely jog my memory.


Also keep in mind; trying to measure the tan wire for voltage, you may get nothing until you first un-plug that wire from the sender at the tank. It will not hurt the car to have it un-plugged for the time it takes to measure. Key on tan wire un-plugged you should see around 3 volts output from the rear of the gauge. Yes; Brown wire, rear of gauge plug = 12 volts battery + (key on).

Here's the photo of the gauge. Measure d the one that was on the dash before changing it and it was about 33 ohms.

[samiralfey]

Ummm...guys...I checked behind the dash and it wasn't plugged in...

Now it works and shows almost empty as it should.

Thanks all for your advice. Now I'm going to go and sit in a dark corner for while...

[jamesq]

We've all done it....
Kudos to you for admitting it!
James Q