68 Fuel Guage pegged past full

[Richard D.]

Guys:

Fired up my 68 and the noticed the fuel guage pegged full right (past the full mark, in the "3 o'clock postition) for some reason, where it had been working fine before. Has a bit more than a half a tank of fuel in it.

Believe the tank, guage, and float to be original to the car.

Was wondering if someone can tell me what makes a guage do that, meaning that if it goes bad for some known reason it makes it read that way, like the float crapped out (I believe it creates some kind of electrical signal as it moves through it's range, but not sure exactly how it works).

All other guages and clock are working OK, and I have a new Lectric Limited harness in the car installed about 6 months ago.

Was wondering if anyone can tell me how to test the guage, or the signal that goes to the guage to help troubleshoot this.

Thanks for any advice.

Regards,

Rich

[Jack H.]

Re: 68 Fuel Guage pegged past full

This is typically the result of the wiring to the gas tank sender unit shorting to ground (can also happen at the dash guage where the wire to the sender connects).... The result is the gauge (basically a specially calibrated ammeter) passed SO MUCH current that the pointer needle crashed into its upper limit 'stop'.

You first need to find and repair the short, In the meantime consider disconnecting the gauge at the dash because continued overcurrent can result in damage to the gauge's inductor windings! Often, a gauge that's seen over current, can be 'coaxed' back into proper operation by gently and repetitively tapping on it to jiggle the pointer needle free from where it's hung up on the limit stop. But, other times, you don't get away this easy as the gauge suffered electrical and/or mechanical damage from being over-driven....

[Richard D.]

Re: 68 Fuel Guage pegged past full

Thanks Jack, now I know where to start.

Can I take a reading with a volt/ohmmeter at the pins at the guage to see what amount of current I'm sending to the guage?

Any idea of what that reading should be (range of current, that is).

Thanks again,

Rich

[Shannon Burgess]

Re: 68 Fuel Guage pegged past full

Jack, I hate to disagree with you, but when my gas gauge was pegged way past full it was due to an open circuit in the wiring to the sending unit rather than a short. The sending unit has around a 100 ohm resistance at "full" and a near zero resistance at "empty". When the wire to the sending unit is shorted to ground, the gauge pegs down below empty, and when the wire is pulled off to an open circuit the gauge pegs way past full. At least this is what I found when diagnosing the gauge problem in my '69. Once I found the break in the sender wire and repaired it, the gauge returned to normal operation.

Shannon

[Dick W.]

Re: 68 Fuel Guage pegged past full

Jack, I have to agree with Shannon. The instrument cicuitry was changed beginning in 1968. You ground the circuit on these cars and it will show empty

[Richard D.]

Re: 68 Fuel Guage pegged past full

Thanks all.

I just may be the ground that's gotten too corroded to pass current.

I do appreciate all the good advise.

I'll check it out this weekend (I hope!).

Best regards,

Rich

[Paul B.]

Re: 68 Fuel Guage pegged past full

Check the operation of your tailights-if they are acting dim, no brake lights when you step on the brakes, not lighting at all, it's your ground as the tailights & gas tank share the same ground with the black wire at the left rear corner of the frame-may also be to the back of one of the tailights to that same ground.

[Jack H.]

I could be remembering backwards....

But, the circuit is simple:

B+ battery voltage feeds one side of the dash gauge and the gauge picks up its center leg ground reference by contact to the instrument panel. Then, the tan wire routes from the dash gauge sender terminal, through the wiring harness back to the sender lug on the tank's sender unit. There's a variable resistance through the sender which is nominally 90-ohmn max to ground and ground is achieved via the black wire connected to the tank sender ring.

So, if you pull the tan wire at the tank, you should be opening the sender circuit and forcing the gauge to read either full or empty (based on circuit topology) and you should be measuring full battery voltage at the disconnected tan wire's connector, with respect to ground.

But, this is a 3-legged circuit with B+ to ground through the gauge at the dash setting one pointer needle reference and the variable resistance to ground through the tank's sender unit establishing the specific offset from reference for the pointer needle (e.g. Full-Empty).