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My gas gauge just recently took to popping the circuit breaker. After a bit of trouble shooting and replacing the old circuit breaker, which is probably still good, I found that by disconnecting the gas gauge wire, everything else worked correctly.

Two questions:
1. Can I test the gas gauge by hooking it up directly to the battery, or with a multimeter?

2. Is it better to buy a new gauge or have it rebuilt?

If any one has a spare gauge they would be willing to part with, please pm me.
thanks

David Grubbs

Have you pulled gauge out of the cluster?


The gauge is mostly a bimetal strip with a coil of heating wire wrapped around it. Wire ends connect via a short length to each terminal. The wire is so thin that it seems if the wire itself had shorted it would burn open somewhere between the short and the bat post. The photo is a 56 but all 51-6 bimetal types are of similar internal layout.

The two terminal posts are kept from shorting to the metal back and cluster by various methods. Of the gauges I can examine, 47 has ceramic or fiber step washers on one side. 48-50 has a formed heavy fiber strip attached to the metal case with the terminals suspended in mid air within wide round openings. 55-6 has a heavy fiber strip on the inside that is pulled by the mounting nuts into a slot in the metal to lock gauge in place. All have a hold nut over another fiber sheet to provide insulation on the outside & then a second nut for the wire. I don't have a 51-4 to look at but suspect it will be one of those methods.

Point is, am wondering if the insulator on the bat post side might have broken or mounting nut loosened and the terminal is able to slide and touch the metal. Logically it would be the one on the Bat post since the breaker is tripping. If the other side shorted it should have only made the gauge peg on full until it eventually burned out.

As something else to be aware of, the temp and gas gauges are fed thru a voltage regulator with contacts rapidly opening and closing. The rate of cycling will provide a pulsed output voltage averaging about 5 volts. If the gauge post did short, I would also suspect the regulator contacts took a real beating and might cause later problems.

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Interesting. The post on the right hand side of the gauge, the one with the power wire, has two nuts on it. One holds the unit tight and the second nut holds the wire tight. The inter nut was loose. I'll look at the gauge more closely and see if there is a short there.

Is there a way to bench test the gauge?

The way Packard advised to bench test was to connect a known good item in the circuit and use it to confirm the suspected bad item. In this case it would be by substituting a good sender to check the gauge in the car. By moving the sender unit the gauge should respond. Since you have a slightly different issue that test is kind of moot.

In this case, first I would do continuity. Disconnect both wires from the gauge and check the terminal position as mentioned above. If you have removed gauge, it will be easier but if not, use an ohmmeter to verify there is no short from either terminal to the metal or to ground. If that is good then go across the terminals. I don't have a 51-4 gauge but using the 56, the ohm reading across the terminals is approx 15 ohms. It seems to vary a bit. Not sure that is a perfect number but since the 56 uses the same sender and also is regulated to 5v, I think the ohms will be about the same on your gauge. Won't swear to it though.

If the gauge is out of the car and you want to check farther without putting it back in, I would use clip leads or jumper wires to connect the bat terminal to it's supply wire off the regulator. With another jumper touched to ground, see if the gauge moves. Leave it grounded only long enough to verify needle will move toward the full mark.

Here is the gauge. I can't find any physical damage using a magnifying glass. One of the things I found interesting is that you can adjust the upper and lower limits by moving the toothed cogs.

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What is the white strip across the terminal holes in the last photo? Don't see anything like that on any of the gauges I have.

Not a strip, but a rectangular hole. You're seeing the white paper behind the backing plate.

Makes sense now. The gauge is held in the mount the same way as the 56 -- except the 56 just has a strip of insulator paper on the back side instead of the full circle..