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Posted by D Sherman [72.47.9.228] on Monday, September 20, 2010 at 19:08:19 :
In Reply to: Gas gauge test posted by Galen [128.229.4.2] on Monday, September 20, 2010 at 15:45:29 :
I'm going from memory here, and you might have to ponder what you have a bit, but with a gas gauge you have 3 terminals. One is where the juice goes in. It should have battery voltage on it whenever the ignition is on. One of the other terminals should go to a permanent ground, usually via a nut on one of the studs on the back that cinches up against the mounting strap. That's a place where there can be problems, so make sure the ground terminal really is grounded. The third terminal goes to the gas tank. The reason for this setup is that it's a "balanced" arrangement. The permanently grounded coil pulls the needle one way with a pull that depends only on battery voltage. The other coil pulls the needle the other way with a pull that depends on the resistance of the sender. This way, the accuracy of the gauge isn't dependent (very much) on battery voltage.
You can do all the testing without needing to know exactly what resistance is where. The first thing you need to do is figure out which terminal is which. If you have an ohm-meter, you should see some relatively low resistance (less than 1000 ohms) between any pair of the three terminals. When you find the pair that shows the highest reading, the one that was left out of that pair is the common, which goes to the battery. You should be able to tell which is the ground based on how it's mounted. If you find one terminal that has no connection to either of the other two, then the coil inside is probably burned out, which usually results when someone converts a 6V truck to 12V and neglects to drop the voltage to the gas gauge.
With everything electrical, the key to troubleshooting is to simplify it as much as possible, leaving out all the potential bad connections, old wires, flaky switches, etc. Get a battery and some wires and hook the common straight to the + side of the battery and the ground to the minus side. The needle should bang hard against the stop on one side or the other (I can't remember which). Now use a jumper wire or screwdriver to ground the remaining terminal, the one that goes to the sender. When you do that, the needle should bang hard against the other stop. If the gauge does that, it's almost certain to be working. They can't really get "out of calibration". Usually they're either burned out, or there's dirt in the works that makes the pointer stick, or there's something else wrong in the wiring of the truck. If you have a spare known good sender handy, you can then hook it up between the sender terminal of the gauge and ground and try working the float back and forth to see if the gauge moves as it should. There's no polarity with these non-electronic gauges so the procedure is the same whether you have a positive or negative ground truck. Just don't hook up a 6 volt gauge to 12 volts or you'll fry it, and don't expect a 12 volt gauge to work on 6 volts. It might work a little bit, but it won't be anywhere near accurate.
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