Fuel Gage

    All GM and several other manufacturers use this wiring for their fuel gauge.  I believe some Chrysler products tie the ground at the tank and gauge with a second wire.
    The Tank has a variable resistor and gauge moves the needle in relation to resistance as long as the voltage is constant.

    Testing:   Never apply any current to the wire from tank to gauge.  You will burn out the hard to find sending unit and might even explode the tank.    If your system is connected as above but the needle doesn't move when you fill your tank, Test as follows:   Disconnect the wire to tank.  The gauge should read full. Ground this wire,  and the gauge should read empty.  If it doesn't move, be sure you have voltage to the gauge.   Check to see that the needle isn't bent.  If not, the gauge is shot and must be repaired.
    If the gauge moves with the above test, pull the sender out of your tank.  Check to see if the gauge moves when you move to float up and down, remembering to ground the sender.  If not, the sender needs replacement.  (The Stewart-Warner senders have a difference resistance and won't read correctly when used.) You are lucky that all GM vehicles use the same resistance in the sending unit for 6 & 12 volt systems.   If the needle moves when tested, but always reads empty,  see if the cork float is saturated or the brass float is full of fuel.  If it works sporadically, Ground the tank at sender.  The tar paper on the straps are not a conductor.

12 Volt systems
    The gauge works fine on 6 volts so you want to change to 12.  The naive clerk at your auto store hands you a voltage drop resistor for a heater motor.  You install it and it works for a few minutes then the gauge goes up in a cloud of smoke. This is what happens when you run the gauge on 12 volts.        Next paragraph is best method to make the 6 volt gauge work on 12 volts.
        Ron, of San Jose,  says'  "Also,  I was fascinated by reading your web page on fuel gauge repair.  I was wondering if you had given any thought to using a 7806 voltage regulator to give you a 6V source for gauges on 12V systems?  I have used these (780x series) on other projects and they are really neat.  The 7806 can supply constant 6.0V at 1.1A and will cost you about $0.63 at an online supplier such as Digi-Key (http://www.digikey.com) .  [I've seen these regulators marketed by Vintage Auto Parts stores for $150.  Now that's a mark up!]    If you own a Positive ground GMC or Chrysler product, use 7906 voltage regulator.    (Thank you, Ron.)

I would like to thank the Royal Mail for printing this page in the September 2000 issue on page #19.

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