Fuel Gage - Team Camaro Tech
Electrical & Wiring Troubleshooting electrical.

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post #1 of 4 (permalink) Old Nov 29th, 00, 05:58 PM Thread Starter
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Location: Cut Bank Montana U.S.A.
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70.5 camaro gage only reads from 0 to 1/4
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post #2 of 4 (permalink) Old Dec 1st, 00, 02:26 AM
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If you use almost all the gas in the fuel tank and it moves normally below 1/4 tank, then there is a 80% probability that it's the sender unit inside the tank. If removed, you can make an ohms test. It should read from 0 to 90 ohms as you move the float simulating "Full" and "Empty" conditions. The gauge can be tested tracing the wire that comes from the sending unit and applying 12 vdc to it, but for a very short period of time, just to see if the gauge moves from "E" to "F". (important; don't let 12vdc connected to this wire for more than one or two seconds). If it moves to "F" then the gauge is OK. To do this it's helpful to have the wiring diagram of your model year car, I think I ordered my diagram from Rick's. Hope this help, Good Luck...
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post #3 of 4 (permalink) Old Dec 1st, 00, 04:58 AM
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Test your gauge by finding the plug in connector for the fuel sender. It should be in the trunk near the fuel filler tube by the tail panel. It will be a single wire (brown in a 69) with a connector, and will go through a rubber grommet in the trunk floor. When you disconnect the connector (open circuit) the guage should read above full, if you put a wire in the connector half going back to the gauge and ground the end of the wire the gauge should read empty or below. If the gauge doesn't respond this way then the gauge is faulty. Don't apply 12 v to either wire in the trunk. Power for operating the gauge comes from the gauge to the sender in the tank, it doesn't get fed from the sender to the gauge. All you will be doing is is backfeeding 12V into the gauge circuit and stopping current flow through the gage simulting an open circuit. There are two coils inside a Fuel gauge (or any other gauge capable of upscale and downscale movement)that are woound in opposite directions. One coil is hardwired to the 12V instrument power source and has a fixed amount of current flowing through it controlled by a resistor on the back of the gauge. This coil always tries to move the gauge to the full position (upscale) on the gauge. The second coil is attached to the sender and tries to move the needle towards empty (downscale). The sender is nothing more that a variable resistor, as level increases in the tank the resistance gets greater (to about 88 or 90 ohms)which causes less current to flow in the downscale coil which unbalances the forces in the gauge allowing the meter to move upscale. As fuel is used up the resistance in the sender gets lower (towards 0 ohms) increasing the current flow in the downscale coil which will eventually overcome the force in the upscale coil and the needle starts moving downscale towards empty. There are balancing circuits within the gauge that prevents the gauge from creeping all the way to full or empty once the two circuits are unbalanced in either direction.

Some cars use an opposite system in that the fuel sender has 0 ohms when full and a higher value when empty, but all early GM products work the way I just described.

Mark Canning
1969 Indy Pace Car
350/300HP RPO Z11
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post #4 of 4 (permalink) Old Dec 4th, 00, 12:27 AM
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"MarkC", You are totally right, Ooops the wire that goes to the sender needs to be grounded for corect testing. Thanks for the valuable technical information you described.
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