did you put in a 3/8 sending unit in place of a 5/16 one?

Carl Stevenson wrote:

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Normally that is caused by a bad ground.

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 x2 High resistance will cause a gauge to read full. Unplug from tank, put something that fits in the end of sending unit plug and ground. Gauge should read MT.

The reostat on the sending unit could be open which causes the gauge to read full. Check for a bad ground wire first or even make a ground with a jumper wire to a clean area on the frame. You may have to lower the tank and remove the sending unit and check it with a ohmeter. Set the meter on the lowest ohm reading with the posive on the positive on the sending unit and the ground to  ground the unit. Have someone move the float up and down slowly to measure the resistance. If no reading the reostat is open and you will need a new sending unit.

If you need a new sending unit use the filter sock from the old one if it is still in good condition. The new filter socks desintgrate quickly with the ethanol in gas and can potentially cause the fuel line in the sending unit to plug.

Al

rkycoast wrote:

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not to hijack your post, I'm trying to get my a-body fuel gauge to work also, got a new old gauge from a friend it had power to the gauge and out for a bit then quit it only has power to the gauge not out now.. test light shows dim on into gauge and nothing on gauge out to tank, what could be frying these gauges.. have tested the sending unit and is 30 ohms as it should be and the wire to the sending is with test light hooked to the tank ground.. What is wrong shouldn't the test light show power on both sides of the gauge? frustrated

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 Fuel gauges are very low amperage,may not be enough to light a test light(unless using LED test light)on output side. I would check to see if you are getting 12v in and 12v out on back side of gauge. If the input side (supply power) is dim on the test light I would say you have a supply power problem,poor connection somewhere. Can also test continuity of gauge by disconnecting all wires from the gauge and using an ohm meter set to beep(sound) between the two posts. An open (broken circut ) will not beep(sound) and closed or complete circut should beep(sound).