Fuel gauge calibration 1971 FJ40

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You may want to check the connections at the tank sending unit, especially the ground wire that screws to the top of the sending unit.

Cluster gauges and senders are a "set". You can't use a later sender with an earlier cluster, or a later cluster with an earlier sender.
03/69-09/72 83320-35020
09/72-01/79 83320-60050
01/79-04/86 83320-60051

Post# 10
This sender works like the oil pressure sender. It has a bimetal points type regulator in it. The float moves the contacts closer together and farther apart as fuel level changes, causing the amount of current to the fuel gauge to vary. If you want to test, just hook up to the sender wire in the truck before installing and raise/lower the float while watching the gauge. The mounting provides the ground so you will also need a jumper from the sender mounting flange to ground on your truck. Here you can see the cam moves the diaphragm on the sender.

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