Testing and Repair of the Fuel Gauge Sending Unit

Troubleshooting the Fuel Gauge

When troubleshooting the fuel gauge check the temperature gauge too. Does the temp gauge read low as well? If so, the instrument voltage stabilizer may be shot. If the temp gauge is OK, then your sending unit or float is suspect. If the float is original it may be full of gas. The Big Three vendors sell the whole sending unit for about $50 US, or you can buy just the float for $7 from:

R. D. Enterprises, Ltd.
290 Raub Road, Quakertown, PA 18951
Phone 215-538-9323 Fax 215-538-0158
web site http://www.rdent.com
Ray Psulkowski, proprietor

Replacing a Bad Float

First let your gas supply run below 1/3 tank. Remove the fiberboard bulkhead in the trunk (the one with the courtesy lamp in it). Don't let the lamp wires short as you unplug them. You'll see the gas tank there, and the sender is the round thing with the black and green wires. Mark the position of the wires, and also mark a reference point so that you can get the sender in the same relative position when it is replaced.

The sender is held in by a locking collar with three "tangs". Using a screwdriver against one of the tangs, unscrew the collar (counterclockwise) and remove it. Then you can remove the sender itself. NOTE the orientation of the various guide tabs on the sender and tank, and how they fit together. This will make it easier to reinstall.

Try not to bend the float arm as you remove and reinstall the sender. There is a calibration screw on the backside of the sender, but I don't recommend touching it -- it is VERY sensitive. You can test the sender with it out of the tank. Hook up the black & green wires, and turn on the ignition (engine off), move the float arm up fully, and watch the gauge on the dash. Since the gauge is a thermal unit, it takes a minute or so to read full when you hold it in that position. But a good sender WILL read full.

The sender float just clips on the arm. Try to look inside the tank to estimate the amount of gas left, and when you reinstall the sender, check to make sure the gauge reads the same. Remember that the fuel system won't pick up the last bit of gas, so if the gauge is a bit pessimistic, that's OK. Reinstallation is the reverse of this procedure. Don't force anything, it takes some finesse to get the float arm/sender back in. Make sure the gasket is in place between the sender and tank. Tap the locking collar home, hook up the wires, and you are done.

Measuring Sender Resistance.

It is best to remove the sender from the tank. This will allow you to get full scale readings for both empty and full conditions. When empty, the sender should be about 200 ohms. When full, it should be about 20 ohms. The sender unit is accessible from the trunk of the Spitfire. Clean out the trunk and remove the hardboard cover at the "front" of the trunk. This will expose the gas tank. Before you try to pull the sender, make sure the tank is nearly empty, as the sender hole is about halfway up on the tank. Have a new gasket for the sender on hand to ensure that there are no leaks after reinstalling it.


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