Not working: suspect the sending unit. A good sender will measure 1000 ohms resistance cold, 60-80 ohms at operating temperature (with the wire lead disconnected, red lead of the meter on the sender’s terminal and the other to a ground such as cylinder head). If the resistance stays very high even when warm, your sender is bad.
Running Hot: surprisingly the standard operating temp is at the 3/4 mark on the gauge. High by modern gauge standards but normal for Spitfires/GT6’s. Don’t let the needle go into the red!
Not working: If fuel gauge are reading always empty or full suspect wiring trouble. Showing empty is electricity not making it to the gauge. Full is a short.
Partial Reading: If gauge constantly reads low or near empty suspect the sending unit. The float will often develop a leak allowing it to become filled with gas and no longer float.
Bad Temperature AND Fuel Readings
If both temp gauge and fuel gauge are reading incorrectly suspect the voltage stabilizer. It is mounted on the back of the Speedometer.
To test the stabilizer get an analog voltmeter and open the center section of the dash. Pull the green wire that attaches to the fuel gauge, and attach your positive lead there, ground the negative lead, and turn on the ignition. If you are measuring 10 volts constant, then the voltage stabilizer is fine. If not check that is is properly grounding (clean all contacts and make sure its mounting screw is tight).
Bouncing Needle: The cable is sticking internally. Remove the cable from its housing. Inspect for fraying or kinks. If you see anything like that, replace the cable. Otherwise, give the cable a liberal coating of graphite. This comes in either a tube or a spray can (be sure to wear some disposable gloves…it’s messy stuff.)
Re-insert the cable and make certain that the ends are seated fully into the distributor shaft housing and the gauge.
Not working: The cable can break internally. The bearings in the tacho can partially seize breaking the cable.
Tachometer (electric, MkIV and 1500)
Not working: Being a very simple system the usual cause for a non-working tach is dodgy wiring. Check grounds and to see if tach is getting power.
Not working: 99% of the time the cable between transmission and gauge is broken internally. Later US cars had an “emissions check” box in the middle of two cables rather than one long cable. This box often seizes up braking the bottom cable. The best fix is to bypass the box with a single cable.
The gauge can also cause the breakage. The bearings in the speedo and tacho can partially seize.