If the fuel gauge reads erratically, or substantial sudden change anywhere within the gauge scale such as falling from 1/2 tank to “Empty” suddenly, the problem is most likely in the sending unit located in the fuel tank. Remove the sending unit from the fuel tank and visually inspect the fine wire wound inside the sending unit. Spacing between the wire should be even, if the wire is “pushed” to one side then the sending unit is defective and should be replaced. The sending unit can also be checked by connecting a battery and voltmeter in series between the sending unit housing and the gauge wire terminal. Do not make this test near the fuel tank as any spark from this test could ignite petrol fumes from the tank! Observe whether the voltmeter reading changes smoothly while the sending unit float is being moved by hand slowly through its full range. If the voltmeter does not move or does not move smoothly then the sending unit is defective and should be replaced.
If the fuel gauge never moves from the ‘Full’ position, disconnect the wire from the sending unit in the fuel tank. If the indicator needle in the fuel gauge falls then the sending unit is defective and should be replaced. If the indicator needle does not fall then the wire from the sending unit to the gauge is earthed somewhere and needs to be checked.
If the fuel gauge does not register at all, remove the wire from the sending unit and earth it against a clean unpainted part of the metalwork. If the gauge moves to the ‘Full’ position then the sending unit is defective or not properly earthed. Next check with a test light to see that you have power where the black wire from the fuse panel connects to the vibrator. If there is no power, visually check the fuse at the panel and the wiring from the panel to the guage. Next visually check the wire from the sending unit to the guage for damage and if possible check for continuity; replace if necessary. If the guage still does not register then the gauge or the vibrator is defective. To check the fuel gauge remove it from the back of the speedometer and visually check the fine wire inside the gauge to see if it has “burned out”; if this is the problem it is usually visible. You can further check the gauge by using an ohmmeter connected to the two terminals on the back of the gauge (with vibrator removed). If there is any resistance then the gauge should be good, if there is no resistance then the gauge is bad and should be replaced. Warning: Do not put 12 volts directly to the fuel gauge as it will cause damage to the gauge! To test the vibrator it must be installed with the fuel gauge in its proper position on the back of the speedometer and the proper wires attached. Using a voltmeter to check the voltage at the terminal where the vibrator connects to the fuel gauge. The vibrator pulses the voltage to the guage and therefore the voltmeter should pulse between O and 12 volts. If the voltmeter reads steady at either O or 12 volts then the vibrator is defective and should be replaced.
