There are only 3 components and 2 wires in the fuel gauge system: The sending unit, vibrator, fuel gauge, the ignition on wire to the vibrator and the grounding wire between the gauge and sender. The first step is to determine which component is faulty and then replace it.
The workings of the fuel gauge are as follows: The sending unit, a rheostat, varies the resistance supplied to the fuel gauge based on the fuel level in the gas tank. This variable resistance is connected to the - side of the fuel gauge. The ignition switch supplies the vibrator with +12 volts. The output from the vibrator to the gauge simulates a steady +6 volts by pulsating the voltage between 0 and +12. The fuel gauge utilizes a resistance coil wound around a bimetal arm attached to the fuel gauge needle. As the float in the tank changes position the current in the fuel gauge circuit changes causing the coil to heat up or cool down thus rotating the bimetal arm which moves the needle.
One of the following 3 scenarios should help you to diagnose the cause of faulty fuel level readings.
Scenario 1: The fuel gauge always reads empty regardless of the actual fuel level
- Sending unit is faulty or not properly grounded
- Vibrator is not getting power or is faulty
- Fuel gauge is faulty
Testing:
Sending unit
Remove the wire from the sending unit and ground it to the body. If the gauge moves all the way to full then the sender is either damaged or not properly grounded. Reconnect the wire to the sender and add a ground wire from one of the sender mounting screws to a body ground. Use a ring terminal and remove the plastic washer from the screw where you mount the new wire to the sending unit. If the gauge still reads empty then the sender is bad and needs to be replaced. See the section below on replacing components.
Vibrator
If the gauge still remains below empty then it's time to check the vibrator and it's 12 volt supply wire. The feed to the vibrator gets 12 volts when the ignition switch is on. With the switch on check to see if there is ~12 volts at the black wire on the vibrator. If not then trace the wire back to the fuse box, inspect it for any problems and replace if necessary. If that does the trick then you are done. If there is power to the black wire but the gauge is not working then it's time to check the vibrator itself. The original style vibrator takes the 12 volt feed and converts it to a pulsating voltage between 0 and 12 volts. Connect a voltmeter to a body ground and the terminal between the vibrator and the fuel gauge. If you get a pulsating voltage then all is well but if it is either 0 or 12 volts steady then the vibrator is bad and needs to be replaced. See the section below on replacing components.
Fuel gauge
Having checked the sender, vibrator, power and ground wires you've eliminated everything else and it's time to replace the fuel gauge. See the section below on replacing components.
Scenario 2: The fuel gauge always reads full regardless of the actual fuel level
- The sending unit is bad
- Sending unit to fuel gauge wire is shorting out
Testing:
Start by removing the wire from the sending unit. If the guage now reads below empty then the sending unit is faulty and needs to be replaced. If the needle does not move then the wire between the sending unit and the gauge has shorted to the body somewhere and needs to be repaired/replaced.
Scenario 3: The fuel gauge gives erratic readings such as a sudden drop in the needle from some position.
- The sending unit is bad
Testing:
WARNING! Do not make any tests involving an electrical source anywhere near an open gas tank. Sparks from the test could ignite fuel vapors from the tank. It is best to test the sending unit on a workbench, away from the car.
Even though it is most likely that the sender is faulty it can be tested with a volt meter to be sure. remove the sending unit from the tank and allow to dry before proceeding. Connect the meter between the battery + post and the terminal on the sender. Ground the mounting flange of the sender to the battery - post to complete the circuit. While observing the meter slowly move the float through it's range of travel. The readings should change smoothly in correspondence with the travel of the float. If you observe and lull in change or a sudden jump the sending unit ids damaged and needs to be replaced. See below for changing components.
This is the second installment and covers how to replace the fuel gauge vibrator. This assumes that you have already determined that the vibrator is the culprit and you have checked that the voltage going to it from the ignition switch is +12 volts. A properly working vibrator takes the +12 volts and reduces it to a steady +6 volts. The picture sequence is not complete at this time but I will add the missing steps soon.
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| This is the replacement fuel gauge vibrator kit. It comes with the transistor type vibrator, a new screw and installation instructions. | The replacement vibrator is a small transistor with 2 wires attached and encased in black vinyl. |
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| After opening the left and right side panels remove the speedometer cable from the back of the speedo. | Loosen this 8mm nut along with the one on the right side of the speedometer. Rotate the speedo slightly clockwise to free it from the speedo panel. |
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| Pull the speedometer panel away and set aside. | Place a cloth under the speedo as you lay it on it's face. Remove the screw holding the old vibrator to the back of the speedo. |
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| Remove this wire and pull the old vibrator off the fuel gauge + terminal. | Install the new vibrator using the screw provided and connect the black lead to the body wiring. Now check the voltage from the vibrator. To do this connect the + lead from a DMM to the white lead on the vibrator and ground the - DMM lead to the body. Connect the battery and turn the ignition on. The reading should be around 6 volts steady. |
If the reading is at ~6 volts and steady then you are ready for the next step. Turn the ignition off and connect the white lead to the fuel gauge. Turn the ignition back on and the gauge should be working properly. If that is the case then you are good to go and it's time to turn the ignition off again, disconnect the battery and put everything back together. Once you have completed the reassembly connect the battery and you are done.
If you don't have ~6 volts at the white lead then you need to do some further checking to determine whether the vibrator is bad or if it isn't getting ~12 volts to the black lead. In the latter case check the fuse and wiring thoroughly again.
©1997-2003, Thomas M. Ainlay
