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Air Cooled VW Fuel Gauges

Mystery of Fuel Gauges


By Bill C.


Picture this; you are sitting on the side of the interstate, 2 miles from the closest exit, which of course has no service stations. Your VW acts like it just ran out of fuel. But you don’t know for sure, since the fuel gauge has not worked right since you bought your Bug many years ago. Trying to find any technical information is just about impossible.

Hopefully, after you read this, you will realize the simplicity of this circuit and be able to troubleshoot and fix it yourself! It is comprised of just 3 components, some wire and spade connections.

This article will deal with the late air-cooled Bug circuit, since that is what I have and am familiar.


DISCLAIMER: The information provided here is from my own personal experiences with fuel gauges and is a documentation of those findings, through frustration to find existing technical info or troubleshooting data. This information may not apply to all models and should not be taken as the ultimate gospel.


Okay, here we go.

Equipment required:

  • Standard hand tools to remove the components

  • Volt/Ohm Meter to check the sending unit (Source: Radio Shack)

  • 4 (colored) test leads with alligator clips on both ends (Source: Radio Shack)

  • 12-Volt Battery source. The family sedan will do, since it is easier to access.


The fuel gauge circuit has 3 components

  • Gauge

  • Gauge Vibrator

  • Sending Unit

SAFETY FIRST!

IMPORTANT: Always disconnect the negative ground cable from the battery before servicing the electrical system.

CAUTION: You are working with GASOLINE (a.k.a. Petrol). Always, work in a well ventilated area away from any source of open flame or spark.

 

Sending Unit: The sending unit with floats is mounted to the top of the fuel tank. It has 2 wires; a brown ground wire that attaches to the grounded tab connector and a yellow (or some other color) “signal” wire that attaches to the insulated tab connector.

IMPORTANT: Loose connectors at the sender or vibrator will cause inaccuracies in gauge readings.


After disconnecting the battery, the sending unit is removed by first, draining the tank (SAFETY). You may have to loosen the tank enough to move it where the sending unit has vertical clearance to be removed (air box in the way?). After disconnecting the 2 wires, simply turn the unit counterclockwise a few degrees and maneuver it out the top. It will have a rubber o-ring that may stay “stuck” to the tank when you remove the unit.

Be sure to plug/cover the opening in the tank to prevent escape of flammable vapors while you work elsewhere.


Sending Unit Test


1.   Disconnect the negative/ground battery cable.

2.   After removing the sending unit, set the unit on clean surface and connect the Ohm Meter leads to the tab connectors at the top of the sending unit. Be sure the meter batteries are good. You will need an ACCURATE reading on x1 scale.

3.   Set the meter to read resistance (ohms) on the lowest scale available.

4.   Move the float(s) to their lowest (empty) position. The resistance on the meter should be approximately 75 ohms.
-If the reading is different, it will be reflected in gauge accuracy.
–If you get no reading (very high resistance), the sending unit is bad and should be replaced (see next step first).

5.   Move the float(s) to their highest (full tank) position. The resistance on the meter should be approximately 10 ohms.
– If the reading is more than a few ohms higher, the gauge will never be able to register FULL.


High Resistance Readings


If you encounter very high readings during the above test, there are 2 contact areas that can be the culprit: 1) The wiper arm of the “rheostat” 2) The float arm (to which the wiper is connected) makes continuity to GROUND on the unit through the “pivot hole” for the float arm. Any corrosion in these areas can render your sending unit useless. You can attempt to clean the corrosion and retesting.

If your sender is bad (readings are out of tolerance by more than 5-10%), you have the options of either replacing it (as low as $20-30 for a Mexican built) or work on it to get the resistance readings in line. You might want to test the gauge and vibrator (below) before purchasing a sending unit; especially since “electronic/electrical” components are non-returnable. If nothing else, it might save you a trip to Roy Rogers.


Gauge and Vibrator Test


These components are more difficult to test with just a meter. So this test will involve a “live” bench test, applying voltage to the whole circuit and monitoring the gauge readings.

 

1.   With the battery ground still disconnected and taking care not to disconnect any of the speedometer wires, carefully remove the speedometer from the dash, after disconnecting the cable. You should be able to let it hang face-down.
– This is a good time to make a drawing of the wire connections on the back of the speedometer.
NOTE: On “curved window” bugs, you have the option of gaining access to  the speedometer cable by removing the dash switch assembly right above the steering column (5 screws and standoffs) and fitting a small hand through the area normally occupied by the light switch.
However, you MIGHT have enough slack in the speedo-cable to pull the unit out without removing the cable.

2.   Remove the wires (one each) from the fuel gauge and vibrator.

3.   Remove the 2 screws securing the gauge and the screw (ground) that secures the vibrator to the speedometer back. Lift the assembly off the speedometer.
– The wire (
yellow?) that was connected to the gauge comes from the insulated tab on the sending unit.
– The wire (
black?) that was connected to the side of the vibrator is switched 12 volts that comes with ignition ON.

4.   Place gauge, vibrator, and sending unit on a large piece of cardboard, so you can carry it out to the family sedan for a power source.

If available, the preferred colors for the test leads for this test are Yellow, Green, Black & Red. Otherwise, just interpret my colors to those that you have. You can use any color if you can keep them straight.

5.   Using the 4 colored test leads, connect the circuit for testing:

  • YELLOW Lead #1 from: Insulated connector on sending unit
    to: Unused connector on gauge (where wire was connected)

  • GREEN Lead #2 from: Grounded connector on sending unit
    to: Grounding screw tab (fork) on the back of the vibrator

  • BLACK Lead #3 from: Grounding screw tab (fork) on the back of the vibrator
    to: Negative post of a 12 volt battery source (later)

  • RED Lead #4 from: tab connector on vibrator (where wire was connected)
    to: +12 volt post of the 12 volt battery source (later)

6.   Carry your newly created “circuit” and cardboard to your battery source (family sedan?) and set the gauge upright so you can view the needle.

7.  Position the float(s) in the EMPTY (down) position.

8.  Before connecting power, observe the position of the needle. It should read EMPTY at this point.

9.  Connect the (BLACK) lead #3 from the vibrator ground tab (fork) to the NEGATIVE ground side of the battery.

10. Connect the (RED) lead #4 (from vibrator) to the POSITIVE +12 volt side of the battery.
 

11. Observe the reading on the gauge. It should read “E.”

12. Move the float(s) to the FULL (up) position.

13. Observe the reading on the gauge. It should slowly move to the “F.”


Interpretation of Test Results


This section assumes you connected everything correctly in the gauge test. Please read the questions and “ifs” carefully.


A.) Needle moves, but is inaccurate:

Question 1: Did the sending unit pass the test?

- If no, rerun the gauge test with a GOOD sending unit.

- If yes, go to next question (Q2).

Question 2: With a YES on Question 1: After connection to the battery for a minute or so, is the vibrator warm?

- If no (Cold), the vibrator is most definitely bad. It depends on an internal light bulb type heater to warm a solid state device to vary the output to the gauge.

- If yes (Warm), depending on the amount of error, you have one option before purchasing a new vibrator. The gauge has some “fine tuning” to it. Run the “Gauge Fine Tuning Procedure” below.

- If that does not fix it, it is PROBABLY a tired out vibrator, since it is a thermal device and more likely to get out of tolerance than the gauge. Even though, the gauge is still a remote possibility, replace the vibrator, rerun gauge test and go to Q3.

Question 3: With a known GOOD gauge vibrator attached, are the gauge readings now accurate?

- If no, run the Gauge Fine Tuning Procedure below, and go to Q4.

- If yes, you are done. Put it back together!

Question 4: Are the readings now acceptable?

- If no, looks like you are going to have to cough up some bucks for a new gauge. You may have to fine-tune the new one.

- If yes, button it up and enjoy the confidence of an accurate gauge. You should be proud!


B.) Needle stuck in one position, never moves with power on or off:

Question 1: With power applied, does vibrator feel warm after a minute?

- If no, replace vibrator and rerun test.

- If yes, run the “Gauge Blip Test.”

Question 2: Does gauge pass “Blip” Test?

- If no, replace gauge and rerun gauge & vibrator test

- If yes, replace vibrator and rerun gauge & vibrator test



Gauge Fine Tuning Procedure


There is a mechanical adjustment on the gauge that will vary the “swing” of the needle under normal operation. I recommend that this adjustment be made with the gauge disconnected.

1.   Viewing the back of the gauge, you will notice a small hole with equally small “gear looking” teeth.
Note the position of this adjustment before moving it.

2.   Move these teeth either left or right with a small jeweler’s screwdriver.

3.   Rerun the Gauge Test and readjust as necessary.
 



Gauge “Blip” Test

Though this test does not test for accuracy, it will at least indicate if the gauge is totally worthless, frozen, burnt up, etc. You should only run it if the questions above direct you.

CAUTION: Do not expose the gauge to this test for more than about a second or two at a time. Extended application of power can lead to gauge damage.

You will need a battery, capable of producing adequate voltage and current, and 2 test leads for this test. The higher the voltage, the shorter the “safe” time before you might overheat your gauge.

 You can use your car battery or even a Black & Decker 3.6VDC VersaPak battery.

1.   Connect the negative (-) battery terminal to the connector on the “E” side of the gauge.

2.   For only an instant, touch the positive (+) battery terminal to the connector on the “F: side of the gauge and observe the needle.

·     If the needle moves to the FULL position, the gauge is functioning enough to at least display a variation corresponding to sending unit output. However, it does not mean that the gauge is capable of accurate readings.

If the needle does not move towards FULL, the gauge is beyond use. It will have to be replaced.

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