Tom's Baja Bug

VW Beetle Repair
and Photos

 

Body/Paint

FAQ 80

Question: The fuel tank sending unit on my 71 SB has a "melted" mounting board in the reostat. The wire wound plastic board has overheated and melted to the point that the wiper arm does not contact the wire at the "full" position. A friend loaned me his spare for testing purposes. It was never installed in my car but had the same damage. We pulled a third unit from a parked 72 SB only to find it in the same condition.

A local VW shop suggested that I had shorted the circuit to a higher amperage line. Twelve volts is still twelve volts regardless where it comes from, so I'm seeking second opinions. One manual says to test the vibrator unit by using a voltmeter to observe the voltage switching off-then-on. Mine appears to operate much like a flasher, with equal time on and off. This means 12V is applied to the sending unit only 50% of the time. Should the vibrator fail and apply 12v to the unit 100% of the time this would double the time the voltage is applied , thereby doubling the heat generated in the reostat. Could this create a "melt down"?

Answer: I don't know how the SB fuel sender works, but yes, if you apply 12V constantly to a circuit that is only rated for 50% duty cycle, the circuit will melt.

For what it is worth, my impression is that fuel senders are low current devices. The circuit is set up with a high resistance somewhere (in my 1973 it seems to be in the gauge). Resistance lowers current flow, but leaves the voltage unchanged. In your case, it doesn't sound like it was hooked to the wrong place, but that the high resisitance part of the circuit failed and shorted (the vibrator). Perhaps. I'm just making an educated guess based on your research.

I wonder if this is a common Super Beetle problem?

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