Fuel Sender Exposed!      by Dave Hall

For some time the Fastback's fuel gauge had been suddenly jumping to a higher or lower reading, and knowing I had a spare tank sender unit if anything went wrong, I removed and dismantled the faulty one by flattening the locking washer on the bottom, unscrewing the nut and carefully pulling off the aluminium tube.

What was revealed was far simpler than I had imagined; two thin resistance wires are supported by a central rod, and the float simply runs up and down as the fuel tank fills or empties, allowing a double ended contact to short the two wires together.  This results in their total resistance varying according to the float position.

The more usual design of tank sender unit has a sweeping contact attached to the float arm, that eventually wears away the finely-wrapped resistance wire; no wonder these tubular senders are generally fairly trouble-free.

A careful clean with some T-cut removed surface gum from the wires and contacts, allowing a smooth change of resistance as the float was moved up and down.  It seems to have done the trick, and is one more useful thing learned about the model.  There may well be a small adjuster somewhere to 'tune' the sender to the gauge, but I haven't bothered to look for it yet.

Incidentally, some models of Bus have a similar tank sender unit.

Fuel Tank Detail

From Geoff Tunks comes this useful picture on what's inside the fuel tank.  It helps explain why rust debris or thick tank-coatings can cause problems apart from blocking filters.

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(c)  VW Type 3 & 4 Club; October 1997