Sender Unit

[Nick Malone]

Good evening,
The sender unit on the Nippy has never worked properly - it even came with a stick marked in US gallons (which is where it spent much of its life from 1945 until 1989 / 1990) so I guess it didn't work for many years over there either.
When totally full, the gauge reads just over a gallon but soon disappears to a zero reading. The float doesn't appear to be full of petrol and the wiring is obviously working OK I think.
Does anyone have a spare one or know where I can get one from please? I also need to know what model I need as I believe they come in many shapes, sizes and specifications.
It's nothing too desperate as I've run the car like this since 1991 but it would be nice to get the gauge working properly sooner or later.
Looking forward to receiving some tip offs!
Best wishes,
Nick

[Bill Dixon]

Nick,
Have you had a read of The Cornwall A7 Clubs technical articles on gauge faults may be of help. Best of luck.
Bill

[Mark McKibbin]

It may be that you have the wrong sender. Is the car still 6 volt?

[Robert Leigh]

I have spare Ruby type sender, which needs checking, but if you are interested I will check it and think up a price. Email me to take it further: lordshipvilla@hotmail.co.uk
Robert Leigh

[Nick Malone]

Thanks for the advice so far everyone. The car is still 6 volt positive earth. The sender is what was in the car when I bought it in 1991 and I guess must have been in since being in the USA given the wooden gauge in the back marked in US gallons. I know the car was taken there after the war by the American airman who bought it in 1943 whilst stationed over here.
If I short out the wire on top of the tank, the gauge goes to "full" immediately so I suspect the wiring, gauge etc are OK. I  have taken the sender unit out in years gone by to see what was wrong. There is no fuel in the float and it seems to slide up and down the resistor wire OK when moved by hand.
Beyond that, I've not got much of a clue about this and I've lived with it all these years without issue. However, it would be nice to get it working properly if I can find a suitable unit to replace the current sender which I think is the faulty part.
Thanks again,
Best wishes,
Nick

[John Cornforth]

One fault mechanism with these senders is the earthing of one end of the rheostat resistance wire to the body of the sender.  This is achieved (at least in the example I have seen) by trapping the wire between a hole in the mazak body and the outside of the insulating tube through which the live connection passes.  I suspect that the different metals set up a corrosion cell, and a poor high resistance connection eventually results.  Dismantling, cleaning and reassembly is the cure.  If you have an ohmmeter, the resistance of an original type sender should vary between about 2 ohms and 25 ohms as the float arm is moved over its range.

It's also worth checking that the fuel tank body is still well earthed to the car chassis/body, as it was originally on rubber mounts and u-bolts, which may or may not still offfer a good connection.  The other connection is via the copper petrol pipe and thence via the fuel pump to the crankcase. Anything more than an ohm or two will upset operation.

[Nick Malone]

Thanks for this information too John,
I don't have anything like an Ohm Meter but I think a friend does so that's worth a look. At least I know the readings I'm aiming for now! I'll also try to run another earth wire from the tank to the car to see if that improves things too.
Best wishes,
Nick

[Andy Bennett]

My faulty sender was cured by thoroughly cleaning the wiper and the wire 'coil' then adding in a clean earth. Now reads well if a little wobbly.
Andy