Fuel Gauge calibration


  1. Hi – On leave this weekso apologies for all the comms .

    I have a Smiths 52 mm Fuel gauge fitted to my Mk1 1275 S and need to calibrate it to the sender in the tank. DSN suggest using a second sendor with the gauge and some resistors to simulate full and empty . Does anyone have any other ideas?

    Regards Bruce

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    I don't know what your 52mm gauge is but presumably it is one of those thermal devices of about 63 ohms and requires increasing current for increased reading. This requires a sender in series whose resistance is maximum for Empty and minimum for Full. Early Mini tank senders and gauges (Mk1 ish) work the other way round ie maximum for Full and minimum for Empty. The early sender is the type which is fixed to the tank by numerous screws.

    In principle you can 'invert' the sender with a bit of active electronics, but the problem here is that the scale on the gauge and also the tank sender itself are non-linear which will mean that the reading at half full will be miles out.

    If you have the later sender then it may be that judicious adding of resistors in paralleland/or series might help to calibrate the gauge but don't be surprised if you have difficulty getting it right both at Empty and Full together.

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    Hi Rodeo1968 – thanks for the reply – The sendor is the bayonent type ( not multiple screwattachment) . I Thought of using a variable resistor ( Volume control to simplify the balance process – similar to a decade box – but not sure if this would work . Just got the attached info from mini site in the USA – seems to say a similar thing to you . What do you reckon- varible resistor -??

    Regards Bruce


    From USA

    You'll have to provide more information.

    The gauge type changed in late 1964. Mk1s before that time used a magnetic type gauge and sender. From late 1964 on, the gauge system included the voltage stabilizer and the gauges were based on bimetallic strips. The components of the two gauge systems are not interchangeable.

    The Cooper-S sender was not different from that used on standard Minis. The early senders bolt onto the LH tank. They had a resistance range of 0 Ohms = empty to 90 Ohms = full. The later senders mount with a bayonet ring and they operate between 240 Ohms = empty to 30 Ohms = full.

    The 90-0 Ohm sender was not unique to Minis. Various other cars (particularly General Motors)used similar senders so you may find a gauge that will work with that (assuming your car is early). A LOT of late model instruments will work with the 240-30 Ohm sender but the response will not likely be linear (i.e. Half a tank displayed on the gauge won't likely be half a tank).

    The important thing to note is that the early and later senders not only have very different resistance ranges… the resistance changes in different directions. That's why components of the two gauge systems cannot be interchanged.

    Post more information on what you're trying to accomplish and what you have and we'll see what we can do to help.

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    I think all these 52mm Smiths thermal gauges (fuel and temperature) are the same except of course for the scales. I recently measured a Mk2 temperature gauge and this had a resistance of 61.8 ohms. At H it needed about 115ma, at N 75ma and halfway between C and N 58ma. I didn't measure C but it would have been about 40ma.

    If you assume that the joke of a stabiliser gives an average of 11volts or so then these figures broadly tie up with the sender resistance figures given on your USA site ie 30ohms for Full and 240ohms for Empty.

    There is a fair chance that your calibration won't be far out. You might be able to tweak the calibration a bit. Personally I would take measurements and then calculate what resistance (if any) is required, rather than guessing what kind of pot I would need and where it should go.

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