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Thread: Fuel Gauge Sender

  1. #11
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    Should be neg earth!

  2. #12
    JDNSW's Avatar
    JDNSW is online now RoverLord Silver Subscriber
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    If it is 1965 it is the earlier type, which was positive earth.

    However, I should point out that both the gauge systems are polarity independent, and work equally well on either polarity*. But the change in gauge systems happened at the same time that the polarity was changed.

    Most earlier positive earth cars have been changed to negative earth because many modern accessories assume negative earth.

    *Although the original post 1967 gauge system thermal voltage regulators are polarity independent, there are now solid state regulators sold that only work with negative earth.
    John

    JDNSW
    1986 110 County 3.9 diesel
    1970 2a 109 2.25 petrol

  3. #13
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    Thanks - that makes sense.

  4. #14
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    G'day All,

    This is what I know about the difference between the Positive earth and Negative earth senders together with experience in repairing them.
    Smith Industry engineers were magnetism experts.
    The positive earth fuel gauges are a form of moving vane meter and have series and shunt resistors across the terminals. The resistance is 40.5 Ohm across the terminals.
    The Positive earth senders up to suffix C (1965) 519838 Series 2a SWB resistance is FULL: 82.0 ohms. EMPTY: 2.0 Ohms.

    The negative earth senders post 1965 resistance is FULL: 14 ohms EMPTY:268 ohms.
    Therefore a negative earth sender will not work with a positive earth gauge. NOTE: The positive earth gauge reads the correct way Independent of voltage polarity, meaning the wiring DOES NOT need to be changed when changing to negative earth system. (Smith/LUCAS Black Magic. I tested this again today to confirm my notes.) Also voltage variation of a few volts up and down does not change the reading.

    However, a negative earth sender can be modified with difficulty to sort of work with the positive earth fuel gauge. Long procedure by reversing the connections inside the sender however, the nichrome wire needs to be silver soldered. The next step is to connect a 150 ohm 5 watt resistor from the sender spade connection to one of the mounting screws for the sender to effectively make the actual resistance of the sender as 8 0hms Empty and 96 ohm Full.
    Therefore the fuel gauge will read with half full tank approximately = 3/4 full on gauge. The greatest rate of change is for the last 10 ltrs to empty because the sender wire is not wound linear. However, this is the most important indication area before you run out of fuel.

    The positive earth senders are hard to find and very expensive to buy NOS. I have repaired them with various faults and to rewind them you need Constantan wire and a lathe. Very time consuming as they are not made to be easily repaired. Also the design regarding the earth connection for the float/wiper is poor. Most of these gauges have worked for many 100,000 miles and badly worn so restoration is not perfect. This is why the gauge jumps up and down and not smooth as you move the float arm. Poor wiper earth.

    So my thinking is "Any fuel gauge is better than non at all!!!"

    Chris

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