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Thread: Fuel gauge or sender.

  1. #1
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    Fuel gauge or sender.

    Well, I'm getting this thing wired up. Turned the switch on to test and the fuel gauge goes to full with no fuel in the tank. I pulled it, nothing looked out of order. Clipped a jumper from the flange to ground and same thing. Moved the float up and gauge moves further up. Checked my order and the information says 6 or 12 volt. What am I missing?

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    Super Moderator LarrBeard's Avatar
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    A. I don't remember what you are restoring. CJ-??. Has it been converted to 12-volt from 6 volt?

    Let me do some poking, but it sounds like a gauge/sender mismatch.

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    Yes, CJ5, but I bought a new speedometer cluster and fuel sender. It says good for 6 or 12 volts.

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    Super Moderator LarrBeard's Avatar
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    Instrument Voltage Reducer

    There is a lot of information out there about fuel sender resistance; some of it is suspect – as in where the writer says there are X-amount of ohms flowing through the meter. But, here is a quick check to see if you have a sender problem.

    Sender resistance:

    Float all the way up – tank full to overflow 0 to 6 ohms

    Float at normal full 7 to 9 ohms

    Three-quarters About 15 ohms

    Half About 22 ohms

    One-quarter About 30 ohms

    Low About 45 ohms

    Float full down (absolutely empty) Greater than 70 ohms

    Now the kicker is that when many car manufacturers changed over to 12-volts from the older 6-volt systems, they really did not change the instruments and senders. Instead they installed an instrument voltage reducer to run the oil, temperature and fuel gauges from a nominal 6-volt source (usually a little less than 7-volts).

    This made the gauges work in 6-volt or 12-volt vehicles; but in 12-volt vehicles only if there was a voltage reducer installed.

    I suspect that you have a vehicle converted to 12-volts - or built as 12-volts. CJ-5's were originally 6-volt vehicles, so they fit this description perfectly. The gauges and senders say “6 or 12 volts”, but what they don’t say is “*Requires instrument voltage reducer for 12-volt systems”. This sounds like your problem!

    You can see from the picture that the voltage reducer was often an "add-on".

    There are a number of products out there that claim to be “12 to 6 volt voltage reducers”. They range from just a simple series resistor – which is an unregulated band-aid approach, to a variety of bi-metal reducers that may or may not match the electrical load of your Jeep.

    Here is what appears to be a good electronic replacement for the bi-metal reducers and it will do a good job for your three gauges. None of the good ones are cheap – but I do not recommend any of the simple series resistor reducers.

    https://www.vintageautogarage.com/12.../vreg06703.htm

    A quick way to check this out is to get a six-volt battery (even one of the big lantern batteries will do) and run the gauges from the six-volt battery. If they act normal, we've found the issue.

    Since all of the gauges run from the same source, you will have oil and temperature gauges that read strangely once you get the critter running.
    Attached Images Attached Images

  5. #5
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    I will look into this. Thanks. I might call Kaiser Willys too since that is where I got all this stuff.

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    Super Moderator gmwillys's Avatar
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    A pretty reliable reducer is the ballast resisters that Chrysler points ignition systems. GMs used them as well, up to the HEI distributors. They are fairly cheap as well.

  7. #7
    Super Moderator LarrBeard's Avatar
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    Quote Originally Posted by gmwillys View Post
    A pretty reliable reducer is the ballast resisters that Chrysler points ignition systems. GMs used them as well, up to the HEI distributors. They are fairly cheap as well.
    The voltage drop across a ballast resistor will vary as the current drawn by the load changes. In the case of just the gas gauge, the current through the gauge and sender will vary from about an amp (full tank) to a tenth of an amp (empty tank). There will be similar, but probably not as drastic changes in the oil and temperature gauge loads. AS the load changes, the voltage to the gauges will change.

    The ballast resistor works well in its intended application because the load current is set only by the coil resistance and is constant (plus/minus very small variations due to temperature).

  8. #8
    Super Moderator gmwillys's Avatar
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    I've been wrong before, it won't be the last.

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    Then I checked the temp gauge, and it doesn't seem to work at all. I put the jumper on the sender and to ground and the gauge doesn't move, so I'm going to have to give a call to KW.

  10. #10
    Super Moderator LarrBeard's Avatar
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    CJ-2A Temp Gauge

    Quote Originally Posted by okiemark View Post
    Then I checked the temp gauge, and it doesn't seem to work at all. I put the jumper on the sender and to ground and the gauge doesn't move, so I'm going to have to give a call to KW.
    Here is more than you want to know about that gauge.

    https://www.thecj2apage.com/forums/w...topic3337.html

    It appears to have originally been a mechanical instead of electrical gauge. You get a gas or fluid in the capillary tube hot, it expands and moves the needle.

    If you have an electrical gauge with two terminals and an electrical sensor on the engine, someone has done some more "new and improving" on the Jeep - kind of like the oil line and brass plug.

    Do you have power to one terminal of the gauge?

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