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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.