I learned the hard way about old Jeep gauges after burning out my temp and fuel gauges last year. Below are 2 images you can look at that might help with your gauges. The information here is specific to my ’65, but will probably apply up to the mid 70s CJs.
The most important thing with these gauges is to be sure you have a good ground to the metal case of the fuel gauge since it acts as a constant voltage regulator (CVR) for itself and the temp gauge. If it’s not properly grounded you will send 12V to 14V into your 5V gauges. This is a bad thing.
This condition will cause both gauges to overheat and eventually will burn one or both gauges out very quickly (they can take this for a short time). See red area marked “case ground area” in the information diagram. Note that your strap to ground voltage will fluctuate when it’s right (essentially providing a crudely regulated 5V to the gauges). It will read 12V when it’s wrong!
Instrument Cluster Lamp Replacement
Below is a simple diagram on how to change your instrument cluster lamps: