Thought this post might help anyone whose CJ temp and fuel gauges periodically misbehave or are wildly inaccurate.
The original setup uses a CVR (Constant Voltage Regulator) in the fuel gauge to drop the line 12V down to an average of 5V. It's not high-tech, just a bimetal set of points with a heating coil around them, chopping the 12V on and off to produce a very, very rough 'average' of 5-7V. This then supplies the fuel gauge itself and (via the little link on the back of the speedo) the temp gauge.
Occasionally, the points get dirty and the gauges under-read or don't work at all. Sometimes the CVR heating coil burns out or the points stick shut, switching the full 12V into the circuit. This invariably barbecues the temp gauge within 30 seconds. As the CVR is already fooked, you're then up for replacing both gauges, and original CJ temp/fuel gauges are getting quite hard to find. There are a gazillion cheap copies on eBay, but they just look shite.
I've got 2 CJs and did this little mod the first time 18 months ago when dust got into the CVR on my CJ7. Now my CJ8 has just fried its temp gauge, I'm doing this again so I thought I'd document it.
This mod bypasses the CVR with a compact 6.2V switch-mode power supply based around a single 5-pin integrated circuit, LM2575HVT-ADJ and a half dozen other components. The parts are worth just under $40 total. If you've got half a brain and can hold a soldering iron without burning yourself, it's about an afternoon's work. Afterwards, your gauges will be as accurate as they ever can be and will be protected from shorting out by automatic current limiting - they will last (effectively) forever.
There are other ways to regulate down to 6V electronically, like the simple old 3-terminal power supply regulators that have been around for ages, but all of them will dissipate a lot more heat than this one. The switch-mode unit is like a perfect CVR, switching on and off thousands of times a second. So it is either conducting fully (makes no heat) or off (makes no heat). There is a small amount of heat generated during the switching transition (there is a finite ramp function) but a tiny heatsink (just a bit of aluminium) will cover it.
I've attached a circuit diagram, a parts listing from RS Components and some photos of my first finished unit. I used a small piece of Veroboard to build it on, then covered the lot in heatshrink. It just sits up under the dash with the rest of the loom. You could make it by just glueing the parts to a piece of plastic and soldering the legs/pins to each other directly.
There are three connections to make. You unplug 12V to the gauges from the back of the speedo, and connect the 12V to the new power supply instead. You then connect the earth from the power supply to the body. Finally you wire the power supply output to the little connector bridging across from fuel gauge to temp. Turn it on and check regulated V - should be around 6. All done!
CJ Temp & Fuel Gauge Fix
