I got this 80 CJ5 with the 151 CU and the temp sender is bad. The replacements I have looked at are 3/8x18 units and mine is a 5/8x18. Any idea where I can get a replacement. Not really sure why this 151 has this thread size. I have a Ford unit in it now from NAPA but the guage needle just barely gets past the C mark. I am going to check the thermostat this weekend.
I had a 80 CJ-7 with the Iron duke, try getting a stock sending unit and some fitting adapters (like what you would get with a new gauge). When I replaced my gauges I had a similar issue, but the gauge supplied me with the correct fitting. Good luck
Did your Iron Duke temp gauge ever get above the C mark on the gauge. I have a new thermostat, the Ford sender and still only at the C mark. The gauge works when I ground the line. Maybe the Duke is just a cool runner.
It could be that the gauge reads that way, but a more likely guess is that a P.O. put in a 180 degree thermostat thinking it was good for the engine. Youy should get the engine up to its operating temperature and put a thermometer in the filler neck to see what the temperature is of the water coming out the upper hose. That engine should probably have a 205 or 210 degree thermostat. If the water isn't that hot, put the right part in. The engine will be much better off.
I put a 195 thermo in last weekend and no change. Also took the sender lead and grounded and the gauge did move to the hot mark as it should..very slowly. I think the Ford sender I got from NAPA just might not be the correct resistance based on the info I used from your Aug 1 2001 article by John Foutz. The Ford unit is 313 ohms when cold and this could just be enough to hold the gauge needle from reaching it's nominal spot when hot. I guess I will look for a 5/8 to 3/8 adapter and go from there.
FYI for anyone having the same problem locating a 5/8 inch dia temp sender. The part number is MOPAR J5751811. The local dealer did not have any and advised they do not make them anymore. I had to go to a dealer in New Mexico to order. Lucky break I guess. The main point is it works in my 4-banger...I hope for a long time since it was difficult to locate.