Renix coolant temp sensor
 

I've recently acquired an 87 MJ with a 4.0 that has an oddly low temperature reading of 150-160. Its been swapped to the newer cooling system, but consistently has a low reading on the gauge. Using an infrared thermometer, I've determined the thermostat is doing its job, as the highest I've seen at the outlet is about 212, and doesn't drop to less than 190. Its got the normal 195 t stat in it. I'm thinking I just have a bad sending unit, as the ohm readings are off. However, nobody locally carries one of the senders for anything more than the idiot light. Dealer worth checking? Or is there an online source for one?

The aftermarket replacement temp senders for the gauge are notoriously known for inaccuracy.

I was afraid of that. The dealers around here either stock nothing, or want insane prices for inexpensive parts. I'll take a chance on the aftermarket I guess. In the meanwhile, I have an HO thermostat housing, and am considering trying to plumb an aftermarket gauge sending unit into that and add a more reliable setup. Is there such a fitting that would allow me to adapt this housing for my purposes?

Yup. Easy as pie.

Your local parts store should have a 3/8NPT to 1/8 NPT "bushing" usually brass in material.