dave1123

You said your temp gauge was acting funny. What I'm saying is it's not hooked up the way that's compatible with your year jeep. I'm saying trash the forward 2 pin sensor. Install a 1 pin sensor in the forward hole, and extend your original wire to reach the forward position. We'll see if that works, then deal with the 2 pin connector afterwards.

dave1123

Okay, here's what I think the previous owner has done. The resistance values of both sensors are pretty close to the same so I think he just grounded one pin and hooked the other pin up to your gauge wire. This would probably be alright until the resistance went beyond the range of your gauge to compensate, then it would read full scale or beyond the top mark. I looked at my FSM for the 2000 XJ and the temp sensor does modify a 5v supply voltage to create the PCM signal so that much is correct. Your 90's gauge uses the sensor as the 3rd leg of a shunt circuit to control needle deflection with virtually no voltage at the sensor itself. This is the same way a fuel tank level sensor works.

cspaltro

Jamrock37, it's been a few years, I assume that you figured out your temp gauge issue. I just went through this same exercise, kind of. I bought a 1991 XJ for my daughter, seemed like it was in pretty good condition when I got it but then as time went on and I was doing some safety work on it before I let her drive it, I discovered that the previous knucklehead owner put an 0331 head on it and then was too lazy to buy a matching valve cover and used the 1991 valve cover which doesn't mate. It was leaking all around. Anyway, long story short I failed to properly secure the clutch fan and the studs came loose while driving and destroyed a rather nice aftermarket all aluminum radiator. So, while updating the entire cooling system I noticed that the previous owner had used a brass T fitting off the T-stat housing and had two temp sensors installed. It looked really bad. I had no clue what the single wire sensor was for, I did know that the other sensor was to trigger the e-fan to turn on/off and also fuel system management via the computer. One wire was ground and the other wire actually carried 12V unlike the newer 2000 ECU that uses 5V signals. I knew that I had to leave that in but I could not use it for my fan because the relay for the aux fan was only rated for 20A, I was installing 3 fans and removing the OEM fans. I was pulling too many amps 40-50A which required a seperate relay wired directly to the 12V battery buss. So, I also noticed that in 1992 they changed from the ruber 90 degree connector to a white plastic clip connector, still just a single pin. I had to go to the junk yard and pull a sensor off an 89 xj. Turns out that I didn't need it, the previous owner just relocated and used a reducing bushing and extended the wire. I'm in Boise ID so it's too cold to test right now, so this summer we'll see how it works. I'm concerned about false readings due to the lack of coolant flow up in the brass T fitting where the sensor head is located, I imagine it's kind of stagnant up in there.
Don't trash any heat sensors that are in there now, they are all important. You just need to figure out where to put them.
About your head, I have an '99-'01 casting so I plan to replace it at some point. It's not cracked yet, but I imagine as hot as these engines get it's only a matter of time. I've talked to the guys at Cascade, their prices are very affordable, and you can go back to an older casting. I believe the 91-95 casting has larger exhaust ports allowing your engine to breath a bit better, the 0331 has smaller exhaust ports and you'll be able to ditch the adapter they installed to keep the headers. When it comes time to replace your head, don't forget about Cascade... even if you decide to stick with a new 0331 so you can get rid of the distributor.