One thing to consider, regarding the ScanGauge, it'll move from one OBD-II ('96 up vehicle) to the next, to the next, and to the next.

 

Wire going to nothing. Ignore my ghetto looking wiring harness, I had to cut it open to find a short.

 

I'll take a picture when the gf comes back with her jeep. Of both the location of the sending unit for the gauge, and the wire where it goes into the firewall. From that you can see if there was in fact an engine swap. Like posted above, if that head has no hole in the head for that sending unit. Then its probably a 99. I dunno, I don't have a 99. I know how some engines are, they could have just left the hole, and put in a plug because it's no longer used. (changes from 96+)

I wouldn't do it at the T-stat, or wherever the single, ecu/temp gauge sending unit is. I'd find the end of the harness that plugs into the sending unit, where the factory connector for the temp gauge is supposed to be. Of course... IF there was indeed an engine swap, and if they did change the harness over as well. Then there's no risk of anything happening.

But till I can snap some pics, here's a picture I found online of the location. Says its from the Renix system, but if that same sending unit was used till 96, it shouldn't matter.

http://www.google.com/imgres?hl=en&s...t:429,r:20,s:0

Keep in mind the shape of the sending unit, and the connector it uses could have changed over the years. like in your picture above.

 

On that '99 motor, there's nothing (factory) to connect that plug to. U need to either install a OE sending unit somewhere (drill hole/tap threads) or do the ScanGauge (or scan tool). '96 down factory temp gauge sending unit is located in the head somewhere in the vicinity of the yellow circle but it's not there on '97 up heads/motors.

 

That looks like the wire/connector that goes to my Temp Sensor for the gauge on my 1996!

 

I tried to get a better understanding and d/l a pdf of the 99 cherokee. Looks like all wires, even for the gauge run to the ECM first, so from there how it signals the temp gauge... I saw two signal wires run from the ECM to the cluster. So I'm guessing it's all computer harbgarble. Not a real wire like an older vehicle. That's way beyond me

 

LOLLLLL Soooooo. I'm left to buy the scan tool? Is there anything else significant about the 2 engines to tell them apart? When I get home I'm going to poke around to see if there really is a place for it.

Would you not advise splicing into the other coolant temp?

 

Not sure that would be a good idea, it might cause degradation of the signal feeding the computer.

 

Similar problem. Temp gage never worked in my '96.
Replaced sending unit and the gage suddenly maxed out, before I even put the key in the ignition ?