I know this is an old post, but just yesterday, I figured out why the temp guages on these things get a little wild sometimes. Best part is that I fixed it for about fifty cents. If anyone is still having the issues with the temp guage being inaccurate and moving all over the place, I'll go into greater detail. Let me know.

 

OMG please TELL ME!!! lol This one has royally stumped me and I've just learned to deal with it. Its bugging the living crap out of me!

 

Yeah, it has driven me nuts for a long time too. Not to mention the money and time I've spent trying to fix it. First off, my RR is a 1996 4.0 p38. Don't know if this applies to any other years. The problem is the main ground strap that runs from the battery to the chasis to the engine block. The temp guage sender is grounded through the engine block, while the becm is grounded through the chasis. These grounds should have the same voltage to ground when using a good digital multimeter. With the engine running, put the red probe on the positive of the battery, and move the black probe from the engine block to the chasis to the battery. You will get a slightly higher voltage when using the engine for a ground. My solution was to run a piece of #8 stranded copper wire with a crimp connector on each end from the alternator mounting bolt to one of the chasis grounds located behind the battery. One a side note, last year I cleaned the connection points of the main ground wire in an attempt to fix the problem. From what I can tell, the problem may be with the ground wire itself or with the crimp connector installed on the end of it. The problem on mine was the short ground wire that runs from the engine block to the chasis. Of course, another solution would be to replace the main ground wire with a new one. I will probably do that eventually, but I had the piece of #8 wire in my scrap barrel and I wasn't exactly optomistic that it would work. But...It did. On a separate note, I had been having problems with my ABS pressure switch acting strangely. This new ground wire fixed that problem too.

 

Interesting about the pressure switch... I replaced my ground a while back and the problem went away for about a week and then came back. I'll have to add another ground and see what happens. Thanks for the advice though!