One common problem with monteros is a temp sensor issue. It pretty much just shows whatever temperature it feels like. My monty has an aftermarket temp gauge that the PO put in. However, it's in a tough to see location and I'd rather have the factory one working. I followed this video for instructions.
[video=youtube;HTqxmSLe92U]https://www.youtube.com/watch?v=HTqxmSLe92U[/video]
It turns out the the resistor R10 wasn't soldered well in the dash circuit board, so it needs to be re-flowed. I've done a lot of soldering so I figured I'd give it a shot. Disclaimer: I'm not responsible for whatever you do to screw up your truck. This is just a guide. Don't follow it or you might screw something up. Now is a good time to replace any burnt out bulbs you may have in there.
Here it is with the key on after sitting for several hours.
Step 1: Unscrew these two screws
The bigger bezel is now ready to come out. There are two spring-loaded clips at the bottom. Lower your steering wheel to the lowers position and give a firm tug (not a yank) at the mid section of this. It should come out.
To access the instrument cluster, remove these 4 screws being careful not to drop them behind the dash.
Dash removed.
As explained in the video above, pull the plastic faceplate off (it's required for reinstall).
Front cover off.
This is what the back of your instrument cluster will look like. There are 5 screws to remove to access the PCB for the temp sensor. Remove these.
5 screws removed.
On the back of that you can see resistor R10 (toward the top left)
right here. It's a tiny thing. I wish I had measured its resistance before soldering so I could know what to replace it with.
Then I hit it with the soldering iron and added a tiny bit of solder.
I put it all back together and went for a test drive. Now the dial is all the way to the bottom, so it was definitely the R10 resistor that made a difference. However, after driving around for 10 minutes, my aux temp gauge went up, but the stock gauge didn't move. I must have fried that resistor. I took it out again and re-flowed the solder, and same result. In other words, the only thing this procedure did for me was further break my gauge so it doesn't move at all. I wish I knew the value of that resistor so I could take it in to an electronics repair shop and swap it out. It kinda looks like I burned the trace to the right of R10. Oh well, it's not like it was working before...
Here are more threads on the subject:
http://www2.pajeroclub.com.au/forum/showthread.php?t=7613&page=3
http://www.pajeroclub.co.za/forum/viewtopic.php?f=27&t=5333&p=33918