Subject: Montero Overdrive
Question: 1994 Mitsubishi Montero LS (115,000 miles)... Since the cold weather began, the Montero slips into and out of overdrive when on the highway. Coincidentally, the engine doesn't warm up (we may need a new thermostat). When I engage and disengage the overdrive button on the shift lever, the "O" display does appear and disappear on the dash display. Could this be a transmission issue or do you think it may be a computer problem related to the cold engine?
Answer: Sandy,
If your engine does not start to warm up after a 5-10 minutes of driving (even in freezing temperatures), you definately need a new thermostat. Not only will you be able to feel heat quicker on those cold mornings, but your engine will run much better the quicker it gets to normal operating temperature.

If you have strange issues with your transmission only during cold temperatures, you may have a computer problem. There is a main computer in your car is called the Electronic Control Unit. Your ECU works closely with your Transaxle Control Module (TCM), which is in charge of controlling your car's transmission. The cheapest and easiest fix you can try first is to get a transmission fluid change. You should get it changed at least every 35,000 miles.
It is possible you have a problem with our ECU. Read this article for further information:
http://90gsx.com/eclipse/cap/caps.htm

A malfuctioning ECU can really cause some strange problems. Have it inspected if all else fails.
Good luck!