I will assume you are asking about the differential or "final drive" ratios, not the transmission or transfer case ratios.
Second, since you call it a Montero and not a Pajero, I'll guess you are in North America. I think most of the North American Monteros came with 4.625 ratios in the differentials. Some may have had 4.875 ratios.
Lower 5.29 gearing is available, but Mitsubushi only used this in rigs that had a small diesel engine. These vehicles were NOT sent to North America. These gears will fit in Generation I Monteros with V6 gas engines (1989-1991), but they are very hard to get and expensive. These gears are still available (as of 2008), but the only source I could find was a person in Australia that would order them directly from the factory in Japan. Search these Mitsubushi forums if you want more information:
http://www.4x4wire.com/forums/postlist.php?Cat=&Board=UBB69
Mitsubishi Eclipse was created in 1990.
Mitsubishi Toppo was created in 1990.
Mitsubishi Diamante was created in 1990.
Mitsubishi GTO was created in 1990.
timing diagram for 1990 1.8 Mitsubishi eclipse
YEAR 1990
Yes it can.
The fuse for the computer in Mitsubishi Montero 1990 is located under the plate called fuse panel. This is where you will find the fuse.
Close ratios in transmission.
no, but heres a list of cars with the same style engine as your lebaron, not saying theyre exactly the same, wiring is probably different, but the basic engine is the same, from wikipedia * 1990-2001 Mitsubishi GTO (aka Mitsubishi 3000GT, Dodge Stealth) * 1989-1995 Plymouth Acclaim/Dodge Spirit * 1987-2000 Dodge Caravan/Plymouth Voyager * 1990-1993 Dodge Ram 50 * 1988-1989 Dodge Dynasty * 1988-1990 Dodge Raider * 1991-1996 Dodge Stealth * 1987-1995 Chrysler LeBaron * 1989-1991 Chrysler TC by Maserati * 1988-1989 Chrysler New Yorker * 1990-1993 Dodge Daytona * 1989-1994 Dodge Shadow ES * 1992-1994 Plymouth Duster * 2000-2005 Mitsubishi Eclipse * 1991-1999 Mitsubishi 3000GT * 1999-2003 Mitsubishi Galant * 1988-1990 Mitsubishi Sigma * 1990-1998 Hyundai Sonata * 2001-2005 Dodge Stratus/Chrysler Sebring Coupe * 1990-2002 Mitsubishi Diamante * 1991-1996 Mitsubishi Verada (Australia) * 1993-2002 Mitsubishi Magna (Australia) * 1990-2006 Mitsubishi L200 * 1990-1996 Mitsubishi Mighty Max * 1988-present Mitsubishi Pajero (aka Montero/Shogun) (Japanese and Middle East markets) * 1998-present Mitsubishi Challenger (aka Montero Sport)
The 1990 Mitsubishi does have a computer. You can reset the computer by removing the positive cable from the battery. Keep the cable off for 30 seconds and return it to its original position.
The automatic transmission temperature sensor, on your 1990 Mitsubishi, is located on the top of the transmission. The sensor will have to electrical wires going to it.