Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
Welcome, I'm Chris (aka Moose) I prefer working afternoon till just after midnight.
If you use L and get up to speed then shift to D, do the remaining upshifts occur and gears work?
It would be very tough for me to say it is an internal issue since it was fine prior. The first thing I would look into is the range sensor and shift linkage out of adjustment which will effect the way the fluid in the valve body is directed to the clutches and bands. Have you checked this yet or can you?
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Are you checking the fluid level while running in park at operating temp?
Dry it holds 10.4 quarts, during a service where the converter is not drained you replace about 6 quarts.
Since the linkage is fine our net thing to check is the line pressure. Is that something you have the tools to do?
Since the concern was not with the linkage external then you had a fluke failure of something internal that we would need to diagnose, then remove and inspect, then repair and rebuild.
What would you like to test outside the transmission?
It seems you are asking for someone else to help. Opting out now. Best of luck.