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1991 Montero transmission

My 1991 Mitsubishi Montero (over 300K miles) has developed this shifting into overdrive difficulty. It takes about 15 to 20 minutes of driving time before it will shift to overdrive. It operates normally after the shifting takes place. It occurs after the car is parked for hours and every morning. I have taken it to a transmission shop which said it was possibly an engine sensor problem but not a transmission problem. I took it to my mechanic who did change out my thermostat which was sticking but was told it was a transmission problem. I was told by another tranmission shop that it will not shift into overdrive until it is okay with the engine temperature. If I let the car sit there and idle before driving it does not subtract time from the necessary 15 minutes of driving time needed for it to shift into overdrive. Any ideas?

On newer cars it won’t shift to OD until you are at a certain temp. I doubt that is the case in a '91 but I am not very good at this. Sometimes changing a temp sensor will help. The mitsu trannies have a tendency to go bad from the OD and then 3-2 & 1. There is a sensor pack in the tranny that some people have been able to change without tranny disassemble and it worked.

The transmission tech is correct that the transmission computer reads the temperature data from the coolant temperature sensor and will not allow overdrive until the it determines that the engine is at running temperature. If the Coolant Temperature Sensor is reading too high a resistance for the actual engine temperature, the computer will not allow overdrive until the engine gets to a temperature above the thermostat set point. So you might have the engine mechanic measure the resistance of the CTS cold and warmed up to determine if it is calibrated correctly. Alternately, you might just have it replaced and see what happens. Also some computer scanners can output the data from the engine computer. Thus you can see what the computer is doing which can point the mechanic in the right direction.

Let us know what is finally determined.

I am leaving it at the trans shop today so it will be cold for the technician on Tuesday AM. I’ll let you know if they find anything.

I own 2 1991 monteros,and mitsu tech.what i have found in the past was the relay located at the right frt kick panel (will sometimes stick,and the trans does have its own temp sensor built into it.which if you watch on a scan tool and graph it ,youll then know what its doing.and if its the prob.#2 mitsubishi has a said parameter for the ECU(to keep the trans out of OD .until the proper temp is met,reason being mits does not want OD engaged till warm.(keeps wear on the clutch pack to a minimum and extends trans life. on occasion ive found the button on the shifter to open intermittant(try tapping the shifter while issue occurs.)if it trys to shift ,then you more area is the TV cable underhood for excessive play or binding or too tight.TV cable means throttle valve.good luck.