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YOTA Master
'93 2wd P/U
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2,440 Posts
Automatic transmissions will not shift into overdrive below a certain engine temperature. And if you are driving and it is in overdrive and the temperature drops below that point then it will drop out of overdrive. So if your thermostat is not functioning correctly and it is really cold out side then your engine could hover around this temp and drop in and out of overdrive.

There can also be a blockage in the coolant pipe or hose that circulates coolant through the sensor block that will prevent the sensor from reading the correct temperature.

Also there is a year or two of that model(don't recall exatly what year) that had a problem with a ground splice in the wiring harness that would increase the resistance on the sensor circuit and cause the ECM to read the temp as lower than what it actually is resulting in much the same problem.

Hopefully this info will get you going in the right direction
 
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