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I don't think that the regular service is related to the warning light comming on. Normally the trans will run hotter if the engine is running hot. You will want to make sure that the engine temperature is normal.
There has been a revised test for the trans temp sensor. The sensor may just be bad.
Here is the test for it according to the bulletin.
NO.: TSB-92-23-003DATE: December, 1992MODEL: 1987-93 MONTERO
SUBJECT:TESTING PROCEDURE FOR A/T FLUID TEMPERATURE SWITCH - SERVICE MANUAL REVISION
This bulletin describes how to test the operation of the automatic transmission fluid temperature switch. It also revises the A/T fluid temperature switch checking procedure shown in the 1992-93 Montero Service Manual, page 23-35. Details of the service manual revision can be found in this bulletin.
All 1987-93 Monteros equipped with automatic transmission
To test the operation of the automatic transmission fluid temperature switch, follow the procedure given below.
If the A/T warning light turns on and the transmission does not overheat, a fault may be present in the A/T fluid temperature switch. To check for a faulty switch, turn the ignition off and proceed to step 2.
Refer to the chart.
If the switch requires replacement, proceed to Step 6. If the switch does not require replacement, proceed to Step 9.
There is a way that you can request experts from the home page, (they sometimes put us on randomly) or you could state that it is for Super Mechanic in the first part of the question.
Glad I could help, thank you and have a safe and Happy New Year.