Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.
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.
DATE: December, 1992
MODEL: 1987-93 MONTERO
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.
|1.||Run the vehicle until normal operating temperature is reached. Check the A/T fluid level. If necessary, add A/T fluid until full. Refer to the service manual for A/T fluid specifications. |
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.
|2.||Remove the connector on the A/T fluid temperature switch, start the vehicle, and observe the A/T warning light. The light should now be out, indicating no faults in the A/T fluid temperature switch circuit. Turn the ignition off and reconnect the switch connector.|
|Note :The A/T fluid temperature switch is located on the transmission body, behind the inhibitor switch. |
|3.||Remove the transmission dip stick and insert a temperature probe (pyrometer) into the dip stick tube. Make sure the end of the probe is inserted far enough to reach the transmission fluid. |
|CAUTION :Do not allow contaminants to enter the dip stick tube. |
|4.||Observe the reading on the temperature probe. If the fluid temperature is 257°F (125°C) or less, and the A/T warning light is on, the fluid temperature switch is faulty and should be replaced. If the A/T warning light is on and the fluid temperature is 258°F (126°C) or more, the switch is normal.|
Refer to the chart.
|5.||After checking the A/T fluid temperature, remove the temperature probe from the dip stick tube and reinsert the dip stick. Make sure the dip stick is clean before reinsertion. |
If the switch requires replacement, proceed to Step 6. If the switch does not require replacement, proceed to Step 9.
|.||Replace the switch by removing the switch connector and the switch. Before installing the new switch, apply a small amount of sealant to the threads (semi-drying Threebond 1207F or equivalent). |
|WARNING :THE AUTOMATIC TRANSMISSION FLUID MAY BE EXTREMELY HOT WEAR PROTECTIVE EYEWEAR AND GLOVES WHILE PERFORMING THIS PROCEDURE. |
|7.||Install the new switch and tighten it 29 N-m (22 ft.lbs.). Attach the switch connector. |
|8.||Run the vehicle until normal operating temperature is reached. Check the A/T fluid level. If necessary, add A/T fluid until full. Refer to the service manual for A/T fluid specifications. |
|9.||If the switch is normal and the transmission overheats, check the transmission oil cooler, pipes, and hoses for blockage. Refer to Group 23 of the service manual for information on transmission oil cooler components.|