I have an 87 300E which had similar problems. All were solved withe repairs made to the controls. The controls are the buttons you push and knob that you turn.

The job is not too difficult and you start by removing the ash tray. Remove the screw behind the ashtray that holds the wood grain panel cover and then continue to remove screws untill the you have access to the controls.

The problem will probably be with the wheel that sets the temperature.

Do a search on all of my posts because I give a detailed proceedure on how to fix it. I did the job some years ago and now forget the details.

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Earl



1993 190E 2.3
2000 Toyota 4x4 Tundra