1980 300cd. When I rotate the thermostate dial to a high temperature number, on my heat/ac control panel and push a fan button, only cold air is produced. The engine is at normal operating temp. Any easy fixes or test I can do before spending hundreds of bucks at the shop?
Thanks. John

 

check the mono valve.

it is either blocked or faulty (always close).

 

1980 HAS THE BIG HIGH DOLLAR A/C SERVO AND AMP....

 

Thanks for the response. You guys are great. John

 

I have a similar problem. I have good heat at slow speeds and no heat over sixty mph. If it is the mono valve, where is it and how do you check it.

 

just feel the hose going in to the heater core. if it's hot, then it's working.

A/C heating should be on.

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