The Ac/Heat on my 1994 C280 began to blow hot and cold air. With the AC on, it will blow cold, then blow hot. I can see the compressor clutch engaged and the freon flowing in the glass viewing window. Even with system cut off, unbearable hot air comes out of vents. Any suggestions apprecited.

1990 300sl
1994 c280

I assume this car has a mono-valve which opens and closes to let hot water flow through the heater core.

Its default position is open and it requires voltage to close.

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2012 E350
2006 Callaway SC560

Thanks for the response. If it is the monovalve, do you know of a way to diagnose the problem?? Is there a way to make it shut off the supply of hot water completely (by disconnecting the wires?) Would disconneting the wire make it stick in the open for heat mode or the closed for cold air mode?

If it has a mono valve, disconnecting the wires would make it stick in the open position. With the AC on, feel the heater hose on both side of the mono valve. If it's hot on both sides, you know the valve is open. If it is open, check for voltage at the plug.

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2012 E350
2006 Callaway SC560