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Old 12-31-2005, 02:50 PM
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No Heat 87 420SEL

I have suddenly lost all heat when idling or moving. No heat when the defroster is turned on, (it justs blast cold air) or when pressing any other buttons on the CCU. The blower motor behaves properly, the A/C compressor kicks in when it is supposed to, and the A/C will blow cooled air when the temp dial is turned to the blue. I just canít get any heat! I pulled the mono valve and cleaned it and tested it and it works properly. Iíve got the CD manual but cant figure out where to start trouble shooting this problem. All the postings Iíve found relating to a no heat problem usually indicate that the defroster setting still blows hot air but thatís not the case here. Any suggestions? Im gonna pull my glove compartment and look around back there but Im not sure what to be looking for. Thanks

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Old 12-31-2005, 06:07 PM
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Possibly the monovalve is stuck / disconnected? See if the hoses are hot on both sides of it.
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1997 C280 Sport, 144,000 miles
1991 300SL, 189,000 miles
1987 420SEL, 149,000 miles
1986 190e 2.3 16v, 151,000
1968 W30 442, 78,000 miles
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Old 12-31-2005, 07:56 PM
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I disconnected the electrical leads to the monovalve and got heat! I put a meter on the monovalve lead and It turns out I am getting a constant 12v current to the monovalve regardless of what buttons are pressed on the CCU which is keeping the valve always shut. There is no cycling of the current. Where to look now?
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Old 12-31-2005, 08:16 PM
Craig
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Quote:
Originally Posted by jriewer
I disconnected the electrical leads to the monovalve and got heat! I put a meter on the monovalve lead and It turns out I am getting a constant 12v current to the monovalve regardless of what buttons are pressed on the CCU which is keeping the valve always shut. There is no cycling of the current. Where to look now?
I'm not very familiar with your car, but it sure sounds like a CCU issue to me.

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