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2001 M-B E320 (W210) Currently 11,000 Miles
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501 Posts
Discussion Starter · #1 ·
Picked up a 1987 420SEL yesterday that mechanically seems pretty sound. Drove it this morning, using the heater. Engine tempature gauge showed normal range after driving but the heater tempature was only luke-warm; never got really warm or hot. The air conditioning works great, dropping the ambient tempature by almost 40 degrees.

Where should I start in determining the lack of 'heat'?

Thank you,
 

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'82 Euro 500SEL, '85 Euro 500SEC AMG WB Cabriolet,'86 Euro 500SEC RUF
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9,498 Posts
Mono Valve would be the first place to check. It is between the false fire wall and the real fire wall to the driver's side of the battery, and has 4 screws with a shiny dome cylinder protruding from the square mtg. Use a magnetized screw driver so you don't loose one of the screws down between the firewalls.

I need to check mine in the SEC as I have heat until I move the car up to highway speeds. Here in So CAL I just haven't need the heater or defroster in the last 6 weeks that I have owned the car.
 

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2001 M-B E320 (W210) Currently 11,000 Miles
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501 Posts
Discussion Starter · #3 ·
Mono Valve would be the first place to check. It is between the false fire wall and the real fire wall to the driver's side of the battery, and has 4 screws with a shiny dome cylinder protruding from the square mtg. Use a magnetized screw driver so you don't loose one of the screws down between the firewalls.
Got my monovalve repair kit and installed it today. It's 85 degrees outside, so I can't be sure if that was the problem until I check it tonight. I noticed that the old monovalve had a torn diaphram, so I'm pretty sure this should solve my lack of heat problem. I was surprised how easy the replacement process was; thanks to all in this thread and another that I asked about the monovalve.
 

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1986 Euro 300SEL
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2,151 Posts
If you had a torn diaphragm, that's your problem. Causes the monovalve to stick either open or closed (can't remember which), causing constant heat or constant cool.
 
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