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Old 01-16-2000, 05:13 PM
James Stephens
Posts: n/a
My 1988 300SEL has a heater problem. It will intermittently work at the MAX setting. When the car reaches operating temperature and the Climate control is in the MAX position the heater works sometimes. When the dial is then moved into the white normal operating zone it blows cold air. When I move the dial back to MAX it still blows cold air. I can then turn the engine off, immediately restart it and the heater will again work on the MAX setting. What can be the problem here. Any help is greatly appreciated. Thanks in advance.

James Stephens
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Old 01-17-2000, 07:36 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Sounds like a hydro-locked mono heat valve. The insert can be replaced for $35-40.
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Old 01-19-2000, 05:49 PM
Registered User
Join Date: Mar 1999
Location: Williamsburg,VA, USA
Posts: 106
similiar problem on my car. Mono valve fixed it. About a 20 minute job. on my 91 300SE the valve is located on the passenger side between the firewalls under the hood. May have to move the hoses that heat the windshield fluid, but didnt have to disconnect anything. Not sure if it was necessary, but I drained the heater just to be on the safe side. good luck
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Old 01-21-2000, 07:06 AM
Registered User
Join Date: Apr 1999
Location: Cincinnati, OH, USA
Posts: 59
Can you explain more about this valve and how it works? I have a 92 500SEL and it seems that the heat does not come out as strong as it should. I hear the blower running strong in the dashboard but I have yet to be able to get a forceful stream of air coming out of the dash vents. I have tried Auto set at 78F, Auto set at 78F with red light between center vents on. When I put the defrost on it blows out strong like it should. Heat for the rear seats only comes out at the floor ducts and not at the ceter armrest duct found between the front seats. Like above it seems that when I turn the car off and restart the heater sometime regulates correctly. Any ideas?

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Old 01-21-2000, 08:42 AM
Andras Nagy
Posts: n/a

I believe the center outlet in the rear is for A.C. only. Check your Owner's Manual. Mine blows only during A.C. operation, and that's what my Owner's Manual says it should do.

The heat comes out of flat ducts under the front seats, blowing back into the back seat compartment.

All this makes sense, for the MB engineers know that heat rises, and A.C. falls. So these ducts are located correctly....Andras
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