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Old 11-17-2002, 02:33 PM
Coming back from burnout
Join Date: Jun 1999
Location: in the Pacific Northwest
Posts: 2,274
85 300D No Heat Modulation

On my 1985 Mercedes 300D, I get Heat at full Blast, especially when I push the leftmost button on the Center Console Climate Control( Is this Defrost?). Icant seem to get Warm Air. I only get Very Hot Air or Very Cold Air (A/C).
Is this a damper problem or electrical problem or am I using the system incorrectly?
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Old 11-17-2002, 07:00 PM
Posts: n/a
im havin the same problem on 84 500sec, no i dont think ur using the system wrong if u push the EC button 2nd to the right, and have the temp wheel in the red, you SHOULD get heat...other wise ur in my boat! or should i say mercedes! ha \

Scott L.
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Old 11-17-2002, 10:30 PM
Registered User
Join Date: Apr 2000
Posts: 459
You most likely either have a problem with your monovalve or the air-return tube behind the glovebox. First thing to do is remove your glovebox and inspect the foam tube that connects to the air return on the dashboard. Most likely its crumbling apart and needs to be replaced. I used 1/2" pipe insulation from Home Depot. This is a cheap and easy fix.......

BUT, you could also have a problem with the monovalve. Order a monovalve repair kit for around $25 and throw the old one away. Obviously, do the cheap/easy fix first and then move on to the more complicated possibilities. If you need more help, check the archives, this is a VERY common problem.

'84 300D, 169k
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Old 11-17-2002, 10:37 PM
Coming back from burnout
Join Date: Jun 1999
Location: in the Pacific Northwest
Posts: 2,274

Its winter now and I use the heat a lot and I am getting Baked at times....
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Old 11-18-2002, 06:10 AM
Posts: n/a

Greg offered a very good response. The only thing I would add to it is that if you are on a budget, rather than just put a kit in the monovalve, take out the four screws and look to see if the diaphragm is leaking. This is a five minute job. If you're not familiar with the monovalve, let us know and we will tell you more.

Good luck,
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