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Old 10-21-2006, 08:32 PM
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Join Date: Aug 2004
Location: Sunny Arizona
Posts: 8
300SDL...A/C works. heater doesn't

Well, it's finally fall here in Phoenix. I went to test out the heater the other day (so I could keep the sunroof open, of course) It didn't heat up at all, even with the temp set to 28C or whatever the highest reading is. Is there a solenoid valve that allows coolant to flow through the heater core? Where would I find this piece. Unfortunately, the PO was a bit of a hacker. I've already started redoing some of the work which wasn't done correctly before. I've even had to replace the turbo, as the old one's impeller was starting to hit the side and dump oil into the manifold .


1978 Trans Am "Roxy"
1987 300SDL Dark Orient Red "Hilary"
2002 VW Golf TDi "Birgit"
2005 Suzuki GS500F motorcycle "Zoe"
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Old 10-22-2006, 12:47 AM
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Join Date: Jan 2006
Posts: 19
Not sure if this applies, but when I got my 240D I noticed my passenger side heat would never turn off. I looked up the part and found someone mentioning buying a cheap valve from the hardware storethat you could put to block both vents during the summer months. I went to look and found one was already installed, so I just turned off. On mine it's right on the firewall under the hood in front of the passenger seat.
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Old 10-22-2006, 01:20 AM
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396
Originally Posted by bigpaws1984 View Post
Is there a solenoid valve that allows coolant to flow through the heater core? Where would I find this piece.
You're describing the "monovalve". It's located between the two firewalls just to the inside of the battery on the passenger side. You may not see it because of the hoses that cover it. It has a round stainless steel top and outside this top is a flange with four screws.

Be careful removing the screws. If you drop one, be prepared to go to China to retrieve it.

When all four screws are out, the piston and diaphragm can be removed. Check for any tears in the diaphragm. It's the most common culprit. If you have a tear report back and we'll tell you where to get a replacement.

If the diaphragm is OK, then further tests of the valve are necessary to determine if the valve iteself is the problem or the CCU is the problem.
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