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Old 09-06-2003, 06:48 PM
Registered User
Join Date: Jul 2001
Location: Molalla, Or
Posts: 60

I've been reading the forum with respect to erratic heater/air conditioning problems. My 95 C280 is also blowing hot & cold air at various times with wild fluctuations of fan speed. Is the monovalve a possiblility? I checked in the parts shop and it doesn't recognize that part name. Does it have another name? Does the C series have them? Is it something I can check? How? Any help would be greatly appriciated.
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Old 09-06-2003, 08:21 PM
Posts: n/a
goFirst of all your car does not have a monovalve. This was the type heater valve used on the 123 and 126 cars of the seventies and eighties. Yours has a valve that is not rebuildable.

Secondly, since it is acting eratically, you need to see that the fan that draws air over the temp sensor is working. I'm not intimately familiar with the 202 cars, but I expect that there is a small grill near the rear view mirror. With the key on, see if it will draw smoke. A cigarette works good for this. If you don't smoke find some other way to put smoke there.

If it does not draw smoke, you need to find the fan and check and possibly replace it.

Good luck,
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Old 09-08-2003, 08:06 PM
Registered User
Join Date: Jul 2001
Location: Molalla, Or
Posts: 60
Located source of strange humming

Thanks for the responce. I followed your instructions and it didn't appear as though any smoke was being sucked with any force. As I was doing this I noticed a regularly intermittant humming noise coming from the area of the trunk. It sounded like an electric motor (similar to the sound the power windows would make) that engaged for one second then quit for one second and repeated for as long as the key was turned on.
I tracked it down and discovered the source was behind the thin wall of the trunk on the passenger side, wrapped generously in foam padding. It's definately the source of the noise because every time it engages the whole unit twitches in your hand. There are two sets of electrical wires and a couple of vacuum lines running to (or out) of it. It's black with a grey cover, about 2 inches wide x 5 inches long x 4 inches deep. It's part# is 202 2800 12 48, but the parts store doesn't recognize the number.
Do you know what this is? Is it behaving as it's supposed to?
Thanks, Bob
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Old 09-09-2003, 10:06 AM
Registered User
Join Date: Jun 2000
Location: Plano, TX
Posts: 2,512
The humming box sounds like the vacuum pump for the central locking system. I bet the rest of the doors are a bit slow to react when you lock or unlock the drivers door? You have a leak in the vacuum system of the central locking. The pump pulls the system down into vacuum and shuts off. As the system leaks, the pump cycles on to restore vacuum. You need to either repair the system or disconnect power to the pump. It will rapidly burn out due to excessive cycling as a result of the leak in the system.

Can't offer advice on the climate control, I'm not familiar with the 202 chassis. You don't have the center vents pointed directly at the temperature sensing grille, do you? That'll cause some erratic behavior every time.

- JimY
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