1996 C200:Dashboard vents....HOT only

[grahamrennie]

I'm a newbie here - first post and I wasn't sure whether to post here or in engine section.

I've a 1996 C200 sedan, and just recently the dash vents have given over to producing hot air only. Very useful when we are on the cusp of Summer here in the Antipodes!!!

I've done some general research on the web, noting comments about 'vaccuum lines behind the glovebox' and also references to the 'heater control valve' in the engine bay, but its all a bit too general for me to really glean anything substantive.

The only bit of additional info. I can provide is that I was driving the car one hot day recently, so I put the AC on. A short time later a noticable - but not overpowering - smell came into the cabin, which I would best describe as like the smell of steam, and which remained for the rest of the 45min journey. The temp gauge went up a little for a while, but not unusual for the hot conditions outside and the verrry slow traffic I was caught in at the time....it returned to normal when I broke out of the traffic. No change to the water level in the reservoir tank either. From that time on I've been getting the hot air...doesn't matter what I do with either of the roller temp selectors.

Any clues where I should start looking? (I live in a semi-rural area with no Benz shops around...I'd like to understand the nature of my problem before sending it to a workshop)

Thanks in anticipation!

 

[grober]

Chances are its the duo-valve that's malfunctioned its found in RHD models in front of the brake servo under the bonnet . Failure to shut means hot water from the engine is always circulated to the heater. It could be a failure of one of the many pneumatically operated flaps in the heater unit itself but apart from checking that the vacuum supply is functioning there isn't much you can do without taking the heater to bits- to do this you need to dismantle the dash!!!!! So a start with the possible= the duo valve here's a DIY LINK different car but principle the same. http://www.mbclub.co.uk/forums/howto/26888-howto-clean-duovalve-w210.html

 

[grahamrennie]

"..without taking the heater to bits- to do this you need to dismantle the dash!!!!! "

Ohh...those words scare me! Anyway, Graeme, thanks for your advice and reference to the DIY link - its very comprehensive. I have located the valve in my car, so I'll have a look at that first. I wonder if I can do it with losing any bits?!?! (I will heed the warning in the link!)

Thanks again
Graham

 

[grahamrennie]

Ok, I dismantled the valves, noting that they were a bit grungy and sticking a bit. I gave the valves a good clean and replaced the valve unit.

I started the engine, let it warm up then put on the A/C and got cold air - 'chilled' on the driver's side but a little warmer on the passenger's side. I was quite happy with that. While the engine was running, I actually touched the top of the valve units, at least one of them was vibrating very slowly (like the valve was moving constantly in the cylinder) while the other was static.

Next time I started the engine, it was back to Hot again - although I can still get cool-ish air from the driver's side by fiddling with the dashboard control, but the passenger's side just remains on hot permanently.

So the valve cleaning seemingly wasn't successful. The vent controls seem OK, just the hot water through the duo valve seems to be the problem? Is there any other thing I should look at first, or just get a replacement valve?

 

[d w124]

Last time I had this problem,knocking the duo valve got the problem sorted