I am slowly progressing with my now ongoing heating saga. Eliminating things as I go along. It has been months now - So again I am asking for guidance from the knowledge source.
Just to recap, I had constant heat in the car (1987 300SE EURO - which is with Manual climate Control- not PBU) , so I first replaced both inserts on the duo-valve. They were bad . That fixed it on an intermittent basis. I next purchased a used valve, put the good inserts into it's "top" and replaced the top of my existing unit with the "new one". Things are different now. If I never use the heat - All is well. If however, when car is running I turn the heat on by rotating the temperature dials and then turn it down again - The heat will not go away.
The temp sensor in the light dome sucks air.
I checked the valve inserts with direct 12v - they click as supposed to.
I checked the electrical connector to the valve with KOEO. I get 12.4v with the dials in the off position. then pulsating with dials in white or red. Finally returning to 12.4v when dials turned down to OFF again. So it seems that the system works correctly with KOEO but somehow does not when the car is running at temperature. Where do you think I should look ? Could the valve be malfunctioning when hot ? Could the dials be at fault Somehow not functioning when they are "hot" ? Or might the problem be elsewhere like a faulty ground. I am at a loss here.
I should mention that I purchased the inserts online and realized they came in a blue box (forgot the make) rather than the Bosch yellow. I do not think that has anything to do with anything but am mentioning this because every detail matters. Any help would be appreciated

gidpor,

I had a similar climate control nightmare that plagued me for months on my U.S. spec '88 300SEL However I had a combination of problems: 1) On defrost only, fan blower would cut out intermittently. After Push Button Control Unit replaced x3, found wire damaged at plug on firewall just directly behind fusebox. Found this by tapping fuse box area with screwdriver handle. 2) Had constant cycling of hvac doors. Replaced switchover valve inside r/h side of center console. 3) Had uncommanded and sometimes unbearably hot air flow into passenger compartment. Sometimes I had split hot/cold air combinations. Hot air coming from top side vents and cold air coming from bottom side vents. This problem would only occur after driving approx 20 to 30 min. Interior temp sensor replaced x1, Aspirator fan x1, The Monovalve insert replaced x2, Monovalve housing x1. After the Monovalve housing was replaced all was fine for approx. 2 weeks. My problem came back after starting an 800 mile journey with my family packed into the car. Not a happy camper! I again searched these threads and could not find a similar problem. My thinking was that there was some sort of a signal loss problem to cause the system to default to heat. After my not so pleasant trip, my mechanic took out the complete Monovalve assy. again and took it apart. There is a check valve located in the bottom of the assy., if the check valve sticks it could let hot water enter into the heater core. The check valve in mine was found contaminated with sludge/debris that probably made the valve stick open. The coolant system was flushed when I bought the car in Jan/04, was flushed again (approx. 7 mos. later) when the original monovalve housing was replaced. The check valve within the housing seems to be very sensitve to any type of contamination. The monovalve assy was cleaned and replaced and the coolant system flushed again until positively clean. No reoccurance with uncommanded heat for 3 mos. now. Hopefully, problem is gone for good. Also hope my experience helps you.

Brgds,

BENZSTER

Wow. You must have nerves of steel to have kept at it like this! It does give me new hope, after much frustration.
A new approach to solving this very nagging issue is always more than welcome and I am willing to try just about anything. Of course I ran downstairs to look at the extra valve I have on the bench as soon as I read your post. I did see how the check valve functions - It seems to block water from the Aux. water pump when instructed to do so (?). I say this with great caution, believing that the valve inserts further down the line would themselves block all water from entering the heater core when activated. Nevertheless I will pull the duo-valve body later this week and check . My system has been flushed twice in the last four years, but perhaps the valve assembly has collected it's own share of junk over years. Hopefully this is the culprit and will do for me what it has done for you.
thanks a lot.

New to the forum. Am not at all tekky: switch on and steer is the limit of my knowledge. Help me please. I have a 1986 230E only 93k miles. Bought it three years ago and have always had terrible heater problems. Comes on and off at will, twenty minutes of heat often then v cold for hours on long journeys. Stop car, switch off and restart often gives heat again for another ten minutes, etc. Drove across Europe 1600 miles each way in winter several times with this problem. Have had duovalve (?) replaced twice, neither of which did any good at all.

Are you saying I need to get something flushed out, and this might solve my problem. Have printed out your entries, and will take to mechanic. Grateful for your advice if you think solution proposed in your entries will also solve my problem.

Otherwise my car is excellent and the best purchase I ever made.

Hi there & Welcome to the best Forum,

I am not familiar with your system but...
Do you have ACC (automatic) with Push Button Control unit in the dash,
or is it manual control ? - If ACC That could be your problem right there.
Does your car have a Mono Valve( half a problem) or a Duovalve (double trouble..).
Was the valve itself replaced or just the Inserts (the shinny looking piston/s)?
The flushing was described by Benzster and worked for him. My climate issues seem to not be related. However, It is very good practice to do this every couple of years in order to clean and maintain the system. In your case perhaps this was not done for a long while. When doing this also get rid of fluid in the block by removing the dedicated bolt on the pass side of the block. Please use MB fluid (antifreeze) as replacement. In Europe this is usually Blue. Elsewhere it comes in orange as well. Any details you supply will help us all in trying to assist you.

I recently added to this thread with another
Ronak, Mission Trading Co. / Or Bosch ? - take a look at that as well, perhaps it will lead you to the promised land. Ha Ha. Actually with a benz you NEVER get to the promised land.

gidpor, thanks for your rapid reply. My car is automatic transmission. Oops, just realised what you mean. It's manual-controlled dials on the fascia for the heating, blue thru' to red. No AC. Had the valve-thing replaced two times: the whole unit, I think, kept the old ones as I'm sure they're working OK, and were changed needlessly, perhaps. Still, it all gets rid of money, doesn't it? I think it's a duo-valve as there are two clear outlets: like a fat Y-shape. Fits under the bonnet (hood) just in front of the windscreen (windshield), centre of the car. Mechanic proposes to disassemble control panel inside car to have a look, but this of course will cost money.

Grateful for your thoughts and any others.

the attached photo is a mono-valve - if it is double wide etc then it is duo valve. You need to make sure the insert's rubber diaphragm is intact . Simply and carefully remove the four (or six) screws that hold the top of the valve in place. If in doubt , replace.
If intact you need to make sure the pistons are able to move and not frozen.
the way to do that is to hit the connector male pins (on top of the valve) with direct 12v. If you hear it ( them) click, they are good.
Now you look at the female pins of the connector. check them with a volt meter. KOEO. you should get 12v+ across the pins with the dials set to total blue - when you change the dial towards the red the meter fluctuates finally coming back to where we started with the dial wheels back in the blue. All this can be done before you rip apart the center console and get into expensive labor costs.
If these tests are all good you need better people than me to help you out -
On some models there are all sorts of vacuum actuators. Any failure there could create may-ham in the system.

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Thanks. Will convey this to the man.