Yet another w126 climate control probelm! - PeachParts Mercedes-Benz Forum

mplafleur · #1 10-16-2006, 12:56 PM

Let's see if anyone has had this one before...

I get heat, but not all the time. I get cold air out of the defrost while in defrost mode. I'm sure the monovalve is working, as I can feel heat in the center console and knee panels. It's just not coming out the vents.

I then removed the passenger kick panels and tested the vacuum pods. They all work, no leaks. I did not verify the vacuum relays yet. I replaced the temperature control module with a spare and no change. I removed the pushbutton module and took it apart. It is in great condition. There were no signs of pitting on the relays, no solder reflows, no cold joints, etc. I could tell that it got hot, but is still operational.

Now it gets wierd. I took the car out for more tests and found that if I get down the road and while at a stop light I turn the car off and restart, I get heat! After a while, the heat goes away and if I cycle the key, I get heat again. It doe not do this if I turn the CCU off and then back on.

This testing was done with the CCU temp wheel in the locked-on heat mode. I found that once I had heat and if I then rotated the wheel to cold, it would immediately go cold, and if I immediately rotate to locked-on heat, - no heat.

Yesterday I removed the monovalve assembly with the aux coolant pump. I verified correct operation of both. In full heat, the plunger was open. In full cold, the plunger was closed. In between, the plunger was cycling 2 seconds closed, 3 second open. The pump was spinning when heat was called for, otherwise it was off. I also checked to make sure that the plunger was not getting stuck or was inhibited in any way.

I have more testing to do yet.

Any ideas?

Brian Carlton · #2 10-16-2006, 01:46 PM

You've certainly covered all the bases and I don't have a solution for you.

But, I'll relay an anecdotal story on the SD and see if you can figure this one out.

The SD has had all its bad vacuum pods replaced and all the doors work as designed AFAIK. It also has a rebuilt CCU that seems to function properly and maintain temperature of the vehicle.

The only modification from stock is running full vacuum to the center vents at all times.......using the dash sliders for control of the center vents.

So, when heat is demanded, the legroom flaps and the defroster chime in and provide the desired heat. However, the temperature of the air from the center vents is dead cold. Completely different temperature of the center vents when compared to the legroom or the defrost.

With common supply air, how does the system get heated air to the legroom and the defrost but unheated air to the center vents?

carnut · #3 10-16-2006, 03:18 PM

Remove the mono valve top cover ( 4 screws) pull out the diaphram and stretch it. You will find a torn diaphram! classic symptom of torn diaphram!

mplafleur · #4 10-16-2006, 09:18 PM

Quote:

Originally Posted by carnut

Remove the mono valve top cover ( 4 screws) pull out the diaphram and stretch it. You will find a torn diaphram! classic symptom of torn diaphram!

Tell me why...

What happens with a torn diaphram? It doesn't stop the actuator from actuating. The plunger still moves and seats in the valve body.

UNLESS... This is what I was thinking yesterday...

If there is a small tear, is it possible that coolant is entering the solenoid cavity under pressure and forcing the plunger body down into the closed position? The solenoid isn't actuated (heat position), so the only thing keeping the plunger up and open is the spring. If it isn't strong enough to counteract the coolant pressure building up behind it, then the monovalve will slowly close.

or

?

1985 300SD Sady · #5 10-16-2006, 10:11 PM

Quote:

Originally Posted by mplafleur

Tell me why...

What happens with a torn diaphram? It doesn't stop the actuator from actuating. The plunger still moves and seats in the valve body.

UNLESS... This is what I was thinking yesterday...

If there is a small tear, is it possible that coolant is entering the solenoid cavity under pressure and forcing the plunger body down into the closed position? The solenoid isn't actuated (heat position), so the only thing keeping the plunger up and open is the spring. If it isn't strong enough to counteract the coolant pressure building up behind it, then the monovalve will slowly close.

or

?

I know first hand torn diaphragm in the monovalve can do all sorts of crazy stuff.

Im sure you could have checked it in the time it took you to think of how it could be causing your problems, and then you will know for sure if that is the culprit.

mplafleur · #6 10-16-2006, 10:44 PM

Quote:

Originally Posted by 1985 300SD Sady

Im sure you could have checked it in the time it took you to think of how it could be causing your problems, and then you will know for sure if that is the culprit.

Oh, I know it's torn. I saw it a couple of days ago. A very small tear.

I have a hard time buying a whole replacement kit for $50+ when my problem is a 10 cent part.

Right now I have the monovalve disconnected (electrically), with no change in operation.

Brian Carlton · #7 10-16-2006, 11:00 PM

Quote:

Originally Posted by mplafleur

I have a hard time buying a whole replacement kit for $50+ when my problem is a 10 cent part.

Think you can manage $20.?

http://cgi.ebay.com/ebaymotors/Heater-Valve-Kit-Mercedes-Benz-300-d-cd-td-sd-123-126_W0QQcmdZViewItemQQcategoryZ33548QQihZ020QQitemZ300037474124QQrdZ1

mplafleur · #8 10-17-2006, 07:09 AM

Quote:

Originally Posted by Brian Carlton

Think you can manage $20.?

http://cgi.ebay.com/ebaymotors/Heater-Valve-Kit-Mercedes-Benz-300-d-cd-td-sd-123-126_W0QQcmdZViewItemQQcategoryZ33548QQihZ020QQitemZ300037474124QQrdZ1

Hey, I'd been looking for one for a month! Apparently, search for monovalve doesn't do it. This auction was titled heater valve.

1985 300SD Sady · #9 10-17-2006, 05:56 PM

Quote:

Originally Posted by mplafleur

Right now I have the monovalve disconnected (electrically), with no change in operation.

This makes sense.

I see what you mean about spending so much on the replacement 'core' when its only a cheap torn diaphram. Maybe you can find a replacement diaphram somewhere, I know you can replace the diaphram.

Brian Carlton · #10 10-17-2006, 05:59 PM

Quote:

Originally Posted by mplafleur

Hey, I'd been looking for one for a month! Apparently, search for monovalve doesn't do it. This auction was titled heater valve.

This fellow sells them all the time. He normally gets about $20. for them. There is always an auction running.

The search is "Mercedes heater".

mplafleur · #11 10-18-2006, 11:31 PM

Auction's over. It's mine now...

mplafleur · #12 10-24-2006, 12:26 PM

Been driving for two days now with the new monovalve kit installed. The heat is fine and does not go away.

It's amazing what kind of seemingly unrelated problems occur in the heating system with a tiny little split in the monovalve diaphram.

rhodes2010 · #13 10-30-2006, 12:24 PM

Quote:

Originally Posted by Brian Carlton

The only modification from stock is running full vacuum to the center vents at all times.......using the dash sliders for control of the center vents.

So, when heat is demanded, the legroom flaps and the defroster chime in and provide the desired heat. However, the temperature of the air from the center vents is dead cold. Completely different temperature of the center vents when compared to the legroom or the defrost.

With common supply air, how does the system get heated air to the legroom and the defrost but unheated air to the center vents?

Sorry to bring this up again. But....
The original post described a certain problem that was solved by installing a new monovalve diaphragm.

My problem is exactly as stated above and quoted by Brian. I put straightvacuum
to the pod for dash vent air full-time. Now I get cold defrost and cold dash air. The cold dash air is the biggest bugger.
My doors are real hot, my panel right over the knees is real hot.
But dash air is cold. I did check the monovalve, but did not stretch it.

Is the monovalve replacementr supposed to fix the problem Brian described ?

Brian Carlton · #14 10-30-2006, 12:58 PM

Quote:

Originally Posted by rhodes2010

Sorry to bring this up again. But....
The original post described a certain problem that was solved by installing a new monovalve diaphragm.

My problem is exactly as stated above and quoted by Brian. I put straightvacuum
to the pod for dash vent air full-time. Now I get cold defrost and cold dash air. The cold dash air is the biggest bugger.
My doors are real hot, my panel right over the knees is real hot.
But dash air is cold. I did check the monovalve, but did not stretch it.

Is the monovalve replacementr supposed to fix the problem Brian described ?

I'm quite sure the monovalve is not the issue. The vehicle can put out some very hot air via the defrost outlets and the legroom flaps if requested.

But, once it attains cabin temperature, it has warm air from the defroster and legroom flaps and cool air from the vents. In fact, the air from the vents is so cool that you've got to manually close 'em.

I'm a bit baffled by it.........all the supply air should be originating from the same source..........

rhodes2010 · #15 10-30-2006, 01:13 PM

Me too (baffled).

I am also closing my dash air vents to keep from freezing.

I considered putting the vacuum bypass back the way it was to see if
warm air returned to the dash vents.

So, maybe the dash vent bypass tapped into the source that feeds the
center vent in the rear seat, the one that feeds fresh air no matter what.
That air is always cold too.