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No Heat - Monovalve?

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benzbum's picture
Lucketts, VA
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2011 E350 BLUETEC
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1987 560SL

Okay, I've followed several threads on this issue and just want to make sure I'm headed in the right direction before I start screwing something up. Although somewhat capable, I am not a mechanical genius. I have a 1985 300D Turbodiesel (W123). The weather has really turned cold the last few days and I have needed heat. The blower blows fine on all speeds. The pushbutton unit seems to direct air to the appropriate vents. I say "seems" because I get air on the windshield on the defrost setting and air from the center dash vents when I turn the A/C on. I notice little to no air from the floor vents on any setting. Although troublesome, this is the least of my worries. It is darn cold driving this car with temps in the teens and twenties! Heat from any vent would be most welcome!

Based on something I read in another thread, I removed the electrical wire from the top of what I assume is the monovalve on the firewall just inboard from the battery. I say "assume" because it appears to be in the general location described in other threads, it does have heater/cooloant hoses going in and out and disconnecting the electrical connection on top didn't seem to adversely affect any other function of the vehicle. If I read the other threads correctly, disconnecting this electrical connection should result in full, continuous heat as I gather this forces the valve to remain in constnat heat mode. My result - no change in function. I still get full blower action but no heat. I did leave the car run for 10-15 minutes and then drove it for about 10 miles. The engine temperature gauge moves (slightly in this cold weather), but still no discernable heat from any of the vents.

It sounds like a monovalve issue, if my research is accurate. I just want to see if I am headed in the right direction. If so, it sounds like the rebuild kit may be relatively inexpensive ($40?). In the alternative, anybody have any idea what a new monovalve assembly for this model should run? Does it sound like something else might be the issue? Thanks in advance for any advice/encouragement.

Steve

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Steve McPeak Greater Washington Section

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benzbum's picture
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1987 560SL

Don350SDL wrote:
The symptoms of a bad monovalve are usually too much heat. Whether disconnected or faulty, they move towards open when they break, not closed.

Don

Thanks, Don. Much appreciated.

Actually, I thought something I read in previous posts that indicated the opposite:

"But it sounds like the monovalve may be bad - disconnect the wire from the valve and see if you get full heat - if not, the valve needs replacement. If you get heat, then the pushbutton panel needs replacement "

I think I also read something in another post that also indicated that disconnecting the wire would result in full, uncontrollable heat:

"It is apparently not a problem with the monovalve since if I disconnect
the wire to the monovalve, heat comes into the cabin,
but out of the dashbaord vents, and it is uncontrollable
as regards temperature
."

As I said, I am not a mechanical genius and I'm just trying to read as much as I can before I start tinkering too much. I don't want to appear to question others who are far more knowledgeable than I am, but it appears some information may be contradictory. I'll certainly try the diagnosis with the auxiliary pump. I guess I was just hoping the monovalve was the issue since it seemed much easier and straight-forward to diagnose and repair.

Steve

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Ferdman's picture
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Steve, if the engine water temperature gauge barely moves the thermostat may be stuck open not allowing the engine to reach operating temperature. Even in the coldest weather the engine should reach operating temperature ... though it will take longer than if the ambient temperature was a balmy 80 degrees.

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vlayton wrote:
This bit about disconnecting the monovalve electrical connection = full heat is something I've not heard of or tried. In my experience, when the monovalve fails, there is no/little heat.

Yeah, when I first read it I thought that disconnecting something to make something that wasn't working (the heat) to work sounded a bit goofy. But when I read what I thought was the same thing in several posts, I began to wonder.

Thanks for your sage advice. Sounds like going ahead and replacing the monovalve element may, at the very least, be a good bit of preventative maintenance if it doesn't, in fact, cure the problem.

Steve

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Ferdman wrote:
Steve, if the engine water temperature gauge barely moves the thermostat may be stuck open not allowing the engine to reach operating temperature. Even in the coldest weather the engine should reach operating temperature ... though it will take longer than if the ambient temperature was a balmy 80 degrees.

While the temperature gauge "barely moves" I am confident that the thermostat is functioning properly. The gauge does move as much as it ever has for this weather and for the length of time it has been running for as long as I have owned the car (many years). It has always thrown out plenty of heat in the past. Regardless, even if the thermostat was malfunctioning I would expect there would be some, albeit perhaps minimal, heat being produced through the vents. There is virtually none, nada, nil, zilch. It is nothing but a continuous blast of consistently cold air. Thanks for the suggestion.

Steve

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vlayton wrote:
Gets to 85-90 in short order and sticks there?

Yep. That's about right. The thermostat was replaced a few years ago when I had the water pump replaced but the old one was the same. This has never been any issue other than a curiosity. I've also got a 1987 560SL and the gauge on that one consistently moves to the center of the range (normal), but this one stays near the bottom of the normal range in cold weather and moves only about 1/4 to 1/3 of the way up the normal range even in very hot weather. I've always thought it was just a "cooler car" (pun intended) than the SL. Like I said, though, there have never been any operational issues related to this for the many years I have driven it every day, to include any heat problems.

Steve

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The monovalve default is open (heat) that does not mean they cant stick open or closed, what is te temp of the engine (not guage) the circulating pump does not come into play unless you go to the (rest) option)

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Don350SDL wrote:
Benzbum,

What I wrote and what you wrote agree (read them carefully): The monovalve regulates the temperature by allowing or blocking flow of hot coolant into the heater core. The monovalve's default position is open, meaning it always allows hot coolant to pass (when not energized). From that comes the symptom of full heat, which is what I have always experienced when my car's monovalves went south.

Always in the summer, of course, so the A/C would be warm. :D

Peter makes a good point, though, in that it can still get stuck open or closed. It's just that in my experience open is the more likely.

Don

My mistake. In a hurry, as usual, and didn't read carefully. Logic seems to confirm that the problem is the monovalve:

1.) If the engine is warm; and

2.) If the dash controls are all set for full heat; and

3.) If the blower is moving air; and

4.) If no heated air emanates from any of the vents; and

5.) If the default position for the monovalve is open (full heat) when not energized; and

6.) De-energizing the monovalve (which in a properly opearting monovalve should force the valve open = full heat) results in no heat; then

7.) Must be an issue with the monovalve.

Now I'm feeling real confident that the problem is the monovalve, only now another wrinkle.

I drove the car all day today with the monovalve wire disconnected. No heat all day long (blower blows to beat the band but only cold air). Tonight I'm driving home. The car is at normal operating temperature. I'm about a mile from home when suddenly heated air starts spewing out of the vents. One second, ice cold. The next, a veritable blast furnace! Do these monovalves unstick themselves or should I start looking elsewhere?

Steve

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Heater control valve

Steve,

I had the same symptom,hot then cold,it seemed heat would come out
when I pulled off the highway onto slower road.
My parts supplier,who was a parts manager for MB dealer here in north Carolina,
told me it was my heater control valve,which is the same as what is described as a
"monovalve". The part was $38.00.
Very easy to change, 10 minute job.

James

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vlayton wrote:
Gets to 85-90 in short order and sticks there?

Actually, it only gets to about the second mark on the scale or about 60. The first mark is 40 and the third mark is 80 - mine only gets to about the second this time of year. The manual doesn't indicate the normal operating range and only talks about problems if the temperature gets too high.

As I stated in my earlier post, this has always been the case with this meticulously maintained vehicle. I have over 300,000 miles on the original engine and transmission and it has never given me a single problem. Everything works except the dash clock (and now the heat), to include the cruise control, power roof, A/C, etc. The water pump was replaced several years ago as a precaution when weeping and dampness first started around the seal. The thermostat was replaced at the same time, but it had always operated in this same temperature range. The coolant is monitored and changed according to Mercedes specs by my mechanic using only Mercedes coolant in the appropriate formulation. This was last done a little over two months ago (late September). This vehicle is driven every single day. It has never left me stranded and has never been towed for any reason whatsoever. The engine leaks no oil, all seals are tight and the engine looks as clean, if not cleaner, than the one on my 2001 E320 4-Matic wagon that only has 35,000 miles.

Steve

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Okay, I called my local dealer yesterday and had him pull a monovalve repair kit for me (the parts counter is closed on Saturdays) and I dropped by first thing this morning to pick it up. It was a bit more than $40 ($65 plus tax with my MBCA discount). As stated in previous posts, it was a fairly straightforward process to replace. I did have to unfasten the monovalve from the firewall in order to unscrew the back two screws and remove the valve assembly. It did take more than 10 minutes (more like 30 minutes, but I was taking it nice and slow).

I got everything put back together without incident and drove it into town and back to run a few other errands. Voila! With no warm-up, I had only driven down the road a little less than a mile before the heat cut-in right on cue. The cabin warmed up very shortly afterwards with a nice strong flow of heated air before the automatic climate control kicked-in and cut the fan speed and heat output back accordingly. I was gone a couple of hours, made quite a few different stops running my errands and everything works flawlessly.

Thanks for all of the advice, assistance and encouragement. Now on to my next issue - cosmetic body/paint issue - but that is probably best left to another forum.

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I just replaced the monovalve on my R107 1987 560SL two days ago.

Symptoms:

a) car's operating temperature was just fine.
b) vents/defrost/AC/blower speeds operated just fine -- air goes where it is supposed to
c) BARELY any heat -- and what there was was lukewarm, and only after the car had been running for a very long time -- 10+ miles

I removed the monovalve and it was IMMEDIATELY apparent that it was the problem -- the rubber diaphragm was totally cracked and ripped.

I replaced it with a new Bosch unit (DO NOT get the cheap eBay units for $25) that cost approx $50. Again, IT IS IMPERATIVE THAT YOU GET THE HIGHER QUALITY BOSCH UNIT. I got mine from my indy shop -- they have a good supply of them in stock. DO NOT SCRIMP when it comes to these parts. Spend the extra $15 -- in the long run it will be worth it.

Spent 5 minutes reassembling the monovalve assembly. Ran the car and within 1 mile I had heat starting to come out the side vents. Within 3-4 miles I was toasty hot in that car with the soft top up.

Difference between night and day. This is one of the EASIEST DIY jobs that a person can do, and you will thank yourself for restoring properly operating climate control system in your 123/107/126/124.

The monovalves in the 123, 107 and 126 are fairly inexpensive and replaceable, but they tend to fail after a period of time. The 124 units are MUCH more robust and don't fail as often, but when they do are much more expensive to replace.

Anything beyond a 124, I don't know if they would even use a monovalve, or what form it would take.

Cheers,
Gerry

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