Climate control help - PeachParts Mercedes-Benz Forum

tenknots · #1 11-17-2014, 09:02 AM

I have a 1984 300SD with a frustrating heater problem I'm chasing.

I only get hot air no matter where the temp wheel is set UNLESS I go to the cold detent on the wheel. Then I get cold air. Anywhere else, I get hot air. Fan works properly, though I seem to get lots of air through the defrost vents. Moving the temp wheel to colder does direct air to the center vents though it's still hot.

Here's the weird part: If I disconnect the two-pin connector from the monovalve and meter it, I get 12v most of the time. 12v is supposed to turn off the monovalve and not allow hot water through it. If I got to defrost, I then get 0v as it should and get lots of heat. Any other setting (aside from the cold detent) gives me 12v though often if the wheel is set for cooler temps, I get 7v that switches to 12v every few seconds. But I still get hot air.

Now weirder: If I run 12v directly to the monovalve from the battery, the heat goes off. If I plug the monovalve back in, even though it showed 12v when I metered it, the heat stays on.

I've replaced the monovalve insert. I verified its operation in my hand by giving it 12v. (I wonder how hard the valve should move - I can overcome it by hand easily). I know it works when giving 12v either directly or by using the defrost mode. But anywhere else, I get hot air, even though I metered 12v at the connector.

I've swapped out the CCU once and got the exact same thing. I bought the CCU used from a seller who said it worked.

Is there a component somewhere I'm missing?

How can the monovalve get 12v and still give heat - then when I give it battery 12v it turns off like it should. And why does the cold detent on the wheel turn it off like it should, but no other setting on the wheel does, despite the fact that when I metered it disconnected, I got 12v at the monovalve even when the wheel is set to cool?

If anyone can shed some light, I'd sure appreciate it!

uberwasser · #2 11-17-2014, 10:08 AM

I happen to have this exact same symptom.

For a quick overview, here are the main components in the climate control system:

Push button climate control unit (CCU)
Monovalve
Temperature regulator (silver box behind the passenger side kick/hush panel)
Blower relay (black box right next to the temp regulator)
Blower resistor (toaster coil looking item - not sure where it's mounted on the W126)
Cabin temp sensor

Checking for 12V at the monovalve is not really a good test, since it's opened and closed by varying the GROUND not the power.

When in most any setting on the CCU's temperature wheel the amount of heat supplied to the cabin is controlled by the temperature regulator based on inputs it is receiving from the CCU, cabin temp sensor, etc. It varies the ground side of the connection to the monovalve to flutter it open and close (grounded = closed, un-grounded = open) in order to get the desired flow of hot coolant through the heater core.

When you put the temp wheel in "MIN" or the cold detent as you call it the push button climate control unit or CCU bypasses the temperature regulator and provides the monovalve with direct ground to one of the chassis grounds, I think the one behind the instrument cluster. This shuts the monovalve entirely.

I'm not sure, but I am making the logical jump that in Defrost the CCU bypasses the temp regulator in another way, by cutting 12V to open the monovalve as a sort of fail safe, kind of like how the system is designed so that even with a dead vacuum pump the defrost vents default to open for safety. But otherwise as you're finding you will see 12V at the monovalve pretty much all the time.

On my car I have recently installed a Programa rebuilt CCU that I got lucky and found in a junkyard car. This is about as close as you'll get to a "known good" CCU. Swapping it in did NOT change my symptom of all heat in any other setting but "MIN". So the CCU was not the problem, and I think logically would usually not be the problem in this particular symptom based on my understanding of the system.

At this point, I feel there are really only two possibilities. A bad ground somewhere on the car that the temp regulator is trying to use to close the monovalve (would only apply in the event the ground point is different than the ground point the CCU uses for in MIN), or a faulty temp regulator. I'm leaning towards a faulty temp regulator.

tenknots · #3 11-17-2014, 01:47 PM

Thanks, uberwasser. It's great to hear from someone with the same problem. Maybe we can figure this out before summer!

I have a schematic for the climate control system and I don't see any reference to a temperature regulator. It says the monovalve is controlled directly by the "pushbutton switch unit". Maybe mine is not correct? if it is, the monovalve gets power from fuse 15 and ground (or lack of) from several sources including the off and defrost buttons.

If there is in fact there is a temperate regulator, I'd be suspicious of it in my case. I know there is a silver box attached to the blower under the passenger kick panel, but assumed it was the fan controller.

The other possibilities as you say are a faulty ground, or I'm thinking, a weak solenoid that works ok with direct battery power and the off-button and cold detent, but not with other ground switches. That last one sounds unlikely though.

Incidentally, I did not get continuity between the ground side of the monovalve connector and battery ground as I would have expected, no matter what the setting. A clue? The fact that I am getting 7vdc (alternating to 12vdc every few seconds) sometimes when the temp wheel is set to cool (not cold detent) may point to that also. But why the valve works with the off and cold detent and nothing else is still a puzzle.

uberwasser · #4 11-17-2014, 02:31 PM

It's important to note that we'll have some differences between our two systems as mine is a W123 while yours is a W126 so fuse numbers and the like aren't going to match up.

However, the underlying components are the same for this era of climate controls (the locations are just different, my temp regulator and blower relay are behind the glove box liner rather than down by the blower as on your car).

Here is a picture of a temp regulator (the writing on the front of the silver box may be covered by a thin layer of black felt):

It will be right next to a black box, which in fact is the fan controller i.e. the blower relay. The blower relay is a simple box full of switches that send current to the various coils in the blower resistor to change the amount of voltage sent to the blower. This varies blower speed.

On the other hand, the temp regulator is for lack of a better phrase "the brains" of the system. It interprets your inputs at the CCU, the inputs from the cabin temp sensor, the state of the monovalve, current blower speed, current state of the vacuum switch-over valves, and uses all that info to adjust to your desires.

In reality, the CCU is kind of a "dummy" device. It just passes on your selections to the other systems. In the "no brainer" settings like MIN and Defrost it controls some devices directly since there is no question about the desired output from the system. But in instances where some interpretation of current conditions is required it's all handed off to the temp regulator. That's why it's relatively easy to fix a CCU by reflowing solder at cracked or cold joints - there isn't much going on in there besides point A to B flow of current. The temp regulator, on the other hand, is much more complex and has integrated circuits, capacitors, etc. that would need to be replaced to renew such a unit. As far as I know, no one has done a DIY guide on fixing on up. Programa, who rebuilds CCU's, also rebuilds the temp regulators. I plan to order one soon, since I've eliminated the CCU as a problem with my Programa CCU.

I'm hesitant to upload the wiring diagram that I have because I believe it's specifically for the W123 cars and again the fuse numbers, and maybe even wire colors, may be different and I don't want to steer you in the wrong direction. However, if you cannot find a better wiring diagram for the W126 climate control setup then the one I have may do in the meantime. Check it out below with that important proviso in mind. Click the link, save the image, and then you can zoom in to read the small text.

W123 CLIMATE CONTROL DIAGRAM

uberwasser · #5 11-17-2014, 02:41 PM

A picture of the inside of a temp regulator, for reference. Note the IC's. It would be hard for a DIY'er to identify if one had failed, to source replacements, etc.

Credit to forum member JamesDean for the picture, he's done some rebuilding of these. Not sure if he's offering repair of them as a service at this point, however, like he does for cruise amps and the like.

tenknots · #6 11-17-2014, 03:37 PM

Great information, thanks!

A little studying has me thinking the aux water pump may have some play in this. As a way to eliminate it, I'll unplug and see what happens.

In the meantime, thanks to your advice, I've ordered a temp control unit. Normally, I'd just find one at a salvage yard, but Programa had them on sale for the unheard of price of $25. I'll let you know what I find.

uberwasser · #7 11-17-2014, 06:48 PM

Where did you find that sale? Please post a link, I'd love to get one from them at that price!

tenknots · #8 11-18-2014, 01:11 PM

Maybe I got the last one - I don't see it listed anymore at http://www.programainc.com/, but I did get a confirmation email from them. If it doesn't work for me, you're welcome to it for what I paid plus shipping.

SD Blue · #9 11-18-2014, 05:01 PM

Here is the FSM for the W126: Untitled Document

From what I've seen so far, the W126 is very similar to the W123 for the same year range.

A couple of other things that might be coming in to play. The In-car Temperature Sensor is located above the mirror and a collapsed/rotted intake tube may be part of the problem. The other is the Heater Air Temperature Sensor in the heater housing behind the console. Both of these provide an input (variable grounding) to the Temperature Controller.

On the W126, the Temperature Controller is located to the RH side of the Blower Motor housing. Dropping the knee panel will get you access.

The aux. water pump is primarily for providing flow through the heater core at idle, when the main pump provides less flow.

uberwasser · #10 11-18-2014, 05:09 PM

Quote:

Originally Posted by tenknots

Maybe I got the last one - I don't see it listed anymore at ProgRama Remanufactured Electronic Control Modules 2-3 YEAR warranty, but I did get a confirmation email from them. If it doesn't work for me, you're welcome to it for what I paid plus shipping.

Thanks. Looks like the W126 and W123 parts have different part numbers. Might be related to different temp sensors in the W126? Not sure. Anyway, I'll play it safe and get the part Programa says goes with my car.

And on another note, the one you got only has a 30 day warranty where the one for the W123 has a 3-year warranty.

That all said, I really do hope it helps! I plan to order up a Programa soon and will post the results.

aps13 · #11 11-18-2014, 07:10 PM

I am having the exact same problem on my 83 300SD W126, I am waiting patiently for one of you to solve this mystery for me. I have not worried about it much just due to the fact that 10 months out of the year the AC is the only thing that is necessary in Sauna I have decided to call home. LOL. I have over 410,000 on mine and she is still running strong. Looking forward to hearing what you discover.

JamesDean · #12 11-19-2014, 09:01 AM

I figured I would chime in. I do still offer repairs on these types of climate control units.

More often than not I see cold solder joint issues on the center console push button unit.

The 126 (to 85), the 123 (81-85), and the 107 (81-89) all use this same kind of system. (Roughly for the years).

From my best recollection the temperature regulators and blower motor control units are interchangeable. The push button units are not.

Here are some recent units that I've done.
R107 560SL (1988) Climate Control Refresh - Imgur

W123 300D (1983) - Climate Control - Push Button Unit - Imgur

W126 Climate Control Repairs - 10/25/14 - Imgur

I do not know how much I would trust Programa rebuilt units. I have had a few customers from the cruise control side of this approach me with non-functional systems only to find that the rebuilt Programa amplifier was the cause of the problems. $200+ is a lot to charge someone for something that does not work, or does not last more than a few weeks.

I see Programa is asking $25 for their repair of the temperature regulator unit. That kind of seems suspicious. Most of their work that I've seen has been "fix it so it works" not "fix it so it works and lasts." I know on several of those models that there are a number of electrolytic capacitors that easily total up to $5-10 in parts cost alone (especially the large axial types). You add time to replace those and re-work the rest of the board you are looking at say 30-45 minutes, plus testing time. Nothing I've seen from them (at least on the cruise control side) would give me confidence in their other lines.

It should be noted that I will, soon, be building a test setup for these "3-piece" HVAC systems. Its just a matter of finding the time to design it up and build it. Much of the testing I do right now is in my dad's 300SD. Testing the temperature and blower control units is easy....testing the pbu is a pain in the butt....testing things at these temperatures (14F this am)...is..yeah going to build the testing setup soon.

Anyway, if there's anything I can do to help, just let me know.

funola · #13 11-19-2014, 09:55 AM

Quote:

Originally Posted by uberwasser

.................................

W123 CLIMATE CONTROL DIAGRAM

Where did you find this very legible W123 diagram? The one in my W123 FSM CD is totally illegible.

funola · #14 11-19-2014, 10:04 AM

Quote:

Originally Posted by tenknots

I have a 1984 300SD with a frustrating heater problem I'm chasing.

I only get hot air no matter where the temp wheel is set UNLESS I go to the cold detent on the wheel. Then I get cold air. Anywhere else, I get hot air. Fan works properly, though I seem to get lots of air through the defrost vents. Moving the temp wheel to colder does direct air to the center vents though it's still hot.

Here's the weird part: If I disconnect the two-pin connector from the monovalve and meter it, I get 12v most of the time. 12v is supposed to turn off the monovalve and not allow hot water through it. If I got to defrost, I then get 0v as it should and get lots of heat. Any other setting (aside from the cold detent) gives me 12v though often if the wheel is set for cooler temps, I get 7v that switches to 12v every few seconds. But I still get hot air.

Now weirder: If I run 12v directly to the monovalve from the battery, the heat goes off. If I plug the monovalve back in, even though it showed 12v when I metered it, the heat stays on.

I've replaced the monovalve insert. I verified its operation in my hand by giving it 12v. (I wonder how hard the valve should move - I can overcome it by hand easily). I know it works when giving 12v either directly or by using the defrost mode. But anywhere else, I get hot air, even though I metered 12v at the connector.

I've swapped out the CCU once and got the exact same thing. I bought the CCU used from a seller who said it worked.

Is there a component somewhere I'm missing?

How can the monovalve get 12v and still give heat - then when I give it battery 12v it turns off like it should. And why does the cold detent on the wheel turn it off like it should, but no other setting on the wheel does, despite the fact that when I metered it disconnected, I got 12v at the monovalve even when the wheel is set to cool?

If anyone can shed some light, I'd sure appreciate it!

How were you measuring voltage at the monovalve? The 7 V reading is suspicious. You should see 0 or 12V across the monovalve connector terminals (the correct place to measure). It is also a dangerous place to measure (so be careful) since a short there when there is voltage will blow a trace in the PBU.

An easy test to see what the temperature control unit is doing is wire up a light bulb across the monovalve and vary the temp control dial. If you see the duty cycle of the bulb changing with changes of the temp dial, then your monovalve is suspect.

mach4 · #15 11-19-2014, 10:14 AM

Your problem might be as simple as a bad connection. I was headed across AZ once with the temps in triple digits when the heat came on and was pumping 150 degrees out the vents....not fun. It was the connector. My quickie on-the-road fix was to twist the monovalve wire in such a way as to to put sideways pressure on the connector which was enough to make a connection and get me to my destination. The home fix was to take apart the connector and clean the connectors good and then "crimp" the connectors just a bit with a pliers to make a tighter connection.