Climate Control. Seemed like a good idea at the time no doubt. Brian Carlton told me about a 300SDL he would give me except it's on Long Island. Says he likes it but doesn't have the time for it anymore ... and ... said it has a Climate Control nightmare going on.

Mine is continuing to go bad. Long story, could spell it out later, for now I'm in WA state, going to Cal tomorrow. Right now it's heat all the time. Last winter heat was sporadic. There's a hose underneath that goes into the mono valve area, it's plenty hot, my thought is to get some of those same sized hose bits and put a regular old home depot ball valve into the middle of it. For now.

On the way up we had 70 degree weather most of the way. Gets cold at night.

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1986 300SDL, 362K
1984 300D, 138K

I put a valve on all of my cars that don't have a valve. It's the next best thing to AC and the results are stunning.

I also put one on my SL because MB controls water leaving the heater core rather than water entering. And, . . .the stock MB electric valve defaults to open when car is off / power removed, this allows for thermosiphon when the engine is off leading to a hot interior on a restart.

I use a 3/8" pipe ball valve and a 3/8" pipe about 6 - 7 " long cut in half ( be sure to debur / chamfer the ends. The pipe OD is slightly larger than 5/8" but smaller than 3/4" allowing it to work with both sizes. Be sure to leave the pipe slightly rough so the hose has something to grab onto.

Given your heater control system isn't working and MB generally uses water flow to adjust temperature, give some of these below a look. Also have a look on Rock A and the Four Seasons web site, for cable control, use cars from the early 70's and dig around.

FOUR SEASONS 74627 $ 17 73 Ford Bronco
5/8" in line cable operated

FOUR SEASONS 74662 $ 33 72 AMC Matador
5/8" 90* cable operated

FOUR SEASONS 74809 $ 10 99 Ford Ranger
5/8" Inline vacuum operated ( apply vac to close ) w/ 4-Port Valve < used on systems that need water flow through the heater supply and return lines when heater is off.

FOUR SEASONS 74859 {#74622} $16 99 Ford Ranger
5/8" Inline vacuum operated ( apply vac to close ) w/ 2-Port Valve

Generally, but not always. W201's for example use air blending. The heater valves are fully on whenever heat might be required. A vacuum controlled blend flap diverts incoming air either through the heater core, a/c core, or some combination, depending on the wheel setting, outdoor air temp, and indoor air temp. The flap can be positioned at any angle, and a flap position rheostat provides feedback for closed loop control. I'm not sure how the W124 system works, but these models tended to be similar.

W126 has no air blend door. The monovalve and A/C compressor are cycled to control vent temps.

If the Monovalve is working properly, it effectively closes off ALL coolant flow through the heater core when in cooling mode.

Adding a ball valve is a pretty half-assed (and unnecessary) solution. Repair/replace the monovalve control circuit or the monovalve cartridge. The monovalve coil is energized to close it, if the pushbutton unit is acting up it may not be energizing the monovalve.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I think it would be more accurate to say that the ACC runs the compressor full time and uses heat to increase the temp as necessary to maintain the selected temp. Except when MIN temp is selected, in which case a hard ground is supplied to the monovalve.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Ambient temperature plays into what's happening. If the outside air vent is open, the circulating fan is running in medium or low speed, and the ambient temps are low, the compressor will be cycling as well due to the coil being cold enough to trip the evap temperature sensor.


If the cabin temp is not reached (too hot for the setpoint) and the compressor is commanded on, the monovalve is held closed until the ACC goes into temp regulation mode.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

The compressor does NOT cycle for the purpose of regulating temperature. The low temp cut-out switch exists only to prevent the evaporator from freezing; temperature regulation is not its function.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

I would rather have a functioning Climate Control thing the way God intended it but there are a few things I'm not getting yet. I found a vid about the monovalve replacement, can't find it again. The guy was testing the current at the pins on the monovalve cartridge. He was getting full voltage at both ends of the hot and cold spectrum on the control wheel. He proceeded to swap the monovalve.

I didn't quite get that. If the current should be lower in AC mode, or heat mode, I forget, doesn't seem that the monovalve will dictate that.

Seems something in the dash might be remiss.

I did a dicey thing on buying a monovalve replacement. I've gotten BMW parts at a good price from a parts dealer in Berkeley - he's also a font of knowledge so it's useful to buy from him. He can also get Mercedes parts - I asked him about the monovalve, gave him the part number, he came back and said they used the same part for some BMW heaters, had had no bad reports from them and that he had a few on the shelf for $40ea.

The brand was Ronac, he said MTC bought them and he was under the impression that quality had gone down under the new owners. I like this guy and like to support him so I took a chance and bought it

But, oops, no, not a good part. That's why I have full time heat now.

I'm almost wondering if the whole assembly needs replacing as electromagnets get tired and weak with age. The gas turn-on switch in gas dryers is electro magnet activated and they routinely go bad with age.

__________________
1986 300SDL, 362K
1984 300D, 138K

https://www.startekinfo.com/StarTek/outside/12253/program/Climate/83-600.pdf

Page 9 starts the monovalve discussion.
It says the monovalve is operated by switching ground/minus.
When it is ON it is CLOSED. When it is OFF it is OPEN.
No current, valve is open and coolant flows through heat exchanger.
Sounds like it opens and closes periodically when temperature is being controlled.

__________________
1992 300SD 140.134 Sold

MTC is a known-bad part. There are hundreds of reports of the MTC cartridges not working from the day installed here and on various other forums. Your first recourse should be to replace with an OEM cartridge and see how things fare. They've gone up in price since the $35 days, but it seems to be about the only reliable way to get a working cartridge. You can also swap the diaphragm and rubber parts from the MTC cartridge to an OEM core assembly for a working solution, but that requires having an OEM core to swap the parts onto.

Electromagnets will last forever if they're not overdriven or abused. The electromagnet coils in gas dryers fail because they are marginal to begin with and are in close proximity to a heat source. They suffer insulation failure or simply burn out. DC coils last considerably longer than AC coils do as well, so you shouldn't be worrying about that part of the system.

As wwii pointed out, the ground side of the monovalve circuit is switched. If the key is on and the ACC is powered, you will have 12V present at one of the leads all the time. Since it is the ground side of the circuit that is switched, the only way to test it is to leave the coil in place and test each wire to the chassis with a multimeter. One will read 12V, one should read near 0V. If both wires read 12V referenced to the chassis, you know the ACC is not switching.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

An alternative to the monovalve:
https://klimakit.com/product/monovalve-and-coolant-circulator-pump-eliminator-upgrade-kit/

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

I used an Audi heater valve on my w203. It made the AC a lot more effective. $3 I ever spent.

__________________
CENSORED due to not family friendly words

Something to think about all right.

I'm wondering how hard it is to access that stuff between the two firewalls. Also their page refers to 'most W126 through '85. My SDL is an '86. I think it's the same though.

__________________
1986 300SDL, 362K
1984 300D, 138K

You've got a lot more knowledge of electro magnets than I.

I attached the neg lead of my voltmeter to batter neg with a jumper cable. One lead reads battery voltage and the other .49v no matter what I do with the hot and cold control. I suspect any testing of it apart from 'does it operate' the system will not be easy. I could perhaps get the heater control module off a gasser with half the miles. If the monovalve or the Klima replacement doesn't yield good results I could always try new control parts.

Do you know anyone who has used the Klima?

__________________
1986 300SDL, 362K
1984 300D, 138K

Don't use a digital meter for voltage diagnosis, it can pickup stray / leakage voltage clouding testing.

Connect an incandescent test light across the harness terminals, plug the valve in and see what you get.