Like most, the climate control in my 1984 300D was always mystical, particularly the AC would blow cold then hot, then sometimes get cold again. This was even when clicked "max cold" on the thumbwheel, where the mono-valve is forced "on" (i.e. closed, control wire shorted to ground). I long suspected the mono-valve was erratic. Pretty sure I had re-gutted with $25 MTC parts, but since I have another 300D I lose track. I know I never paid $300 for the "correct" Bosch guts, and recall fussing with the guts in both cars many times. At least once the diaphragm tore to leak coolant, which is another downside of the factory design. I have tried clamping off the heater hose above the oil filter but that never seemed help give cold air for some reason, unless water was still flowing past the clamp.

On my 1985, I plumbed a later W124 mono-valve since more available, placing it where the silly "aux water pump" sat (unneeded and long-gone), replacing all firewall heater hoses with silicone hose and tee. But, before I ran the wiring to it, my daughter ran a red light and I haven't replaced the crashed fender and hood yet, so no experience.

Since then, I installed a vacuum-operated water-valve in my 1965 Chrysler (not factory AC), using new parts, so figured ditto for my 1984 300D. I know M-B did so in later cars, and one can buy a kit to retrofit a 1984 thus, though M-B parts aren't cheap. Instead, I used a generic new water-valve ($8 ebay) and vacuum-switch from junkyard 1980's Dodge (have box of them). You could use a M-B vac-switch, which they term "switch-over valve" or "code" for the interior climate ones (strange translations). That is easier if the connector matches the mono-valve (unlikely). But, M-B connectors are big and robust, so easy to unsolder the pins. I soldered on new wires w/ heat-shrink and sheath for the vac-switch. On my Chrysler, I used a cute little vac-switch from a Porsche bought years ago on ebay ($12), so include that photo. As always, I used purty blue silicone hose for both coolant and vac-switch to last a lifetime.

The new water-valve is NO (i.e. Normally Open w/ no vacuum, spring-return) and the M-B logic is "ground to close mono-valve", so I plumbed the SPDT vac-switch with common to the water-valve, NO (port normally open) to vent (white fuel filter for small gas engines), NC (port normally closed) to vac-source (tee'd off main supply). Electrical wiring is same as the mono-valve: constant +12 V one terminal, low-side switched other terminal. The vac-switch worked with either polarity. I used the same tapped firewall hole for the mono-valve bracket, w/ strap to the water-valve. The 5/8" hoses are a straight shot. I show my long-gone aux water-pump, replaced w/ 5/8" hose. Note I installed a 1985 CA frame-mounted air filter in my 1984 to eliminate the air-filter-bracket-cracks problem. I had to cut the outlet tube 1.5" shorter to match the lower inlet of the turbo-charger than in a 1985 CA (my other 300D).

I like that the water-valve is exposed so no longer a mystery what it is occurring. I wondered if the mono-valve was controlled on-off or as a proportional voltage (partially open valve). One might trace the control circuit to know. If the last component is a mechanical relay, then surely on-off control. It appears on-off control, with pulse-width modulation ~3 sec period, since the hot-water valve opens and shuts continuously with thumbwheel in white region on 80 F sunny day. I wonder if such constant on-off might wear out the water-valve since not designed for that, but I usually click "max AC" on the thumbwheel since that barely suffices in our >100 F sunny summers which keeps the water-valve shut. I only tested it on one drive so far in early Nov, but seems like I get full AC now. If true, that means the rest of the clutter of electronic boxes and vacuum-switches in the over-complicated analog climate system are probably working correctly (a bridge-too-far in the 1980's).

I should add, that by the time I finished typing this and tried to add attachments, PeachParts had logged me out. But, I was clever enough to first cut & paste to notepad so didn't lose my effort. Not the way to treat people who contribute free content. Perhaps they think all posts should be useless 140 character drivel like most websites, but we try to be more detailed here, so please give us time.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Very nice write up!

Looks very nice and a much needed change for these cars. I'm going to try your solution and see how it goes.

This is something i've always pondered. The factory mono valve is a turd!

Proof that you actually can cleanly get rid of the monovalve setup!

And... If you're not running R12, it'll probably help your AC. It's made many Nut Sac summers more bearable...

I'm not sure how this set-up could negatively affect R12.

THANX Bill ! .

I'm O.K. but my brother's W126 has an issue where the heat is normally full blast through the dash vents, we spend a lot of time in the Desert and he doesn't like the windows open so it's miserable .

We're about to take another road trip and I sent this to him, maybe we'll try it when we get back .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I have even seen people plumb in a manual water valve to shut off hot coolant in the summer months. One photo was even a home cut-off valve with hand-wheel, but don't need to be that silly. As an initial step, install a vacuum-operated valve like I did, and plumb the vacuum to the main vacuum source to keep the valve closed all the time in summer. You need that plumbing anyway, then you can add the vacuum switch later.

I drove the car again a few days ago and needed heat, but the vents were blowing slightly cool, even with the thumbwheel clicked to max red. Under the hood, I saw the coolant valve full-open (no vacuum) which was correct for max-heat, but the AC clutch was engaged. That tells me my AC is working great since it was beating the heater. BTW, I use Duracool refrigerant and the factory Harrison R4 compressor. At least I can now see what the coolant valve is doing, unlike the mystery with a mono-valve.

Thus, there is a problem in either the push-button box or the TempRegular box (above glove-box) or their sensors. Not the first time I've been "in there" and have many spare boxes. Seems I'll need to go bigger and rig up cables to test all I/O signals to these boxes. I think I changed the cabin temperature sensor already, but might have been in my 1985. An electrical engineer on here (James Dean?) used to rebuild the boxes for a fee, replacing all the old tantalum capacitors as that is often the problem. The system is an early automatic temperature control, using mostly analog electronics, and proved "a bridge too far" in the 1980's.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Thanx Bill ;

I copied and sent my brother your entire tech post with pictures, no comments from him yet .

I've been trying to get him to let me install ball valves for a while because that's the cheapest fix but he's an odd one (pot/kettle etc. ) and often leaves things un repaired, especially when it makes riding in his vehicles like wearing a hair shirt .

Go figure .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

BG:

1) In your opening post you indicate that the water valve is a generic unit; do you know if it had an OEM production application, or is indeed simply a "water valve, vacuum operated" that was not ever OEM?

2) Have you investigated the response of the valve to varying vacuum strength, i.e., will it behave as a proportional valve, or does it tend to jump from fully closed to fully open with only a small change in vacuum signal?

3) Do you know if the valve would be suitable for use on the hot (feed) side of the heater core, rather than the MB location on the cold (return) side?

Great write up! What silicone tubing did you use for your vacuum lines? Did that replace the rubber connectors and the plastic tubing?

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Theseus

Instragram: doss_project
1984 300SD - 930 Blue, OM 617
Murfreesboro, TN

Theseus owned a ship in Greek mythology, and when a part of the ship needed replacing, it was replaced. Eventually, everything was replaced. Is it the same ship? (I own the car of Theseus. )

I've ordered the new Klimakit mono valve replacement. I like the direct swap ability. Yes, pricy. But I think Klima has done a nice job in mocking up a new valve body and using newer off-the-shelf solenoid components.

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'85 300TD "Puff The Magic Wagon" - Rolling Resto
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter

Nice job. Looks like it’ll last. I think we are going to be doing more jobs like this as time goes on. It’s hard to believe the cars are 40 years old now.

I just got the updated URO mono valve kit from Pelican. I put it on the shelf but I intend to drop it into my SD at some point.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

As post 12 states, later M-B used this same system of vacuum-switched coolant valve. Klima kits those parts, if you want to stay M-B. I guess that M-B doesn't make such parts, just brands them, so not important to me.

Re post 9 questions (sorry, just saw this):

The ebay listing didn't specify applications. They were selling a bunch, so likely a liquidator who got them from say a trucking incident sell-off or bankruptcy. The PN is in the photos if that helps. In my 1964 Valiant and 1965 Dart, I did similar, using a small plastic valve off ebay listed for ~2010 Ford, only $8 also from a liquidator, but I had to fab a metal bracket.

I did test the valve with a hand vacuum pump, just to insure the diaphragm didn't leak. As I recall, it sort-of popped open, similar to the way the door lock actuators in our cars do, though those are push-pull vacuum instead of working against a spring.

It would likely work on the supply side of the heater core. I say this because in my 1960's Chrysler cars, the two heater hoses run parallel from the front of the engine to the firewall. With factory AC, one was run over to a coolant valve on the inner fender. But since their taps on the engine are close together, as are the two heater core tubes, I am sure many people mixed up the hoses. I've never heard of the valve failing due a mix-up. Perhaps even the factory workers mixed them up (Billy-Bob on graveyard shift).

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Bill ;

Those valves have two different size hose nipples to prevent mixing up the hoses .

I'm sure this was a boon in the assembly plant but when working on older beaters this means it's no simple thing to by pass a leaky heater core .

The good news is : these valves are available new in cable and vacuum controlled configurations making them ideal for replacing the EVIL SERVO FROM HELL on Klima I equipped cars....

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I've had the new Klimakit valve in my hands to install on the W126 300SD. It looks to be very well designed and appears to be a drop in replacement. I'm waiting on a replacement since the Postal Service used it for training purposes. The electrical connector was broken off. Klimakit is replacing it.

On my '76 450SEL, I had a ball valve for heat control. Shut in summer, cracked in spring and fall and mostly open in winter worked just fine. Temp was controlled by switching fan speed and which vents the air came out.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120