W123 Mono-valve Resto-Mod

[BillGrissom]

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.