Hi,


I'm borrowing an '85 w123, seems to have a few climate problems.


When air blows out, it only blows defrost. When anything else is selected it either doesn't come out or goes to defrost. I can hear the blower motor working, just no air.


When cruising down the freeway, almost no air comes out. As soon as I reduce speed on an off-ramp, suddenly all the hot air blasts out-again, only to defrost.


I have a w126 300SD parts car with a good climate control system that worked when I parked it 6 months ago. Are most of those parts interchangeable?

Thanks!

There's 5 electric solenoids that are controlled by the Climate control system, those solenoids switch vacuum to 4 or 5 vacuum pods in the dash. I believe the fail-safe is to blow to the defroster. Chances are you have a leak in the line going to the climate control system from under the hood. It passes through the firewall to the left of the brake booster (looking from the engine bay) but I don't remember off-hand what color the hose should be.
In order to get my system working, I've completely replaced every single vacuum pod in the car, as well as all the vacuum lines and fittings, and tested/replaced the solenoids (Switch over valves/change over valves I think they're called)

As far as parts interchangeability, I'm not sure.

Your evaporator housing is probably full of gunk causing the fan to have problems. Cleaning it is really going to involve a huge amount of work.

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

This is usually a problem of broken solder joints in the Pushbutton Controller. The circuit boards are arranged in an "H" pattern and they develop cracks in the solder where they meet.
Here is a thread talking about the problem. (posts 37 & 38 have pics)
DIY page. Introduction to Resoldering Climate Control Unit
If your not handy with a soldering iron, and don't know someone who is, an alternative is one of the rebuilt ProgRama units.

As far as interchangeable, I'm not sure as there appears to be different part #'s for the W123 & W126.

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Sam

84 300SD 350K+ miles ( Blue Belle )

All the above...and;

Your center vent vacuum diaphragm may be leaky or you may have a big leak in the system leaking away all your vacuum.

Best place to test is right behind the climate control.

I just refurbed my system. Resoldered joints in Climate control unit, changed center vent vacuum pod and replaced two open solenoid valves (1&3) on my 85. Check posts 92 and on. Start by removing the climate controller and check each valve electrically and with a vacuum pump. My 1&3 looked overheated. I suspect these are energized more often than the others.

Got a 1985 300D turbo

Your speed dependent hot air issue on the off ramp sounds like a monovalve issue. Take it out and inspect it for tears.

Good luck.

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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

My solution:
> Replaced factory climate control unit (CCU) with *MUCH* better Programa unit ($).
> Tested each vacuum element with a MityVac. Any vacuum element that would not hold vacuum was replaced ($). See below**
> Tested each of the five switchover valves behind the CCU using 12v to trigger open/close. Any switchover valve that did not function properly was replaced ($).
> CAREFULLY STUDY THE FACTORY SERVICE MANUAL AND UNDERSTAND HOW THE CLIMATE CONTROL SYSTEM OPERATES BEFORE ONE ERRONEOUSLY CONCLUDES THAT ONE CAN JUST "FIGURE IT OUT" AS ONE GOES.

**Lastly and for your own sanity and pocketbook, be smart with the MityVac. Don't fall for the completely stupid advice posted here and elsewhere that a rubber vacuum line used for ANYTHING can be completely plugged with a golf tee or screw. What's the downside? Say, the time and expense of replacing a vacuum element that was actually holding vacuum but because of the tee/screw, was falsely determined to be leaking.
How to properly plug a rubber vacuum hose (whether it is part of your diagnosis process or used in the system)? Take a piece of new hose to Ace Hardware and find a solid metal plug that is hard to shove into the hose. Buy a couple and use them as needed.

Another MityVac tip: When you pull vacuum, don't just watch the needle stay at some spot for a few seconds and call it good. No. Pull the vacuum, let the needle settle, wait a minute or several. Needle dropping slowly? Nice to know...

Based on my years of diagnosing CCU problems on two late model W123s, the process involves a significant amount of time, care and patience plus tools and money.

Peace out.

If you're replacing your CCU - make sure to either add a very small fuse to the auxiliary water pump (2a I think - but don't quote me on that) or unplug & bypass it entirely. If that pump seizes, it will draw too much current and burn out traces in the CCU.

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

Where are these five electric solenoids? I found that my '84 300SD parts car climate control unit is the exact same as the '85 300D. I just had to move two nut clips over to the new unit, everything matched up perfect.

That parts car climate control unit worked great, but no change occurs when installed on the new car.

Having driven the car a lot more, here is an update of symptoms:

Speeds below around 60 MPH, air seems to only come up to defrost. And it's only hot, no matter where I spin the wheel, unless the wheel is to the blue and i'm in the EC mode. In any other mode even when selected cold temps, only hot air blows out, and only through the defrost vents. Above 60 MPH, very little airflow comes out defrost, and not very hot anymore. I freeze on the freeway, and when I jump off suddenly I'm opening windows. It's awesome.

5Cylinder-thanks for this response. I want the quick and easy fix!!!! Obviously I probably won't find it. Being a loaner car that, when I'm done with it I'm going to sell, time and money are not things I want to invest in this car. My time and money should be going to my w115 I'm doing an engine swap on and other things...

UPDATE.

I found the solenoids. I made a test wire harness from the battery, found solenoids #1, 4, and 5 dead. I went to my 300SD parts car and found slightly different looking solenoids, but same wire connector, two-pin, and two vacuum lines-I figure same function. They bolt right in-but the vacuum connection is loose so that's no good. I went ahead and replaced all and started the car. I now have blower out of the two side vents! Still nothing from the middle two. But I also got a temperature change in a mode other than EC so that was awesome.

OK, for my loose vacuum lines, just slightly loose, I'm going to put small zip ties around each fitting for a better fit...think that will work? Any other ideas? My other thought was safety wire but I thought that would cut through the hose.

Definitely had major vacuum leaks because the car wouldn't shut off after that, had to press the button under the hood.


Thanks...

Went through this recently. Post 99

Got a 1985 300D turbo

Sounds like you need to check your center vent vacuum pod. The easiest way is to mity vac test it right at the solenoid. The hot air blast when slowing down is likely a torn monovalve diaphragm. Easy to disassemble and inspect.

Zip ties work great. Check out my w126 center vent repair here. Not quite the same as the 300d but same general idea.

Fixed AC center vent flap vacuum pod 300sd w126

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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

Appearance can be deceiving. The W126 has more flaps (therefore more vacuum actuators) than a W123. The CCU's may look the same, but they are not identical.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.