Something wrong with w126 climate control system?
 

Idk what exactly is wrong with the system, but I'll describe several problems with it that I experience- they may be related? First, with the three sliding switches directly under the front center vents: the one on the left seems to not do anything- whats it supposed to do? the one on the right also seems to not do anything- is it supposed to open a door in the air system, because it doesnt seem to affect the airflow in any of the vents- maybe both the left and right switches are broken? Only the switch in the center works- it controls air flow in the center vents- when its closed no air, but when open, fresh air always blows in, even in the winter when it's on recirc and I want only heat to blow in through the vent. The passenger side front air vent always has air coming out when the blower is on- no switch can stop the air flow. The driver side air vent never blows air out- no switch can turn that one on. Also, a smell of gasoline seems to be coming in somewhere through these vents- idk where from. I don't think there's anything wrong with the climate control electric module, but if this makes any difference, only when I turn the climate temp switches for the driver and passenger sides to below max heat, when I'm driving, sometimes it sounds like the blower motor or something around the glove compartment makes a buzzing noise, but it never does this when the temps are switched to max heat. Is there a fresh air door that's jammed or something? Is there a network of air flow tubes that has messed up doors (either stuck closed or open) or something? Does anyone have a diagram of the w126 cabin air system? Can you tell me any more info about this complex air flow system so that I can take it apart and fix the problem? Thanks.

Welcome to the forum, and to the wild and woolly world of W126 climate control......

Okay, first those vent switches on the center console..... The left and right sliders are supposed to control air flow to the vents on the far left and right. Sort of counter-intuitive. One would think it would control the center vents, but no.


Anyway, if you can't get air out of the center vents, it's probably the vacuum pod that controls that flap. The whole thing is controlled by engine vacuum, and it's a goofy setup. The electronics box sends signals not only to the blower, compressor, and monovalve, but also to a vacuum operated valve bank that uses electrical solenoids that control the vacuum signals to the flap pods......The electronics box decides which flaps open, and it's done depending on the settings. For example, heat will never come out of the center vents when functioning correctly. Only the sides, floor, and defroster.

Standard repairs to get it working would probably replacing the vacuum pods, and perhaps seeing if any of the manual flaps are damaged....... You will probably need to do your monovalve diaphragm at some point, as this controls the flow of engine coolant into the heater core and when bad causes heat in the summer and none in the winter.

The gasoline smell is worrisome, I would have your fuel system inspected soon. The blower intake is in a spot which tends to suck in engine smells such as oil and fuel leaks........

I have been sliding those levers for a year wondering what the left and right ones do. Bless you tiny Panzer.

If anyone feels like pointing me to the next logical step..

My compressor does not kick on.

I replaced the Push Button controller (it wasn't even telling the blower to come on)

Freon Level is at Capacity according to my mechanic cousin who hooked up his freon tank to it, High low pressure was good.

I attempted to bridge the circuit on the dryer...nothing.

83 300sd

A/C relay? Fuse?

the fuse in the box is fine. there are two what look like relays on either side of the blower motor under the glove box. one is aluminum and the other is black. I unplugged the aluminum one while the blower was on and now the blower won't work either. Joy joy joy.

You probably just blew the blower fuse, which is located near the main fuse box, but in a separate little black box off to the side that contains a single 40 amp fuse strip. Those usually go bad anyway. Couldn't find one at the MB dealer, but a VW dealer had it.....

So you replaced the CCU huh? Did you get a new one or one from a junkyard? If it's new then we can rule out other CCU issues....

As to other fuses, they can appear good and still be bad if they are the originals. The old style German fuses get oxidized on the ends and stop working. Try spinning them around a few times in their sockets to chew off that oxidization.

If still no luck, hook a volt meter to the AC compressor clutch wires. See if there is voltage present when you turn the AC on. If not, it's probably the relay, or possibly the high pressure cut off switch, assuming the CCU you put in is good. Also, try this with the temp wheel ALL the way to cold until it CLICKS..... This will rule out a bad interior temp sensor.

If there is voltage, but the compressor still won't kick in, then the clutch is bad.

PRINT! and off to find a voltmeter

Interesting
I changed out all of my cylinder fuses when I got the car.

My AC is not blowing cold...it is the euro ac so its not climate controlled
I don't know if the compressor is working..nev er checked under the hood yet.

did notice the #11 fuse a blue one controls the blower
I twisted it a tad but they are new & it looked ok

I did not know that there was a blower fuse or relay for the compressor.
Maybe this went bad.

You mentiion one off to the side of the big black fuse box...
I'll have to check..would a 83 500 SEC euro have one?

I'm not all that tech but would like to try before bringing it into repair
it would be nice if it was a fuse



Thanks

I just don't know. My knowledge of these cars is limited by which one I own, and the handful I have worked on.....

Compressor kicks on!!!
 

1983 300sd converted to R134a by PO. Ester Oil.

It was the ETR, I tightened the small brass screw on the etr and now the compressor kicks on.

The expansion valve gets cold and condensation forms on it. The low side pipes get cold.

I hear constant clicking at low RPMS from inside the cabin, I can't hear it when I am outside the car, even with the hood up.

the air coming out of the vents is ambient temperature.

When I push the middle button, I hear the vacuum change and the blower speed increase. The air is ambient temp.

What would yall do from here?

Next
 

The fresh air vent is behind the glove box, it wasn't shutting all the way. I have "propped" it completely shut/open (no fresh air) until I get the AC completely functional. I'll replace the vacuum pods once the Super Don is a meat locker.

Monovalve is the next thing down the line.

It is located next to the battery in the engine compartment.

Monovalve is perfect. I could sell it on ebay.


Next thing is to try and clean the evap.

Is there anything else?

If the clicking is erratic it may be the switchover valve: a black box where all the vacuum lines attach and which operaqtes the various blend doors which direct the air flow to different places. Mine clicks at first, until the car has run a little while. Why I don;t know.
If it is steady, it may be a simple as leaves or a small twig in the blower compartment, rubbing on the fan.

So where can I find this black box switchover valve? I have a 1984 500SEL w126 to be exact. And to elaborate on the buzzing/clicking noise, I can hear it sound even when the blower motor is powered off, so I think that might be it. It seems to buzz mostly when I am accelerating from a stop and the temp control is set below max heat.

Hey, did you ever get this fixed? I am working on mine again.
The switchover valve is to the left of the glove box, and down. Remove the glove box liner and you can see it; it will be several inches in length vertically and have a bunch of vacuum lines attached to sither side. it looks sort of like a vacuum version of a wiring terminal strip. this controls the vacuum flow to the different pods, to make the air come out of the different vents. if you remove he knee bolster (carpeted part underneath the dash on passenger's side) you can get better access.
Since I can't get heat to come out of the floor vents, only the defroster and side vents, I know I either have a bad vacuum pod or a problem with the switchover. gotta tear into it. My switchover has been clicking repeatedly on cold start; if I go a few miles down the road before I turn on the climate control, no clicking. Hummmn.....

Unfortunately, I never really got to the bottom of this. I just learned to live with the little problems. Now that its about 10 degrees outside, I won't be working on this again until spring. What I've taken notice to now that I've had this car for longer is that the clicking I speak of only occurs when the cabin temp control is set below max heat. If I jiggle around the temp wheel the clicking will stop. Also, I've accepted that heat will only come out of the defroster and floor vents and not the center vents (the air only gets slightly lukewarm coming out of those). I suspect the problem to be the unit of elaborate vent doors behind the climate controls. When I got a peek at it, some doors seemed jammed open while others jammed shut or even completely missing. I'm not completely clear on the problem, and I won't be until it gets warm enough outside for me to find out ;)

Also, I can't get to the bottom of why the air coming out always seems musty and humid. The humidity from rainstorms seems to get buried into the system for days. For like a week after a rainstorm, I have to keep the AC on constantly with the heat just to keep my windows from foggin up. Any input on this issue?

You say the heat will never come out of the center vents when funtioning correctly? I had the same problem as the person who started this thread, but I do have heat coming out of the sides, floor and defrost. I thought my heater was broken. It has been cold, so I have not tried the air conditioning yet. I know it works, but I just have not needed it. IF heat does not come out of the center vents what does?

THe gas smell means you need to tighten the bolts on your cylinder heads. I bought the new gaskets, but it was 20 degrees out side and decided I could put up with the smell. So I tightened the bolts about a month ago. The smell went away, and the gaskets are still in my trunk. Give that a try?

climate control
 

you may have a vac leak or the vac pump is weak

wow,ive been following this thread,and my 89 300ce is doing the same thing,no airflow comes from the center vent,even when its fully open,there is a noise coming from behind the glove box,i thought it maybe the blower motor,but when the blower motor is off i still hear it,when i switch the blower motor to high everything sounds normal.imgoing to check the black box,its vacum controlled,im wondering if this may be whats leading to my surging up and down idle,from a vacum leak,should the vacum unit be replaced?is this what most likley causes the weird noise behind glove box?

ive been following this thread and its been very helpfull,im having the same issues,the center vent will not blow even on high,there is a wierd noise coming from behind the glove box,thought it was blower motor,shut blower motor,still makes sound when cold in morning,when switched on high cannot hear noise,fan sounds great on high,auto and off position give off weird scratchy noise clicking sound,from behind dash.also the fan doesnt change positions when pressing climate control blower locations,air blows from all vents except for the double center vent.

As far as I know, heat never comes out of the center vents when the system is working properly. Side vents, floor, and defroster vents only.

Does W126 heater/AC box in a '91 350 SDL interchange with 82 300SD?
 

Thanks. Also, pls point me in a better direction to ask this question if this is not the appropriate place.

The 126 heater boxes are prone to failure of the flaps which turn the heat on down at your feet; the hot air comes out of the defroster vents instead. It has one vacuum pod which opens a door on each side of the box simulatneously to warm the floor at the driver's and passenger's feet. The linkage to each door is plastic and the plastic parts break. It's a tough job to access (remove the instrument panel, etc.) so i am living with the situation.
The only other faults with my car are at least one vacuum pod is leaking again, and usually if I turn the clomate control on before the engine warms some, the switchover valve clicks repeatedly.

Also be warned that the plastic "pop-clip" that connects the vacuum pod on the driver's side of the heater case to the flap arm can break. At a certain point, if you really want things to work properly, a rebuild of the heater box makes a lot of sense. You can replace the disintegrated foam throughout, replace all of the vacuum pods, replace all plastic linkages, and get a replacement/re-soldered push-button unit, blower motor, etc. See my prior posts for details. If I were to buy a diesel W126 I would plan on doing this entire procedure before expecting reliable HVAC operation. You could also consider replacing parts like the heater core, expansion valve and evaporator. All of this stuff can and will break (heater core, expansion valve and evaporator not as much), so replacing parts piece-by-piece will probably just lead to lots of frustration.

Just be warned that you do not want to break the plastic "extension arms" for the footwell flaps when you take the unit apart. It is easy to do, and the driver's side flap lists for roughly $250 at the dealer.