Something wrong with w126 climate control system? - PeachParts Mercedes-Benz Forum

84MB500SEL · #1 05-12-2009, 08:16 PM

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.

tinypanzer · #2 05-12-2009, 09:17 PM

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........

thayer · #3 05-15-2009, 09:21 AM

Quote:

Originally Posted by tinypanzer

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

Cal Learner · #4 05-15-2009, 10:13 AM

Quote:

Originally Posted by thayer

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?

thayer · #5 05-15-2009, 04:56 PM

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.

tinypanzer · #6 05-15-2009, 06:19 PM

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.

thayer · #7 05-18-2009, 10:28 AM

PRINT! and off to find a voltmeter

CamelotShadow · #8 05-19-2009, 05:43 PM

Quote:

Originally Posted by tinypanzer

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.

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

tinypanzer · #9 05-19-2009, 06:09 PM

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

thayer · #10 06-01-2009, 01:37 PM

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?

thayer · #11 06-02-2009, 11:46 AM

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.

thayer · #12 06-03-2009, 09:32 AM

Monovalve is perfect. I could sell it on ebay.

Next thing is to try and clean the evap.

Is there anything else?

Randy · #13 06-04-2009, 04:37 PM

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.

84MB500SEL · #14 06-16-2009, 11:56 AM

Quote:

Originally Posted by Randy

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.

Randy · #15 12-14-2009, 10:14 AM

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.....