I've got a 91 300SE and NO HOT AIR will come out of the vents or onto the floor and it's starting to get cold around here. It blows plenty of outside ambient temp air through.

If I press the defroster button, plenty of hot air comes out of the upper vents.

I read on another forum that the Monovalve might be the problem so I changed it over the weekend but that did not fix the problem. I whipped out my handy tester and checked voltages going to the monovalve and indeed they would change when the button was pressed on the dash which leads me to believe the monovalve was not the problem.

Currently it seems like the CCU might be the problem and I was wondering if there's anything else I should try before replacing the CCU and if I do have to replace the CCU can anyone give me some instructions on how to get started. In addition I plan on having this car for a while so a nice shop manual would be in order - recmmendation on where to buy one?

I must admit the Benz is a little intimidating by not exposing much in the way of screws etc to remove panels. Should I be working from the passenger or the drivers side to remove the center panel to access the CCU panel. I have another CCU sitting here and am ready to change it if need be.

thanks in advance!

- jeff

Do a search on W126 ACC vacuum actuators. It's more likely one or more of your are shot than a bad pushbutton unit.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

Jeff
Remove the ash tray (like if you were going to empty it), then look in where the ashtray was and you should see 2 phillips screws. Remove these, then the part the ash tray goes into (the bracket for the ashtray) can be removed. Next the wood panel can be slid down a little and removed.

Gilly

sixto - I went through the service manual last night and I'm not sure why you suggest that one of the vacuum actuators are not working since it's apparent they're working properly. From reading the manual and I do admit I may have misunderstood something their function is to divert air to certain vents.

They are all functioning properly on my car. When I press buttons on the dash the air is diverted to the proper location.

My problem as previosuly described is a lack of hot air. The control wheel on the CCU appears to do nothing. I purchased another CCU which I will install this weekend.

I will ask this question of the assembled masses and it's this "when you press the left most button for the window defroster does the temp thumbwheel affect the temp or it is only supposed to blow hot air?" - I ask because mine blows hot air only when I press the defroster button.

I will run tests decribed in the manual prior to replacing the CCU, but it's looking more and more like a failure in the rheostat on the temp thumbwheel at this point. I read about the repair procedure here in other thread but was able to get a functioning used CCU at a good price.

does anyone know off the top of their heat the voltages passed to the monovalve whe it's in the off and on positions?

and I understand that in the absence of current to the monovalve it is open - thus allowing coolant to flow through the heater core.. is this true?

In addition does air always run through the heater core or does one of the vacuum actuators redirect fresh into through the heater core?

any comments / observations?

thanks in advance..

This is quite typical of Euro cars that have auto climate control--YES, on Defrost, the temp will go to MAX heat and blower fan.

If the other positions (the "true" auto-control positions) tend to not give the desired temp (or don't match the temp wheel setting), then possibly the interior temp sensor is bad, or on an MB there is an "aspirator" system of some type, depending on the year/model. What this does is to "suck" air past the interior temp sensor to give a better sampling of the interior temp.
On some models this is done by a tiny fan built into the temp sensor, on other models the aspirator fan is in a remote location, such as behind the dash, and connects to the interior temp via a tube, and on others the tube is instead attached to the intake air side of the climate control, which will then draw air if the climate control fan is running. Sorry I don't know off the top of my head which system the late-126 chassis uses, I believe the remote-location blower behind the dash like a 124 chassis (the sensor itself is built into the domelight/sunroof switch area, the 4 or so small slots in that lamp assm.

Gilly

acording to the repair guide - the temp sensor is supposed to be mounted on top of the dashboard somehwere.

I will double check.

back to my basic question. Does one of the vacuum gates redirect fresh air through the heater core or is all air redirected through the heater core and via use of the monovale the heater core is assumed to be at ambient temperture thus not affecting the temp of the air fed from outside the car?

- jeff

Can you rephrase the question?

Gilly

ps only the older 126's had the temp sensor on top of the dash, I'm pretty sure it's in the domelamp assm.....just look! 91 right?

Hi Folks,

I am curious as to how this plays out. I have a similar problem with a 90 126. That is, no heat, intermittent performance of switches, etc. etc. I assume the heater control valve is on the inside of the firewall?

well - I have read that if I unkook the connector to the monovalve it should be in the open position - allowing coolant to flow through the heater core. I will try that first.

I will also check the ohm range of the signal to the monovalve to see if it's within spec.

If those two check out then there's a good chance it is not my ccu since the ccu would be emitting the correct signals.

The other more dreaded possibility is that it's the vacuum pot that redirects air into the heater core to begin with. I will check the ohm range of the signal emitted by the ccu to that pot to see if it's working properly.


The manual is very good in descriptions of the test procedures.

and of course it could be the CCU itself in terms of the temp thumbwheel rheostat just not working and not putting out the correct signals.

I am hoping it's just the ccu and not the vaccum pots because from all my reading here - they're a real pain to replace.

- jeff

I can make no sense of what is being asked here. Redirected from where?

The last part is the most confusing:

The heater core is assumed to be at ambient temp? Ambient temp is the temp of the outside air. Of course the heater core affects the ambient temp, it increases the ambient temp, that's what a heater "does".

Gilly

ok - my bad - let me try again.

is ALL outside air redirected through the heater core or is there a vacuum pot that redirects air through the heater core before any of the air exits through the vents?

- jeff

I am happy to report that the problem was with my CCU. Man that was a snap to replace. It took me all of about 20 mintes.

the only downside is that now I have to make a trip to the bloodsuckers at Mercedes of West Chester (Pa) to get them to enter the code in my radio.

bummer - they'll charge me $90 just for that... minimum one hour charge..

- jeff

Glad it works Jeff.

You don't need the dealer to enter the code, you do that yourself. You should have the CODE card somewhere, just enter it! If you have no CODE card, look through those maintenance books, sometimes you'll see a mysterious 4 digit number written in there, that's probably it.
OR if you're sure you don't have it, just try CALLING the dealer and ask if they can run a VMI and get you the code number.

Gilly

I went into Smythe European the other day to get my radio codes and they wanted to charge me twenty dollars for the codes, cash only, no credit cards accepted. I politely declined.

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