Automatic Climate Control in Young-timers

As long as I have served the owners of our well-engineered and reliable cars, I have always been keenly aware of the necessity – and the pleasures – of a fully functional climate control system. Unfortunately, some systems work better than others. When Mercedes-Benz began the transition from manual to automatic systems in 1976, with the Automatic Climate Control 2 (ACC2) system, the systems included vacuum-actuated vent controls which would prove to have problems later in the cars’ lifetimes.

ACC2 was available on the W116 series from 1976, on the W123 5-cylinder and 6-cylinder models from 1977, and on the W107 series in 1978. The system was used until the end of production on W116 cars, until 1980 on the W123, and until 1981 on W107 cars. The W126 and mid-1980 W123 used ACC3 (Automatic Climate Control 3), but the vacuum-actuated vent control mechanisms changed only a little bit. In subsequent systems, the airflow out of the vents was controlled automatically, rather than by the driver.

While perfectly functional when working right, if the vacuum actuators that control these vents start to leak, the airflow goes wonky. Down here in Florida, at least, that’s seriously bad news.

How the system operates – in theory

So, you tell your mechanic that your ducts aren’t closing and opening properly on your 1987 420SEL when you put it into recirculation mode. If it’s just a matter of replacing an actuator, right? That shouldn’t be too expensive. True, the actuator isn’t very expensive, but getting to it and replacing it, that’s a different matter.

Above, top to bottom: The defrost vent actuator from a W126, disassembled to show the return spring, vacuum linkage, electric solenoid, actuating diaphragms, and the operating lever. The vent actuator assembled. The vacuum actuator in place with the electrical, vacuum, and lever linkages hooked up. All photos by Pierre Hedary.

To help you understand what’s involved, let’s first consider how the system operates. When you push the center vent button on your climate control panel, or Auto Hi on ACC3, an electric solenoid valve opens a vacuum valve. This vacuum valve, using a vacuum source from the engine, then opens the center vents. When you turn off the center vents, the electrical solenoid valve disengages, the actuator dumps its vacuum, and the vent closes.

That seems like a complicated way to do a simple thing, but wait, it gets more interesting. On all Mercedes automatic climate control systems, there is a center vent actuator and a defrost actuator. When vacuum is applied to the center vent actuator, the center vent opens. When vacuum is applied to the defrost actuator, the defrost actuator closes.

An actuator of any type fails when there is a pinhole in its rubber diaphragm. Once a pinhole occurs, the diaphragm cannot hold a vacuum and the actuator will ultimately fail to open or close its respective vent. The defrost actuator and the center vent actuator are tied into the same vacuum circuit. If one leaks, neither one will function. That means that if you turn on the center vent, and the defrost actuator is leaking, the center vents will never fully open. In the same way, if you have a leaking center vent actuator, the defrost vents will never fully close. In addition, when you go into maximum AC mode, and your recirculation actuators are triggered – this occurs when you turn the temperature control wheel all the way to blue – if they are leaking, they can affect the opening of the center vent and defrost actuators.

Mercedes climate control vacuum actuators

Let’s look at how the actuators work, and where they’re mounted.

Mercedes vacuum actuators are a very simple design. They are usually pink and white, with one or two chambers. Each chamber has a rubber diaphragm, and a vacuum port. They snap together, and each bottom section has four legs that snap into retainers on the top section. Inside each actuator are a spring and a small arm, mounted to a plastic disc, which the diaphragm slips over. The diaphragm seats in the lip of the bottom piece, and is sealed by the pressure of the two parts against it.

There are two types of mountings for each actuator. The first type uses three small legs that slip into little holes. The legs are held in place with rubber washers and star clips. They are press-fit into place so needless to say, they can be difficult to remove and reinstall.

Examples of this type of actuator include the following:

• 107, 126,124, 116 and 123 center vent actuator
• Early 123 (before 1981) defrost actuator
• 107 and 116 defrost actuator
• 123 (before 1981), early 107 (with ACC2), 116, and 1981 126 recirculation actuator
• 123 (before 1981) and 116 footwell actuators

The second type of actuator has a rotation lock on it. In other words, you rotate it in its bracket to lock it in place. This type of actuator applies to the following models and components:

• 126, 123 (from 1981) and 124 defrost actuator
• Late 123, 126 (from 1982), 107 (from 1982) recirculation actuators
• 123 (from 1981) and 126 footwell actuators
• 126 fresh-air vent actuator

The first type of actuator is the harder of the two to install. The second type is quite easy and is the more straightforward type to replace.
The solenoid valve grid

All of the actuators are opened with an electric solenoid valve. These valves are very easy to replace on some cars, and a bit more difficult and pricey on others. Early solenoid valves on cars with ACC2 were dome-shaped, with a two-pole electrical plug. These almost never fail, and there are only three for the climate control vents. They are located behind the pushbutton control unit, which is a bit difficult to remove. We don’t usually worry about these until we have made sure that the many feet of vacuum line in this system are all okay.

ACC3 changed the game quite a bit. Now all the plug-in solenoid valves (five on the 123 cars, six on the 126 cars, and four on 107 cars) were screwed down into a nice little grid. The grid on 123s is located behind the pushbutton control panel and can be challenging to get to. The 126 panel is located on the transmission tunnel on the passenger side. The 107 panel is in the bowels of the dash behind the glove box. These panels have individual valves held in place with Phillips head screws. The valves themselves can and will fail, leading to the same symptoms as a failed actuator. Therefore, it is imperative that these be checked along with the actuator. Checking is simple. After verifying all vacuum connections at the grid are perfect [yes, perfect – no loose hoses, cracks or ill-fitting lines], then look at each valve. A colored vacuum line comes out of it, which can be followed to its actuator.

After tracing the line, we unplug it at its end, and hook up a vacuum gauge. With the climate control pushbutton unit plugged in, the car is started. With the actuator position selected, If the solenoid valve works, the vacuum gauge will indicate a vacuum. If the gauge indicates no vacuum, then the supply line is checked to make sure it’s intact.  The solenoid grid is also checked to make sure it’s receiving vacuum. If all is well with those extraneous variables, then the solenoid valve is at fault. It can be replaced with a new unit, which can be purchased from Mercedes-Benz for about $60.

On the 126 and 107 cars, in 1986 Mercedes switched to a small valve body that controls all the actuators on 126 and 107 cars. The 124 and 201 had this from the beginning of production. The good news is that these generally give little or no trouble.The testing procedure is the same, but if one circuit fails, the whole valve body is replaced.

Locations of climate control actuators

Now we come to where the various actuators are located, and how much skill is needed to replace them. The skill ratings will be broken down into easy, moderate and difficult. We will focus on more common actuators.

The 123 center-vent actuator – This one is difficult, and most 123 series cars are reaching the age where these sorts of parts can fail. The location of this actuator is inside the dash, behind the center vents. Its three little legs are mounted into a vertical bracket. The best way to get to it is to remove the glove box, and the center vents themselves. One has to be careful of the climate control temperature sensor, however. It is right above the actuator itself.
While Mercedes says that the dash must be removed to replace it, someone skilled with his or her hands can use a small blade to cut the legs off the actuator, and slide it back. It is pinned to the air flap with a plastic retainer. Retainers and rubber washers should be replaced at the same time. Going through the center vent holes with a 5mm socket or a small wrench to press the retainers on has been a trick I have employed many times, but I keep my other hand in there to hold the actuator in place. Needless to say, it helps to have long arms and dexterous hands. I always perform an intensive visual survey before really trying anything.

Shop time on this job, depending on what course of action is taken, could range from two hours to eight hours. A really good technician can usually replace one of these in under two hours without removing the dash, but if your tech knows he can get it right by removing the dash, let him do it.
The 126 center vent and floor vent actuators – This replacement also is difficult. I don’t recommend that anyone try this who doesn’t already know how to disassemble the interior of the 126.

The center vent actuator is located to the right, behind the pushbutton control unit. It lives in a small housing held in place with four 5.5mm screws. The difficult part of this job is hooking the actuator arm in its air flap. There is only one way to do it, and that is with a pair of needle nose pliers, through the access hole. The little housing for the actuator can be removed, and this makes pressing its little legs in place much easier. I always order a new rubber gasket for where the housing bolts to the evaporator case.

My only advice here is that the mechanic must be very careful with the plastic retainer that holds the actuator arm to the center vent air flap. The mechanic needs to be very familiar with the procedure, and also be respectful of your car’s interior. The floor vent actuator simply rotates in place and is located in a visible location in the same area. This job can run three to four hours, depending on the condition of the car and how careful the technician is.

126 Recirculation Vent Actuator

126 Recirculation Vent Actuator Close up

The 126 and 124 defrost actuator – This is an easy job on cars without a passenger airbag. On cars with a passenger airbag, it is a moderately difficult job. After the glove box is removed, the actuator is visible to the left, with two chambers and two vacuum lines plugged into it. It will have a small arm clipped into an air flap. After unclipping the arm the actuator is rotated counter clockwise to undo it from its retainer. Once it is removed from its bracket, and the vacuum lines can be undone, while noting their postition. The lines are hooked up to the new actuator, it is slipped it into its bracket and turned clockwise, to lock it in position. Then it can be clipped to its arm. Operation is checked by starting the engine, pushing the center vent button, and watching the arm move back as vacuum does its work. On airbag cars, the airbag can be unplugged and removed from its mounting as long as the ignition is off. In other respects, the procedure is the same.

The W123 center vent actuator is buried in the dash, sort of like a funny-colored, odd-shaped fish living at the bottom of the Pacific Ocean.

123, 126 and 107 recirculation actuators – Moderate skill is required on these models. On the 126 and 123 cars, the recirculation actuators are set up in a pair behind the glove box. Early versions of these cars from 1981 and before use a three-legged mounting. Later ones rotate in place. These are easy to remove and replace, once the duct work has been carefully removed. Great care must be exercised when removing the actuator arms from the plastic recirculation linkage. 107 cars have four recirculation actuators. Two are behind the glove box and two are behind the instrument panel. These are also twist-and-lock units after 1981.

The 126 center vent actuator is located on the evaporator case, on the right side of the transmission tunnel.

123 defrost and floor vent actuators – This is an easy job on later cars. The defrost actuator is located right above the accelerator pedal. It rotates in, and has an arm to which the actuator clips. The lower dash panel must be removed to get to it. The floor vent actuator is nearby, behind the evaporator case. This is also a twist-and-lock movement but when removing it, I am very mindful of its fragile linkage.

The defrost actuator on the 420SEL; if you look down in the corner, you will also see the center vent actuator.

124 center vent, and 107 and 124 center vent and defrost actuators – These are very difficult to replace, but I’ll tell you where they are so you have a better understanding of how they work. The 107 defrost actuator is behind the instrument pod, to the right. It has an arm that extends into the evaporator case. A linkage ties the two defrost flaps together. The 116 defrost and center vent actuators are hidden in a small area behind the center vents, together. These are only really accessible by removing the dash, or by mechanics who don’t mind working in small spaces. Finally, the front part of the evaporator case must be disassembled to reach the center vent actuators on the 107 and 124 models. These should be replaced when the evaporator is replaced. Labor time on these jobs can be intensive, so make sure you choose a careful mechanic.

The twist and lock setup of the recirculation actuator on our 1991 420SEL demo car.


Now that you have a good understanding of this job, you understand it is not easy. But to make your Mercedes as comfortable as it was when it was new, it is a good idea to replace failing climate control actuators. While some might consider the mounting of these actuators difficult, or redundant, remember that even the best cars have their idiosyncrasies. Repairing little parts on your car that break will only help you appreciate the best parts of it even more!

 The defrost actuator on the 420SEL; if you look down in the corner, you will also see the center vent actuator.

Pierre Hedary owns and operates an independent shop specializing in young-timer classics, in Titusville, Florida. Contact him at 407-765-2867 or mbownersfor [email protected].