My first step was to see if the system could work.   Overseas readers may not know that in Australia it is illegal to sell freon to the general public.   R12 has not been available for many years.   Therefore, I booked the car into a mechanic who does A/C work to have the system converted and gassed up with R134A.   First steps were quite positive, when the Klima relay was bypassed, the system would blow quite cold if you jiggled the ignition key a little.    That left some obvious repair items though.   I had a new ignition switch (the electrical part) fitted, and ordered a new relay.   I also ordered a rebuild climate control unit.

While the ignition switch was out of the car, I planned to put in a new tumbler.   It used to be possible to purchase these from the Mercedes-Benz classic center.   They would then set them up to use the right key for your car.   The part has been NLA (No Longer Available) for years on the W107, but I purchased it for my 300SE about 18 months ago.   In that time, it is now NLA for the W126 except for the 1990/1991 models.

The new freon had dye inserted, so I was able to check if the system was holding pressure.   In general, it was, but there was a small amount of dye around the compressor.   In particular, between the two halves of the compressor.   The mechanic who gassed up the system had seen success with a product that can be added to the A/C system to seal up minor internal leaks.  We added that component and I waited for the relay and climate control unit to arrive.

The Climate control unit is quite easy to fit.   I have done this job on my 300SE when I was troubleshooting its climate control system.   First, the ashtray is removed, then the radio.   The dash panel can then be pulled down and out to expose the climate control unit.

it is easier to remove the connections to the other switches first.   Care must be taken to keep the fiber optic cable with the switch, so the back lighting will work when it is re-attached.

In the series 2 cars, the climate control unit is screwed into the back of the wood panel, and there are two big connectors that connect the unit into the cars wiring harness.

I am getting quite fast in removing this part of a W126 dash after doing it quite a few times on the 300SE.  Taking the dash apart also allowed me to fix a few other annoyances.   The first was the illumination of the gear selector when the lights are on.   It is a tiny 1.2w bulb that slots into a hole in the underside of the unit at the front.   Not only was my bulb blown, but it had come out and was floating around in the dash.   The second was plugging in the cigarette lighter properly so I could charge my phone.

I had couple of these bulbs in stock, so it was an easy change.   The Ashtray illumination was flickering as well, so I changed that bulb while I was there.   After putting the dash back together, I had a MBCNSW twilight drive to attend.   I had originally planned to take the 250SE, but the weather was rather poor.  Based on this, the 560SEC was the better choice, especially as I could test the Climate Control.

My results were generally positive, but with some more work to be done.  My 560SEC climate control sort of works.   The climate control unit regulated the temperature well, although once or twice it went from cooling to heating.    The new relay worked fine, and I had no problems with the ignition switch.   The three main problems I had were that the A/C just wasn’t all that cold, the centre vents are not working properly, and the electric fan never kicks in for the condenser.  I’m pretty confident that the centre vents will be the vacuum pods under the dash.   That is a job for another time.   As the weather heats up I need to get the electric fan working as the car gets quite hot during prolonged idle.   After coming so far, I also wold like to get the A/C cooler than it is now.

Next step is to take it back to test the freon level, and see if that sealer has worked at all.   I also need to see if the Aux fan will work if plugged into 12v directly.

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After trying a used Climate Control Unit, new fan resistor etc, I finally bit the bullet and bought a rebuilt climate control unit.   This solved the problem.   In retrospect I probably didn’t need to buy the resistor and I certainly didn’t need the used CCU.     It also pays to shop around as a rebuilt CCU was much cheaper directly from the source than the usual online parts vendors.   I got my from ProgRama Inc.

This job should have been simple.     As I discovered earlier, the blower fans are different LHD to RHD.   This meant I re-used the housing from my old one.   What I didn’t realize until I had it all fitted was that the fan direction is obviously different too.   My new blower was blowing, but no air was coming out.    In the end I had to reverse the terminals of the controller to get air flow through the vents.

The aftermarket controller was slightly different at the back as well, so I had to file down part of my housing to make it fit.

The next steps will be to have the new A/C compressor I have purchased fitted.   This is not something I am equipped to do myself.   In any case, you need a licence to handle refrigerant in Australia.

Overall I like the W126 climate control system when it works.   However, I would have preferred a system with manual climate control.   Mercedes-Benz Australia to my knowledge ordered all the 2nd generation models with Automatic Climate control.   This would mean finding a private import with a manual set up.   A rather difficult task.

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I was able to check this by bypassing the Klima relay, and the compressor popped the fuse even when the engine was not running.   This did not happen when the compressor was disconnected.

I have since sourced a new compressor which will go on in November when the car goes for its rego.

The blower motor was also very noisy and would turn on and off on its own.   I first thought the issue was with the aspirator fan, which is a small fan to blow air over the temperature sensor.   These are known to get very noisy with age.   Only problem was somebody had completely removed this fan from my car.      Next, I was able to determine that the main blower fan was the problem.     These are still available for a reasonable price, unlike the 107 fan that took me years to find one that was not seriously overpriced.    The one I got was for a left hand drive car (they are different in the 126), but this is not a problem as the plastic housing can be re-used and the actual fan that wears out is the same.

The blower was refitted and didn’t work.   Next step was to test both motors on a spare battery.   When connected up to 12 volts both motors worked well, although the old was was noisy as expected.    Next step was to check if the motor was getting voltage, and it was. 

This meant that either the resistor that controls the blower speed, or the climate control unit itself were not working.   The resistor can be seen in the photo above and is known as the porcupine.   The blower is fed 12v through a dedicated, high amperage fuse.   The CCU supplies a current to tell the porcupine how fast to run the blower.   I wasn’t able to detect that control voltage from either of the two CCUs I have.  This may have led to trying another CCU, but I decided to change the porcupine first.  They are much cheaper than CCUs and known to fail.     Now I have two porcupines, two CCUs and still not even a working blower motor, let alone working climate control.

I did manage to find a company that offers rebuilt CCUs at a much more reasonable price than the main parts supplier (and looking at the pictures, seems to be the source of their inventory).   I have ordered one of those, and hopefully it will at least get the system and working, ready for the new compressor.  They suggest some checks on the resistance values from the monovalve and Aux water pump. Finally some good news, they are working as expected.

(Update: October 27, 2017 – The blower motor issues were fixed with an updated CCU/Porcupine)

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This fan is located behind the glove box, so I removed the glove box and the kick panel below it.  However, to my surprise the fan was missing!  The connector for it was there, but no aspirator fan.   Since I have already bought a new one, I will try and re-fit it.    Even though the noise I was getting did not vary with the blower speed, it appears it is from the blower.   Since the blower is on another fuse, I will worry about it later.

The next thing I did was replace the missing relay that is for the Aux fan.   I am not even sure how relevant this is as the Aux fan wiring has been messed with at some point.  Relays are cheap and so I replaced it anyway.

The car is now looking rather stripped down, with no radio, missing console panels, glove box, kick panel etc.  It will probably be like this for a while as I am not any closer to solving the problem.   At least my car will have an aspirator fan.

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The first thing I tried was changing the Climate Control Unit.  This is the push button unit in the dash and they are known to cause erratic climate control if they start to fail.   I was able to purchase a used (but untested) unit for $25.

This change did not seem to make any difference.  I noticed that the compressor was never engaging even when on max cool.  Therefore, the next thing I tried was removing the Klima relay and bridging pins 5 and 7.   The Klima relay controls the A/C compressor and determines when it should engage.   It uses various sensors to control the engagement of the compressor as well as the cooling fans.    This eliminates any of the sensors.    Bridging these pins should bypass all these sensors and simply apply 12v to the compressor clutch.

Doing this and starting the car did result in the compressor engaging briefly.   I even got a bit of cold out of the system!   However a few minutes later fuse 5 blew and the Climate Control System shut down.      Fuse 5 controls most of the climate control functions (except the blower motor), so the next logical step seemed to try and eliminate what could be causing it to blow.   Next thing I tried was to disconnect the mono valve and the Aux water pump.

The Aux water pump is a small pump that increases the flow of hot coolant through the heater.   These pumps are known to draw too much current as they age and are really not necessary in warm climates.    At -20C, having the extra pump does ensure hot coolant from the engine reaches the heater faster.   The mono-valve is used by the climate control system to control how much coolant comes into the system.   (i.e. none for max cool and full flow for max heat).

Fuse 5 still blew, despite eliminating the aux pump and mono valve.   This time, It happened even before I turned on the engine.   Next step may be to test the compressor clutch with an external power source.   This would eliminate some problem with the car’s wiring.

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