[R129 SL] Another air conditioning question

[Peter Shelton]

BerichtHi George,

The 'on' coil temperature is the temperature in front of the evaporator coil!  The 'coil' is a radiator looking part inside the 'clipped' plastic aircon unit that is on the upper part of the dash but behind the dash.  It is actually installed from the front of the engine bay in most models (unsure of the 129 as I have not dismantled mine or any 129 for parts as yet!).   If latent air (in the car or outside) is say, 20 degrees C (68 degrees F), then if the aircon is working on cold (set to maximum temperature), the temperature 'off' the coil should be about 7 to 9 degrees.  It should only take about 3 - 5 minutes to start pumping out cold air.
Set the car to air recirculation, and take the inlet to the aircon unit  (usually under the seats, and measure temperature, measure the coming out of the front vents when both are set to coldest - should be about 7 - 9 degrees.  If you have a hotter day, the temperature should still be similar ranges on the output, if it is working correctly!

I have my hard top stored in the 'top of the shed' and use the soft top occasionally (when it is really raining hard).  I travel with the top down, although the car does let water in even above 60 kph as Merces need a rain deflector on the top of the screen. I travel with the top down in all temperatures,  and just turn the heater on.  Just need a warm hat, ears covered, side shields on my glasses and I'm off!  It is a heck of a lot warmer (as Jan will confirm) driving the Merc in say minus 3 degree C temperatures, with the top down than being on the motorcycle at the same temperatures (wind chill factor).  The bike is exhilarating though and you do not have to lane queue as you can overtake easily with care and not exceed the sped limit (100 kph on the 'open road' in New Zealand). 

I do not use the aircon for cooling as with the top down it is nice and breezy. If I want a tin top car, I use the Nissan Maxima, Toyota Echo, Silver Bullet (1983 280E) or the Orange Roughy (1979 250 Carbie - gas guzzler) if it is really cold or very wet.  I use the tin tops if the cat comes with us, as she (the Siamese) has a tendency to jump out of the 129!

Have a great day and good luck with the aircon.  Cold today and 'nearly sleeting' in 'Eco City'. Jan (The Flying Dutchman and Vehicle Warning Sensor Inventor) is enjoying the summer on the bike - have to make the most of the fine weather!

Regards, Peter S. 
  ----- Original Message ----- 
  From: George Jenner 
  To: Peter Shelton ; SL Mailing List 
  Sent: Sunday, July 15, 2007 12:54 AM
  Subject: Re: [R129 SL] Another air conditioning question


  Hi Jan, thank you for the very fine and detailed procedure to check the problem out.  I know that my fans come on and go off at odd times and I can not tell where the control for the fans come from.  They never run after I switch off the engine (like some other makes do) 
  I would very much like to have the electronic circuit diagrams and the diagnosis procedure my email is:
  gjenner at nc.rr.com  - Peter, where do I check the on/off coil temperature and what should I see when I check?  I get foam in the sight glass most of the time.

  George
    ----- Original Message ----- 
    From: Peter Shelton 
    To: SL Mailing List 
    Sent: Friday, July 13, 2007 7:16 PM
    Subject: Re: [R129 SL] Another air conditioning question


    Hi Jan,  

    Not good about your gearbox. I have spares for 201's and 123's but not for a 129.  I bought a brand new 129 left headlight off "trademe" (NZ equivalent of EBay) last week. Bought as an 'in case' spare.  genuine part - $125 euros.  Amazing what people have in their sheds!

    Jan, after 'frying' your gearbox, maybe you could call yourself the "Frying Dutchman" rather than the "Flying Dutchman"!  It nearly sounds the same?. 

    One of our newer members in our NZ Mercedes Benz Club recently bought a 1974 350SL.  The fuel line leaked and the engine caught fire!   Did a bit of damage but it was extinguished with a Fire Extinguisher.  It was a dry powder (mica) type - what a mess it left and it is corrosive!  Also no insurance for this in NZ and he is an insurance agent/ investigator.  I have nick named his car "The Barbeque"  for obvious reasons.  I suggest we all check our fuel lines frequently as perishing fuel lines can be fatal.  

    Jan, your description of the aircon is very good.  It is important to measure the on/off coil temperatures as this gives an immediate indication of where the problem may lie.  Bubbling in the accumulator sight glass is also an indicator.  

    Great sunny day in Eco City (Manawatu NZ).  The day started off at minimum 2 and it is about 15 degrees now.  It is hot from the sun, as the ozone layer over our country is depleting fast, and the UV filtering effect of Ozone is nearly non existent.  Well, time for a ride in the 500SL, have to make the most of the sunny days. 

    Regards, Peter S.
      ----- Original Message ----- 
      From: Jan Kuylaars 
      To: 'SL Mailing List' 
      Sent: Friday, July 13, 2007 10:30 AM
      Subject: Re: [R129 SL] Another air conditioning question


      George.

      I am not a expert in AC, but basic working is that the compressor compresses the gas to fluid and the fluid is giving of the heat that is produced by compressing.
      The front radiator is for cooling the heated fluid by passing air while driving, or by air stream with the electric fans, during standing still. The heated gas is pumped to the receiver drier to take out the moister. After the drier there is a pressure control switch for shut down the AC if the pressure is to low and a other for running the electric fans if the pressure is to high.

      If you system is not completely filled, you can see air bubbles in the glass from the drier when the compressor is running. 
      When you see no bubbles there is to less or no refrigerant.You can check on the compressor. There should be a different in temperature in the inlet and outlet
      After the compressor stops , and there is a clear vision in the glass, there is to much refrigerant in the system.
      If the amount of refrigerant is perfect, the refrigerant should foam and then stay clear after the compressor stops.

      From the drier the fluid runs to the evaporator inside the car , where the fluid can expand to gas again, and for this expand the gas needs heat.
      The evaporator takes the heat from the room and the room becomes cool. 

      From the evaporator the gas is flowing back to the compressors low pressure side, where the gas is compressed again to fluid and the process starts again.

      So as long as your car is driving and the radiator will be cooled there is no problem.
      When your car is standing still and the engine temperature is climbing the problems starts;

      There are in mine opinion two possibilities, I am not for sure , but I am are trying to think with you :
      First there could be  a failure in the basic operation, and the second one is electronic problem from the regulation of he AC.

      Lets start by the first: if your car is standing still and your electric fans are not working, the fluid is not been cooled and the heated fluid results in higher pressure .
      Also there is a higher temperature in your evaporator. There is expanding from the gas but it don't needs a lot of heat for this because the temperature already is high. The result could be that the emission of heat out of your car to the evaporator is gone. So no cooling.
      This could be resulting in a spiral. After the pressure comes too high the system shut down or the refrigerant will come out the high pressure security valve.
      Result to low refrigerant and not proper working of the AC.

      Possible failure could be the fans, the fan control, the pressure switch or the expansion valve which are not good operating with higher pressure or heat.

      Then there is a possibility for electronic failure , like the Italian college told; The measure of inside temperature and outside temperature in combination with the temperature from the engine. He told us to fix this problem with a resistor placed in series with the engine temperature sensor. Read the earlier messages.

      Also there can be a failure in the circuit itself.
      When you wish I can send you the electronic circuit diagrams and the diagnose procedure.

      Greetings from the standing still Dutchmen with damaged gearbox. 

      I discovered on the bill from my garage that beside replacement from the starter and the steering house, they replaced the rubber pipe from the gearbox oil cooler because it should leak a bit. After that I only drove 40 miles before my gearbox looses oil with pump noise and burned oil on the exhaust as result.
      I must check if the rubber pipe is come loose, and maybe they are the reason of no just oil level or other damage.
      But then I have a problem if they are willing to confess and pay for the damage.

      Jan.










        -----Oorspronkelijk bericht-----
        Van: r129list-bounces at mbcoupes.com [mailto:r129list-bounces at mbcoupes.com] Namens George Jenner
        Verzonden: donderdag 12 juli 2007 15:59
        Aan: SL Mailing List
        Onderwerp: [R129 SL] Another air conditioning question


        I have been experiencing a perplexing problem with my 1996 SL500 air conditioning system.  I have the means to check the pressure on the system and have noticed that when I first start the car the air conditioned runs normally cold.  As I drive and the car heats up the system runs normally, but if I turn off the engine (to gas up) and the out door temp. is hot (in the high 90s) when I restart the car the air conditioner is no longer running cold.  This has led me to check the system pressure at various engine temperature stages and the pressure is in the normal 40 - 50 range when the system is cooling normally but jumps to 100 + when the engine is hot and the system no longer is  cooling.  I do not think I am loosing coolant but need help to locate the problem.

        Has anyone any idea of what may be causing this problem?

        Thanks, George


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