[W126 Coupe] a/c question

[Dick Spellman]

Hi Markus:

Nice bit of troubleshooting step by step!  I'd say you have already 
answered your question.  If the compressor continues to operate (clutch 
stays engaged) but, the high side is dropping as you indicate, it's 
probably the expansion valve.  You'll need to vacate the system of it's 
current R-134a charge (let someone use a machine to capture rather than 
let the system blow off into the atmosphere), replace the expansion 
valve (loads of fun) then install a new receiver/dryer as the last step 
in closing up the system.  Vac it down, let it sit to see the vacuum 
levels hold for an hour or so,  add the R-134 with oil (1 ounce per can) 
and expect you will need 2 cans +/- total.

Only other comments are to vac as soon as possible after buttoning the 
system up to rid the system of any moisture (something the new r/d won't 
have to deal with) and use gauges to set the volume of R-134 correctly.  
At least that's all I'd do.

Dick

Markus Meyer wrote:

> Hi gang,

>  

> I have a 300SEL (W126) that I acquired recently that has been acting 

> up with the air conditioning.  I swapped out the pushbutton unit with 

> another SEL I have sitting around, that didn't do anything.  I did 

> some testing per the W126 manual and thought I had narrowed it down to 

> the Klima relay, but that didn't help either.  Today I was at the 

> dealer getting some brake paste and antenna grommets (items still 

> rather inexpensive at the dealer if you can believe that) and a tech 

> was working on a car there, so when I was done I stopped and chatted 

> with him.  He was kind enough to let me borrow his guages, so I hooked 

> them up to see what was going on.

>  

> Bit of background:  a/c has worked for an hour or so at a time 

> occasionally, works in park/idle just fine and then seems to cut out 

> after driving for a few minutes or so, other times takes longer - 10 

> minutes or an hour.  Nothing seems to make it come back on besides 

> shutting the car off for a minute or two and retrying.  I don't think 

> its the ignition switch because everything else is fine (blower motor 

> continues to blow air, just not a/c air) and jiggling it did nothing 

> for the a/c.   Oh yeah, car was converted to R134a, and I don't know 

> what they replaced besides the compressor which is obvious.  

> Receiver/dryer may not have been replaced as it looks older and still 

> has the sight glass which I understand the R134a systems do not have.

>  

> Okay.  So at the dealer, in park/idle, pressures fluctuated on the low 

> side between 25-40 to on the high side between 150-200, mostly between 

> 150-175.   Tech said that showed the expansion valve was working as 

> the high would go up, low would go down, etc.  Then I revved it up to 

> 3000-3500 rpms for a minute or so sitting there.  Looked at the 

> pressures and noticed the high was now up around 200-250, but quickly 

> would come down again.  He said this seemed odd for a R134a system.  

> He said maybe the expansion valve has a clog, or is sticking 

> sometimes?  I did jumper pins 5 and 7 on the Klima relay and 

> compressor spins just fine.  Something is causing it to stop 

> spinning.  It is NOT the fan (high pressure switch on dryer) because 

> that has been disconnected and hotwired to always run the secondary 

> fan (I will be addressing that shortly).  Maybe the low pressure 

> switch is bad on the dryer?  Maybe the expansion val ve is sticking?   

> Maybe the rpm/speed sensor on the compressor is bad?

>  

> What I don't get is the inconsistency I see.  In park/idle it always 

> seems to work and can't get it to stop.  Once I drive, it could be a 

> few minutes or less, or 10-15 minutes, maybe even an hour until it stops.

>  

> What else can I check or test?  Any ideas?

>  

> Thanks in advance.

>

> Markus Meyer

> mfm at meyerprobateproperties.com <mailto:mfm at meyerprobateproperties.com>

> www.meyerprobateproperties.com <http://www.meyerprobateproperties.com/>

> Meyer Probate & Properties, LLC

> 484-919-9762

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