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  #1  
Old 01-10-2009, 02:37 AM
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Join Date: Jun 2007
Location: Sydney, Australia
Posts: 28
Dying Air Cond symptoms finally figured out

Geday from Australia!

for a long time I have been confused about exactly what was wrong with my A/C...
Here at last are the correct symptoms.

Revs above 2000 - works beautifully - nice cold air
Revs drop - it works for 1 or 2 minutes - if the revs stay low it will CUT OUT
Compressor clutch will disengage (compressor stops spinning) and cooling is dead.

The only way to re-set the system so it works again is to stop the car, turn ignition off and start the car again.

It is the 1987 aircond/ heater system which has the 2 wheels - blue at one end and red at the other - one for each side of the car.
Of course it is running on the R134 juice, but designed for the R12 in 1987. Serpentine belt is new-ish and adjusted right.
Compressor is reconditioned recently.
New switch turns on secondary (front) fan at 100 degrees C (water boiling point)
High and low pressures were right when compressor was fixed. Sorry don't know that bit.

Do any of you super geniuses know the source of my woes? In Australia it's HOT and any traffic slowdown whatever kills my cooling. Thanks to Sixto for an excellent list of possible diagnoses last year, but list was very long and this data clears up all variables

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  #2  
Old 01-10-2009, 03:02 AM
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smoke gets in your eyes
 
Join Date: Jun 2000
Location: Eastern TN
Posts: 20,841
Assuming that car isn't too different from US cars, try this when the compressor disengages - check for +12V at the low pressure switch on the dryer. The dryer has a low pressure switch and a high pressure switch. The low pressure switch is the one without pigtails. Contacts are right on the switch. This switch cuts the signal from the PBU to the Klima relay that controls the compressor when there isn't sufficient refrigerant pressure. This switch is known to go bad. IIRC both wires are the same color so I don't know which comes from the PBU and which goes to the Klima relay. Pull both and check each for +12V. In fact, check it first with the compressor engaged so you know which wire is which. This will determine whether the problem is in the cabin (PBU or evaporator temp switch) or engine bay (Klima relay, OVP relay, high pressure switch, engine rpm sensor, compressor rpm sensor).

My money's on a tired PBU or flaky low pressure switch.

edit - Dang, I keep saying PBU for push button unit but you probably have the highly desireable rotary control knobs that weren't offered in the US. If you have control knobs, there should be a temp control unit somewhere, likely the under the false firewell in the passenger footwell.

Sixto
87 300D
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  #3  
Old 01-10-2009, 11:28 AM
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Posts: 4,263
Is there a speed sensor on the compressor? If it's dirty, it may not be producing enough of a signal at idle.
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  #4  
Old 01-10-2009, 02:55 PM
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Location: Northeast Indiana
Posts: 10,765
Does the aux/front fan run at low speed with the A/C switched on? If not, try shorting across the high-pressure switch (the one with pigtails), it should turn on the fan. If not, there is a white ceramic resistor near the filter/dryer that is probably bad/open. If it does, possible low charge.
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  #5  
Old 01-11-2009, 06:56 AM
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Join Date: Jun 2007
Location: Sydney, Australia
Posts: 28
sixto super genius

Man, that's an incredible reply, even more incredible that I actually understood it.

I think the temp sensor is in the roof with the hatch switch and light controls. A small vacuum hose drags air over it slowly.

The PBU does have wheels, but also has 3 buttons- EC (economy) button (off), *snowflake switch = climate control , and top button (symbol looks like a demister symbol) air cond ON no matter what. (that's the one I use)
I'm pretty sure the cabin temp sensor is irrelevant to this as it gets REAL cold in there when the revs are up

Thanks for extremely helpful replies guys
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  #6  
Old 01-11-2009, 12:07 PM
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smoke gets in your eyes
 
Join Date: Jun 2000
Location: Eastern TN
Posts: 20,841
There's a cabin temp sensor in the roof, an ambient temp sensor by the blower motor under the wiper, a heater core temp sensor and an evaporator sensor. All separate. Evaporator temp sensor is there mainly to cut the compressor before ice forms on the evaporator.

Sixto
87 300D
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  #7  
Old 02-28-2010, 04:58 PM
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Join Date: Jun 2007
Location: Sydney, Australia
Posts: 28
SOLUTION

I am delighted to tell you that it was the KLIMA/KICKDOWN relay which is located behind the battery. There are 2 black boxes there, it is the smaller one.

Pull it out, it snaps right back in.

$650 from Mercedes
$75 from a wrecker

there are a few part numbers which will fit, sorry I don't have them.

Let me know how you go

Good luck... great car! AIR WORKS GREAT NOW!!!

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