Aircond works when started at high revs, otherwise dies
 

Greetings from Australia, where it is just starting to get HOT outside.

MY darling 1987 W124 300D :heart: ... oh where do I start - nothing ever goes wrong with her ... but ...

I spent $1300 (ripped off by Hahn Auto - Botany Road - Sydney) on a reconditioned compressor and re-gas, which did not fix the problem, which is...

:confused:

If I first start the air-conditioner up while she is idling- good for a few seconds then kaput- lukewarm air.
BUT If I first start it up when I am roaring along with the revs over 2000, then it works ok! I can start and stop, and it still works (USUALLY - sometimes it still dies).
It seems crucial to turn aircond on FIRST TIME when the engine is going hard.

It is the aircond/ heater system which has the 2 wheels - one for each side of the car.
Of course it is running on the R134 juice, but designed for the R12 in 1987 (I presume).

Any clues will be very much appreciated!

In US cars with auto climate control (one thermostat wheel), there's a relay (marked KLIMA) that among other things compares compressor rpm to engine rpm. If the ratio isn't right, it assumes the compressor is bad or the belt is compromised and disengages the clutch. Suspects thus are a slipping belt, dirt in the clutch or clutch gap out of spec, problem with the compressor rpm sensor or wiring, problem with the engine rpm sensor or wiring. If the tach/rev counter is working, it's probably not the engine rpm sensor. Functioning cruise control also rules out a bad engine rpm sensor.

The refrigerant low pressure switch, the one with no pigtails, can get flaky with age. Best to replace it when refrigerant is evacuated for whatever reason. This is easy to test - simply bridge the wires into the switch and see if the problem persists.

Also check the overvoltage protection relay. In US cars it's an aluminum box about 3cm to a side and 6cm tall with a red plastic top. There's a blade fuse into the top but the innards can go bad as well.

Sixto
87 300D

First you have to isolate what your problem is. When the a/c dies, is the compressor spinning? or has the pressure cut off stopped the compressor. Did it work at all when it came back from your servicer? R134A put in an r12 system will not cause this issue. If the compressor is turning, also check the suction line, should be larger and insulated, is this line getting and staying cold? May also have a fair amount of condensation on it. If this line is staying cold, and your getting luke warm air.. something in the climate control system is whacky.. maybe monovalve?

Did your old compressor sieze up? You may have what's refferred to hear as the black death. Simply put, pieces of the old compressor go flying into the system and if it not flushed out, may be getting caught in the expansion valve, etc. I have a 300SD that recently got a new compressor on it... worked fine and I developed a similar problem to yours. Turned out I had a blockage in the expansion valve. Hope I didnt confuse you, and gave you some things to look for.

Rick
W2ICA

thanks heaps guys for very thorough answers - but compressor is still engaged until I switch it off. No Compressor did not die - was just weak when replaced, and this was happening before replacement. Tacho, Serp belt and cruise control are fine.

Rick when you say monovalve do you mean the big one behind the middle of the dash? I can hear something slamming shut reassuringly when I turn the fan off ( cabin fresh air fan) But I HAVE wondered if all those valves back there are working.

Bloody intermittent problems can drag on for years!

Monovalve
 

Look here:
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=124133_501&M=603.960&GA=722.317357&CT=F&cat=501&SID=83&SGR=180&SGN=04

Your "Monovalve" (Duovalve) is in the engine compartment.

#s 54 and 56 are the electro-magnetic "Monovalve" [Duovalve](s) [only one applies]that regulate HOT coolant into and out of the Heater Core.
If it's bad it allows hot air to overcome the A/C's cool air output.
I'm leery of it because you have to "Jack Up" the RPMs
(Which would raise the ELECTRICAL output of the Alternator) to get cool air.

For that matter,if the Voltage output from the Alternator is too low, other
factors [as mentioned in an earlier reply]may inhibit the proper A/C system
operation.

have you tried Karlo Kittle motors in Sydney?
here is his number, delete what you need to for local dialing. tell him Ray in San diego Refered you.
61 2 9958 5705