Air Conditioner

[Ben McKinley]

I have a 1978 280se. With what appears to be  york compressor. The Air Conditioner is cold for up to 2 hours of use and then goes hot. But after a  few minutes to half an hour goes freezing cold again, having been turned off. Often it will go hot after it has been turned off and is started again. Is there something wrong or is it just temperamental? Occasionally there is a drip that falling onto the drivers foot if your foot is on the foot rest, from an insulated pipe directly above the rest. (right hand drive car) Is this a leak in the system or just some condensation? 

[pez]

Your drip is most likely condensation from the metal pipes that run to into the heater box, carrying the cold refrigerant. It would be worthwhile to be certain that your drain lines are free and clear and not backed up. 

While there are a number of things you should check on the a/c system, it generally sounds like your system is freezing up and shutting itself down.  Or, alternatively, it thinks it is freezing up [faulty reading on the refrigerant temp sensor] and shutting itself down....that's my two cents!  Try unplugging your temp sensor leads and see if it still shuts down.

[Ben McKinley]

Thanks, are the temp sensor leads the ones plugged into the air drier at the front of the car?

[pez]

At the receiver/drier you have a pressure and temp sensor. These provide info about the state of the refrigerant as it moves through the system. They are worth checking/replacing [ they are cheap ].

The ones I was really thinking of are long coiled wires [no insulation on them] that are under your dash. I believe they are providing temp info the system about how cold the air is in the box - remember, below freezing in the box, and you will not have cold air.

While there are lots of things that could be going on, from your description it sounds like you are freezing up your box with a high efficiency, slow flow, or too much refrigerant situation. It could also be a situation of having some condensation in the refrigerant, too, and it is freezing up.  I would change the receiver/drier and do a true a/c system flush if it has not been done in a while. This will give you a good place to start.

[Ben McKinley]

Thanks I'll have a look.

[Ben McKinley]

Can the air con be shut off if the engine is too hot. The car recently got a new thermostat, which runs at about 90 degrees C (car does not have LPG system). Could this overheating cause the A/C to be shut off? If so would just replacing the thermostat work? 

[WGB]

Not sure about the warmer thermostat but I would suspect that your evaporator is freezing up in the dash due to it being kept too cold for too long.

This will turn off the compressor and it take a while to thaw out which would explain what is happening.

A quicker way to get it to thaw is to turn off the A-C and/or turn on the heater.

Either warm ambient air or heated air will quickly defrost the core and if you are parked it should be accompanied by a lot of water being released as the core defrosts.

Not sure about the M-B system but the G-M ones were adjustable with a small adjustable screw .

You should be able to adjust either by air temperature through the ducts or listening to the compressor cycling in and out. If it is pulling in the compressor and not cycling in and out it is either set too cold or low on Gas.

Another cause of your problem can be water in the pressurised system which can freeze and cause a blockage in the A-C pipes and may be visible ecternally as a frozen band on one of the high pressure lines.

Bill

[Ben McKinley]

Thanks its been getting quite hot lately and the 116 seems to be a sweat box if the A/C doesn't work.  I'll try the heater to defrost the air conditioner, I have seen the water under the car when the air-conditioner works. (one of the few times leaking water is a good sign!). What do you mean by "pulling in the compressor and not cycling in and out" ?

[WGB]

If you leave it idling with the A-C going after a few minutes of cooling you should hear the compressor cut in and out every 20 to 30 seconds.

If it is overcooling it will pull in the compressor and not turn it off until the core freezes (and sometimes not even then).

Bill

[Ben McKinley]

Thanks, I'll try this.

[Ben McKinley]

I've tried the using the heater to defrost the air box, but to no avail. I don't even get any water coming out of the drain lines. (which appear to be clear). Would the system being low on gas be the problem? As the amount of time the A/C works for is getting shorter. And the when the A/C doesn't work the air blown in with just the fan is colder than that with the A/C on. Is there anyway to establish a re-gas is definitely what is needed?

[jamesnb]

Do you have the Automatic Climate Control System (with a temperature dial)?  Does your heater work?
As I understand it, when the AC switch is on, the compressor runs continuously.  The cabin temperature is controlled by adding heat to the system via hot water circulating through the heater core.  If the heater doesn't work properly, the AC will run full blast, uncontrolled.

[Ben McKinley]

No I don't have the automatic Climate Control System. The heater appears to work, as in it provides hot air, and for some time to. I have noticed though when the heater is put on after the A/C has stopped working (in order to defrost the airbox) the middle vents are colder than the outside vents. If the heater was to defrost the A/C how long would it take?

[Ben McKinley]

This is a picture of the sight glass on the air drier/ receiver,

[koan]

This is a picture of the sight glass on the air drier/ receiver,

Is that ice crystals or condensation I can see forming on the receiver?

Is it cold to touch?

koan