[Vwdiesel] 2000 TDI a/c question - and a brief description of the A/C system

Tony and Lillie tonyandlillie1 at earthlink.net
Thu Jun 3 22:51:09 PDT 2010

Here is a basic rundown on how an A/C ststem works:

Any time you pressurize a gas it gains tempurature (heats up). Any time you release pressure from a gas it reduces temperature (cools down). 

You have a gas in the A/C system that gets compressed by the compressor whenever the compressor is engaged via the compressor clutch. When it's compressed, the temp goes up. This increased temp is then above ambient temperature, and some of the increase is removed int eh condensor that sits in front of the radiator. Then the compressed gas is fed into the expansion valve, which releases it into the evaperator. This has been reduced sufficiently in the condensor that it can now drop the evaproator to a very cold temp. After that, it leaves the evaporator and gets pulled back into the compressor.

Somewhere in the system there is also a reciever drier that basically acts as a trap for particles and water. It has an inlet that goes in a bit into a canister, and an outlet that comes right off the top. So, things will drop into the bottom, but not be sucked up.

There are also low and high side switches. The freon acts as a lubricant along with the oil, and if it's too low it will harm the compressor. If the pressure gets too high, you can blow a component somewhere in the system. So, in line with the compressor feed are these switches, the low pressure switch on the "low" side of the system, and the high pressure switch on the "high" side of the system.

HEre's an example with some numbers that I'm just using for reference.
It's 100 degrees outside. The compressed freon adds 100 degrees to that, for a temp of 200 going into the condensor. Then, the condensor drops 100 degrees off it, so now it's 150 going into the expansion valve/evaproator. Since it's dropped 100 degrees of heat, it has the potential to go to 0 degrees. There is a thermocouple (or thermistor) that goes into the condensor to keep it at approximately 40 degrees, to keep it from becoming a block of ice. 

Now onto your car:

Your low side seems about right, depending on the ambient temp. It will vary from about 30 - 60 psi. The high side seems a bit low. That usually signals a compressor getting weak. These later model VW's use Sanden compressors, and I have replaced quite a few. However, don't panic just yet.

You should be seeing about 25-45psi on the low side at 90-100 degrees ambient, and about 160-270 on the high side. You should see 35-55psi on the low side at 100-110 degrees ambient, and about 210-300 on the high side. Since you are in the Dallas area I'm assuming you are doing this around 100 degrees right now?

You mentioned you might be freezing the evaproator. Is air still blowing just as hard out of the vents, just warmer than before? If so, you don't have a problem with freezing it up. If it freezes, you will have a blockage and very little air will make it's way past it to the vents. 

You also mentioned this problem comes on after a while. Is that when you checked the pressures, after 40 minutes or so? If not, I'd definately go back and do that. You may have some sort of restriction in the system that shows up after a while as a very high pressure on the high side, but not when you first start the car. Maybe along the lines of something getting sucked into the system and blocking, but then falls out after the car si sitting for a bit. Also, check this all out when it's as hot as possible, so you can see if the high side switch is getting weak and killing the compressor. I've seen that quite a bit on many different vehicles. 

Hope this gives you an idea where to start, and helps with understanding the system.
Tony Hoffman

-----Original Message-----
>From: Shalyn Shourds <sshourds at flash.net>
>Sent: Jun 3, 2010 10:27 PM
>To: vwdiesel at vwfans.com
>Subject: [Vwdiesel] 2000 TDI a/c question
>It's well into a/c season here in Texas and I've noticed the unit in the 
>TDI is getting a little weak.  That is, however, one area of 
>mechanicking that I've not delved into much.
>As best I can tell, the manual specs are for 203 psi on the high side 
>and 17 psi on the low whilst running at high cool.  I borrowed some of 
>Dad's time and gauges and we found 155 on the high side and 40 on the 
>low side.  Sadly, there's no sight glass.  We looked hard and with all 
>the time I've spent in the engine bay of this car, I would have found 
>it.  Believe me.  The a/c's still dropping out on me after a while.  
>Good to start and after 40 minutes or so, not as cool anymore.  I'd 
>assumed it was the evaporator freezing up because of low coolant, but 
>adding coolant barely changed the readings and the sweating in the seat 
>Any thoughts?  I'm ignorant enough about a/c that I don't even know what 
>questions to ask.
>Thanks much,
>Vwdiesel mailing list
>Vwdiesel at vwfans.com

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