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Discussion Starter · #1 ·
For the first time in three years my personal daily driver vehicle has working AC! WOOOOT!!!!!!
So anyways, since I had to figure all this stuff out myself I figured I would share the information for anyone else who may be wanting to get their AC system back up to snuff.
Part 1, mechanics of how Air Conditioning works. If you understand this it makes figureing out things alot easier. Here we go.
As you all know your system houses refrigerant (either R12 or R134a). The main items of your system are the compressor, condenser, expansion valve, pressure switch, and evaporator. Basic system operation is as follows. Your compressor compresses the gas refrigerant. As you all know compressing anything creates heat. The hot compressed gas flows into the condenser where it is turned into a hot compressed liquid. From the condensor, the hot compressed liquid flows into the expansion valve where it is converted to an extremely cold un-compressed gas again. It flows through your evaporator (like a heater core) and cools the air. The cold gas is returned to the compressor to repeat the cycle. In order for this to all work, you system must be free of leaks and your pressure switch has to work. The pressure switch is the main key in engageing the clutch on the compressor.
Part 2, Electronics (Sorry G60 guys this is for SLC's but the same basic principal applies)
As I mentioned earlier, the pressure switch is the main component to making everything work. As long as there is pressure in your system, the pressure switch will close, if there is too much it will open like a fail safe. Anyways. From your HVAC controls you need to find the trigger wire. On a G60 it is Red w/ Black stripe and on SLC's it is Red w/ Blue stripe. To test to see if your connections are good on an SLC, pull the 4 pin connector on the pressure switch (Pressure switch is located next to the Driver side headlight by the radiator) and check for voltage at the Blue w Red wire. If that has power you at least know the connections and your AC relay are probably good. If not, read this page to figure out the wiring connections at your fuse box.
If you have power at your pressure switch and your compressor clucth still doesn't engage, first make sure it is connected. If it is try bypassing it. To bypass it, use a jumper wire from the blue/red wire to the green/yellow wire. If everything is connected correctly it should engage the clutch. If your clutch engages, you either need more refrigerant or your pressure switch is bad. Good news is that the pressure swicth can be repalced withouth discharging the system if you find out it is bad. If your clucth still does not engage, there is one more test before you can determine if your clutch is bad or not. Jumper the blue/red wire from the pressure switch plug to the solid green wire on your fan control module. This will bypass both the ambient temperature switch and evaporator temperature switch. If your clutch engages, you have a problem with one or both of these temperature switches. The evaporator temperature switch disengages the ac clutch if your evaporator core reaches 34 or less degrees to keep it from freezing. The ambient switch disengages the clutch if the outside temp is 34 or below to keep the compressor from freezing
The ambient temp switch is the one thats in the rain tray by the ECU. The evaporator switch is inside the car and is attached to the front (towards firewall) side of the blower box and has two small black wires coming from it. You can test both switches simply by testing for continuity. To make sure they are even hooked up at all test for continuity on the green wire at the fan control module and the green/blue (maybe green / black) wire at the ambient temp switch (with the temp sensor unpluged). If you get continuity you need to check your wiring. If no continuity, you need to check each switch individually. The ambient switch is easy. With it disconnected just test for continuity. If it's good, the switch is good, if not, you need a new one. Testing the evaporator switch is the same procedure. Check for continuity across the two black wires coming out of it. If this is bad, you need to replace it, and good luck with that one. The whole blower box has to come out and your AC system will need to be discharged.
If they both get continuiety and you still dont have continuity from the green/blue wire to the green wire on the FCM, you have a wiring issue. If the clutch does not engage there is either a wiring problem or it is bad. I am not sure which wire does what on the compressor, but one of them should be 12v. So if all of this checks out Ok and you don't get 12v to that connector then you have a wiring problem from that connector to the FCM. If you do get 12v then the clutch is definitely bad.
Part 3 Wiring from the pressure switch.
The only two you are concerned with to make your A/C work are the blue/red and Green/Yellow. As stated above the blue/red is the trigger from your hvac controls and supplies 12v to the pressure switch. The green/yellow wire heads to the rad fan switch. From there it turns green/white and heads up to the ambient temperature switch. On the other side of the ambient temperature switch it turns green/blue and heads into the dash to connector T4a. From connector T4a it connects to connector T2b which whould have a white/yellow wire and a green wire. These two wires head to the evaporator switch. So, the green blue connects to the white yellow and goes to the evap switch where it turns green and connects to connector T4a. The green wire connects to the FCM and triggers the clutch.
I hope this information will be of help to someone somewhere down the line and if anyone with a G60 wants to contribute, feel free to let me know.


Modified by vw mofo at 10:39 AM 8-14-2003
 

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Re: VW Mofo's comprehensive how A/C works thread. (vw mofo)

Very nice description. Now, i want to see some P-V and T-S diagrams showing the compression, expansion, and cooling steps. Maybe reference the 2nd and 3rd laws of thermodynamics to give your discussion some fun looking equations as well.
Sorry, the chemical engineer in me shines through once in a while
 

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Re: VW Mofo's comprehensive how A/C works thread. (vw mofo)

Ok, so my G60 is on pressure switch #4... What else ever fails?
I never took Thermodynamics, but I understood the writeup professor. Do you still want to do a memory clear on my HP48GX?
 

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Re: VW Mofo's comprehensive how A/C works thread. (vw mofo)

Great write-up man http://****************.com/smile/emthup.gif
edit: I can't spell



Modified by Lopan at 1:41 PM 8-14-2003
 

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Discussion Starter · #8 ·
Re: VW Mofo's comprehensive how A/C works thread. (where_2)

Quote, originally posted by where_2 »
Ok, so my G60 is on pressure switch #4... What else ever fails?

Have you converted to R134a?
If so, you might have too much refrigerant. R134a compresses to a much higher pressure than R12 does. Try charging the system with R134a to about 90% of the recommended R12 charge. Or having some of the 134a reclaimed, we all know that you're gonna vent it, but to be legal, I gotta say have it reclaimed.
 

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Discussion Starter · #10 ·
Re: VW Mofo's comprehensive how A/C works thread. (tipo158)

You know I don't know if it is... If it is, its tied in somewhere in the FCM. That brown switch is no where in any of the bentley diagrams I was looking at.
 

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Re: VW Mofo's comprehensive how A/C works thread. (vw mofo)

Looking at the "From August 1992" schematic, I think the brown switch is the A/C cut-off/third cooling fan speed switch. I don't have the manual here, so I can remember exactly what it is called.
I got the switch replaced this morning and my AC still doesn't kick in until the car has been running for a while. It is a little weird. When I drive along without the AC on, the car runs cool. If I switch on the AC (and the AC starts working), even if I turn the AC off, the coolant temp rises.
Looking through the schematics, looks like VW couldn't decide how they wanted to do A/C on the VR6 Corrados. There is one schematic for the G60 cars, one for the pre-August 92 build date cars, one for the post Aug 92 and one for the 94 cars. My car is a 10/92 build date, so I have been looking at its schematic a lot.
BTW, the 92 and 93 cars have different condensors and receiver/driers. Remember this if you go looking for used condensor and want to put a new receiver/drier on it.
alan
 

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Re: VW Mofo's comprehensive how A/C works thread. (tipo158)

... and if you're sitting at a light in 95* heat, enjoying your AC'd black leather recaro interior, when you hear a hissing sound coming from the dash, and then the cold air turns into hot air... well, then you'd be me last weekend .

I will be looking for a leak in the AC system in my dash sometime very soon. Fun eh?
 

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Orig owner 90 g60, 02 allroad 6SP M w/ 3rd Row, 07 3X White NB Vert, 09 Escalade ESV Plat to tow em
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Re: VW Mofo's comprehensive how A/C works thread. (where_2)

Hi/Low A/C pressure switch # 3 for me. $130. Mine leaked both times.
The cause: Vibration.
Vibration from: Motor mounts.
Last time I let the motor mounts go too long it cracked the flare fitting on the clutch slave (twice)
Remedy: New stock G60 motor mounts (soothing!!)
Remedy #2: Vibratronics poly front mount w/ stock G60 side & rear mounts (little harsh - tolerable though)
Remedy # 3: VWMS front mount w/ stock G60 side & rear mounts (feels better than new!) Where can you get the no longer avail VWMS front mounts?
Michael
 

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Re: VW Mofo's comprehensive how A/C works thread. (petethepug)

BUMP
for possibly the best thread the mofo ever wrote!
had our first (of many) heatwave in toronto monday, and when i pushed in the "norm" button on my AC control, and it didn't turn on... i almost crapped myself
ok i did crap myself.
hopefully mofo's brilliant writeup can help me a little. i don't hear the damn clutch engaging at all. last year, when i left the AC switch on, and clicked my alarm button i would hear a big old "click" from under the hood. now i hear jack with the button on. the switch does work, since it engages a fan. i'm off to check the pressure switch tommorrow. pray for me mofo

and its a damn shame you don't crontribute anymore

-danny
 

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Discussion Starter · #17 ·
Re: VW Mofo's comprehensive how A/C works thread. (CageyBee)

Well, there's only a few things that will keep the compressor from coming on. In your case, you can rule out electrical since it used to work.
I would bet money you are just low on refrigerant. aka you have a leak somewhere.
 

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Re: VW Mofo's comprehensive how A/C works thread. (vw mofo)

well, my system had refridgerant, a good pressure switch, good relays, and it wouldnt engage the damn clutch. after using this very thread and the Bently I checked EVERYTHING including the slide switches (I have a G60) and the last thing remaining was the A/C Thermostat on the evap core. It was the last thing I checked since the mofo said it was a bea-otch to get to...well, it turns out that it was very accessible, right under the pass side dash on the bottom of the HVAC air box.all you gotta do is pull the panel. it's about $80-$100 but I know many people who have pulled their AC systems and this part is probably in their car.
this thread was the perfect jump start even though some of the wire colors were off since keith's car was a bastard...lol
 

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Discussion Starter · #19 ·
lol... I feel bad for the poor bastard that ever has to try and work on that A/C system in the future!
NONE of the SLC a/c system connectors are the same as a G60! Not one. Wire colors are different and the physical connectors are even shaped differently. To be honest, I'm surprised I ever got it to work in the first place! If it weren't for me actually having two bentley manuals I would have never figured it out!
 
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