94 Audi 90cs A/c problem

cardude1122 · 05-18-2009, 05:10 PM

Hey guys, I'm new to the forum and new to owning an Audi. I bought the car a couple weeks ago, and have been fixing it up. I have a problem with the A/c that's bugging me. The clutch will not engage when I turn the a/c on. this car has the automatic climate control.

Here are things I did to rule out problems:
-hotwired compressor to test clutch and it engages
-checked and even changed relay in fuse/relay panel under the hood
-checked all fuses dealing with the a/c or climate control system
-checked harnesses and plugs behind the climate controller
-recharged the system with fresh freon to the correct specs
-checked the high pressure switch

Other things to note:
-if i turn the a/c on on the controller, the indicator comes up on the display saying its on
-i checked the diagnostic codes via the climate control display and the only thing i found was a code in channel 52 for the low pressure switch

Now, this is the thing, before i got the car, the guy had the ac compressor replaced but did not have the system recharged. so i am not sure if that code is in memory from when that happened.

Is there any way to clear those codes without having a scanner?
that would be the easiest thing to try first IMO. other than that, im thinking i need to pull the dash out and check the low pressure switch.

Any ideas or thoughts would be a huge help!! It may spark something in my brain. Also if anyone else has had a similar situation, please share!!!

thanks, jeff

Norty · 05-19-2009, 11:37 AM

Jeff,

Jumper the low pressure switch shut and see if it engages the clutch. If you have a guage set, see what your low side pressure is. That way you can tell if the switch is bad or not.

cardude1122 · 05-19-2009, 01:57 PM

Ok. Is the low pressure switch under the dash on the passenger side, up on the firewall? Or is it somewhere else on this car?

Norty · 05-19-2009, 02:24 PM

Yes, it is located under the dash on the passenger side. On my 91 it's behind the MPI module.

cardude1122 · 05-19-2009, 03:48 PM

well, i sat down and really studied the wiring diagram i had of the climate control and found out how the low pressure switch is hooked up. i did a test and found that the switch isn't open but it is only allowing 4volts to come through on the return line to the ac module, to bypass it i put 12 volts to that wire and HEY THE COMPRESSOR KICKED ON!! so now when i get the time and energy, i'll have to tear the dash apart and replace that switch. if anyone would like to know what wires i checked and used in the harnesses that plug into the back of the climate control unit, let me know.

thanks for the help!! its greatly appreciated!! its nice to come across a good forum, once in a while.

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05-19-2009, 11:37 AM	#2
Norty New Member Garage is empty, add now Join Date: Oct 2008 Location: Otsego, MN Posts: 8	Jeff, Jumper the low pressure switch shut and see if it engages the clutch. If you have a guage set, see what your low side pressure is. That way you can tell if the switch is bad or not.

05-19-2009, 01:57 PM	#3
cardude1122 New Member Garage is empty, add now Join Date: May 2009 Location: Pennsylvania Posts: 4	Ok. Is the low pressure switch under the dash on the passenger side, up on the firewall? Or is it somewhere else on this car?

05-19-2009, 02:24 PM	#4
Norty New Member Garage is empty, add now Join Date: Oct 2008 Location: Otsego, MN Posts: 8	Yes, it is located under the dash on the passenger side. On my 91 it's behind the MPI module.

05-19-2009, 03:48 PM	#5
cardude1122 New Member Garage is empty, add now Join Date: May 2009 Location: Pennsylvania Posts: 4	well, i sat down and really studied the wiring diagram i had of the climate control and found out how the low pressure switch is hooked up. i did a test and found that the switch isn't open but it is only allowing 4volts to come through on the return line to the ac module, to bypass it i put 12 volts to that wire and HEY THE COMPRESSOR KICKED ON!! so now when i get the time and energy, i'll have to tear the dash apart and replace that switch. if anyone would like to know what wires i checked and used in the harnesses that plug into the back of the climate control unit, let me know. thanks for the help!! its greatly appreciated!! its nice to come across a good forum, once in a while.

05-19-2009, 03:48 PM
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