COMPLETE A/C Diagnosis

Old 08-20-2007, 11:58 AM
Marco8's Avatar
Join Date: Mar 2007
Location: Highland Village, TX
Posts: 259


You are absolutely correct. I read your write up once again and saw that if there was inadequate pressure the compressor wouldn't run (pressure switch). So I added refrigerant/w UV dye and as soon as the pressure increased the compressor kicked on and took the charge bringing levels to 16 and 260psi respectively (right in range for the temperature). I looked last night in the dark for leaks but was unable to find any.

So AC is blowing cold and I will periodically check pressure to see if I have an excessive leak.

Again awesome write up and thank you for helping me (and many others) with your endeavors.

Marco8 is offline  
Old 08-20-2007, 12:28 PM
Rennlist Member
Join Date: Jun 2001
Location: ParFRA
Posts: 1,032

<<What happens on a cycling compressor. At rest, the refrigerant in the system is not being compressed and pushed through the lines so the system pressure equalizes at … let’s say, 80 psi. (Just for illustration here, not exact.) The A/C tells the compressor to engage and the output side (high side) of the compressor raises in pressure while the low side pulls down to a low pressure. This pressure change is what causes the coolant to go through its liquid and gas stages – getting that wonderful cooling effect. If the pressure climbs too high – or drops too low, the pressure switch interrupts the current to the compressor.>>

I am not clear on what you are saying here.... Is the diagnosis nothing more than being a little low on refrigerant, or is there a problem with a switch? My compressor cycles on for about 5-7 seconds, then off for about the same time, over and over.
Avenger6 is offline  
Old 08-20-2007, 01:28 PM
Rennlist Member

Thread Starter
BS911's Avatar
Join Date: May 2001
Location: Williamsport, MD
Posts: 828

Yes... That sounds typical of low coolant. If the pressure gets too low it will cut off the compressor to save itself and cycle as you mentioned. If it gets even lower still, it won't even get high enough to trip the compressor on. That is why when people have a really empty system, to get it to started they have to hotwire the compressor to stay engaged enough to draw in the coolant. But there always is the possibility of it being other a faulty pressure switch. If you are already too high and keep adding more, you could blow a line, seal, or something. So you really should know what your pressures are before adding anything. A single low pressure gauge will tell you half the story which could at least help indicate but not prove low pressure. You really need to know low and high pressure. Lets say you have high pressure already but a blockage in the system. That could artificially lower your low side pressure readings making it appear low. But the high side could be running dangerously high. The dual gauge readings will help you know that your pressures are right in relation to each other.
BS911 is offline  
Old 08-20-2007, 03:34 PM
Rennlist Member
Join Date: Jun 2001
Location: ParFRA
Posts: 1,032

Brian - Thanks! I'll take it to the Porsche Center at the factory and have them do a checkup on it and add refrigerant if needed. Having a general idea of what is going on, - thanks to you and your work - I can speak more intelligently to them and possibly minimize the costs a bit. These guys are pretty good about supporting those of us with older cars.....they like seeing them come in, and they appreciate an owner who has at least a little bit of a clue.......
Avenger6 is offline  
Old 08-21-2007, 11:30 AM
verboten's Avatar
Join Date: May 2004
Posts: 371

Thanks to the post I diagnosised my AC that wasn't working. You might want to add my problem.

7.5A Fuse kept blowing due to a bad relay in enging compartment (#13?) I swaped same relay from front fuse box and AC worked. Only 2 dealerships in country had relay $47 and received in 3 days. Otherwise, 3-4 weeks from Germany and summer would be over by that point.
verboten is offline  
Old 04-21-2009, 07:08 PM
ace993t's Avatar
Join Date: Dec 2007
Location: UK
Posts: 17

Hi, Anyone out there able to help? I need a photo of the area behind the radio that is mentioned in Brians brilliant article above - the very last sentence "- If present, look under dash, under ccu for rubber elbow connecting vacuum line to flap mechanism - it would most likely be unplugged due to stereo installer or other inside work bumping it free." The part attaching the vacuum to the flat on my car is missing. (please see my post "Recirculating Air") and I have still not been able to find a part number so a photo may help me fabricate a replacment link... Many thanks

ace993t is offline  
Old 04-21-2009, 09:27 PM
mr_bock's Avatar
Join Date: Jul 2004
Location: FL full time
Posts: 1,413


You ARE The Man!!!!!!!!!!!!!! Great write up. Perfect timing, with the warmer weather coming and all.
I will put this to use as my compressor is not engaging.
A 1000 and 1 Thanks to you!!!!!!!!!!!!!!!!
mr_bock is offline  
Old 08-28-2009, 08:51 AM
Rennlist Member
ljugete's Avatar
Join Date: Jan 2003
Location: Northern IL
Posts: 2,192

Hi Brian, excellent post!

I currently have my 993 at the stealer due to warmer air blowing. I usually have her topped off with freon once a year for a presumed slow freon leak and this has worked well for years.

Yesterday I took her to my tech. who noticed that the pressures were high and he vacumed the system, sucking out a lot of air (he said only one pound of freon remained - usually holds 1.7 lb.). He also noted that the fan was not running at all (he was able to spin it well with his finger). He is now trying to figure out why (I had to leave it at the shop with him).

Any thoughts / suggestions?

Thanks in advance!
ljugete is offline  
Old 08-28-2009, 09:15 AM
Rennlist Member
ToreB's Avatar
Join Date: Apr 2004
Location: Oslo, Norway
Posts: 1,669

What fan isn't working mate? The condenser fan in the front left wing? If so,you probably have a defective low speed resistor. Very common fault and frequently discussed here in the forum. This force the fan intermittently on high and can cause excess pressure in the coolant.
ToreB is offline  
Old 08-28-2009, 10:59 AM
g_murray's Avatar
Join Date: Aug 2005
Location: Long Island, NY
Posts: 1,303

Brian, I take my hat off to you.
Incredible write-up.

g_murray is offline  
Old 04-24-2010, 09:08 AM
01coccobet's Avatar
Join Date: Apr 2010
Location: italy
Posts: 486


01coccobet is offline  
Old 04-24-2010, 09:40 AM
01coccobet's Avatar
Join Date: Apr 2010
Location: italy
Posts: 486

while trying to fix my problem,
still don't get the point.

when I turn on the A/C,for the first very few seconds,it works,then from the front vents the air stop to blow and a little whistle seem like some door is getting closed,while it shouldn't.....and air is trying to pass.

what do you think?

PS with just the fan on,it's everything fine,as I turn on the ac,all this happens

thank you so much.
01coccobet is offline  
Old 04-24-2010, 12:46 PM
Super User
Rinty's Avatar
Join Date: Apr 2008
Location: Calgary, Alberta
Posts: 2,649

Is Brian's AC piece bookmarked somewhere in Rennlist?
Rinty is offline  
Old 04-24-2010, 01:59 PM
Rennlist Member
cpace3's Avatar
Join Date: Jun 2009
Location: Highland CA
Posts: 87

great right up . i thank god i do not have this prob . this souds like a nightmare to work on.
cpace3 is offline  
Old 04-24-2010, 08:08 PM
Zingari's Avatar
Join Date: Nov 2009
Location: Cheshire, UK
Posts: 519

I too have the AC gremlin. After a regas and thinking everything was alright even doing the ballast resistors DIY the system gave up after approx 4 days.

Depsite the AC technician when he gassed it telling me there was no leak a second regas by another supplier confirmed low pressure (extracting only 340g) - but again told no leak!

There is a leak somewhere but I have yet to trace it as I can't see the tell-tale green dye anywhere! Decided to take it back to the Indy given that they had it gassed and told OK when clearly it wasn't so at least I'm owed this back. If they trace it and dependent on where it is and cost I have the confidence to tackle it myself if big labouor costs are involved.
Zingari is offline  

Thread Tools
Search this Thread
Quick Reply: COMPLETE A/C Diagnosis

Contact Us - About Us - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to and affiliated sites.
  • Ask a Question
    Get answers from community experts
Question Title:
Your question will be posted in: