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COMPLETE A/C Diagnosis

 
Old 05-13-2007, 01:09 PM
  #16  
95 C4 993
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Call me stupid but where exactly is the oil temp. sensor?

I have the A/C gremlin as well.
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Old 05-13-2007, 09:25 PM
  #17  
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Not quite coleman... I just tracked every stinking part of the system down over the past few years finding my own problems out.

The oil temp sensor is behind the passenger side headlight. Pop the headlight and it should be right behind and below in the top of the oil cooler.
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Old 06-18-2007, 04:47 PM
  #18  
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Hi Brian,

I did not see one of my favorite trouble-shooting tricks professed by my high school auto teacher... feel the lines to find problem components or restrictions! With the system running, the lines should be very hot all the way to the expansion valve, then cold (with some gradual warming of course) on the way back to the compressor. This trick was crucial in figuring out my system problems yesterday... with everything close to normal except for a slightly low low side pressure (but still in spec) I was measuring pretty hot vent temps (10 degrees F drop from ambient max). I traced the high side path expecting the 11 year old receiver/dryer to be the culprit... but found really high input and a sorta warm output from the conden$er, with the fan running fine and nice clean exterior from the cleaning it received during my aux oil cooler install a few months ago. Oh well, at least the compressor is producing nice pressures!

I don't know if you want to include this trick or not.... but I figured it might help someone pin-point problems. Also, a huge THANK YOU for taking the time to write this guide up... I think it is a great resource. With a little luck my AC will blow cold enough to freeze the bubbling puddle of plastic that was my credit card... and I can revel in the satisfaction of a fun DIY!

Thanks again, Mark.
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Old 06-18-2007, 09:11 PM
  #19  
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Great tip Mark. I haven't heard or tried that one... although it makes perfect sense. I'll have to give my lines a look and feel to see where things are going wrong. My vent temps aren't dropping quite enough and I too suspect either a rec/dryer or condensor or exp valve. What I really need though is time to actually work on the car.

Thanks for the tip and kind words.
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Old 07-31-2007, 02:24 PM
  #20  
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What an incredible write-up, Brian. Excellent DIY threads have gotten few and far between these days. Thanks!
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Old 07-31-2007, 03:20 PM
  #21  
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Brian, as I would expect from you....excellence....well done.....I miss seeing you on the road home in the evening, how is the new house???
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Old 07-31-2007, 06:31 PM
  #22  
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OMG...I just wanted to add another huge THANKS! I'm currently on my second day of 993 ownership and the A/C is on the top of the list to fix.
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Old 08-04-2007, 09:26 AM
  #23  
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Hi, I'm trying to freeze out my A/C gremlins as well.
This is the situation: Condenser fan blowing but no compressor clutch engage. '95 993 C4.

I have checked all fuses OK, manual actuation of the compressor clutch relay in the back starts the compressor. CCU A/C high switch sets all inside fans and air valves OK.
Working my way down the list, sensor resistance readings seems OK:
Evaporator temp sensor ľ 2.9 kOhm (measured between G18-G22)
Oil temp sensor ľ 20.1 kOhm (G18-G12)
But, there is no ground connection on G18 (blue/brown lead)
Is the DIY write-up incorrect? I have studied the schematics and cannot find any connection to ground other than maybe one inside the CCU.

I cannot test A/C pressure, is it possible to measure the electrical outputs of the 3-Level Pressure Switch? I see there's two switches inside this sensor, but cannot identify the function or text on the schematic, does someone know the normal working state of these?

Regards, Tore
Oslo, Norway

Last edited by ToreB; 08-04-2007 at 07:17 PM.
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Old 08-05-2007, 08:24 AM
  #24  
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I'm still working in my garage...
I found a somewhat rough scan of a schematic from on this link.

The 3-Level Pressure Switch status can be tested:
There's a connector on the switch, It's easily found near the windshield, see picture below. Pin numbers are imprinted on the connector.
I tested for continuity between pin 1 (white/red lead) and pin 4. (black lead) This switch was open, too high or too low pressure, thus disconnecting the compressor on signal from the CCU. Shorting these leads engaged the compressor clutch OK. There's my fault.
I reckon this is due to low refrigerant levels from a leak or something. Now it's time to book a visit a the A/C repair shop.

I hope this info is useful to others, and maybe an addition to the DIY write-up.
Regards,
Tore

Picture, 3-Level Pressure Switch position:
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Old 08-05-2007, 09:26 AM
  #25  
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All indications point to a bad CCU on mine but my mechanic fixed it without replacing it by using a toggle switch to manually turn on the compressor. The cable runs from the rear fuse box, through the firewall, under the front seat and sits on the right side of the drivers seat or I tuck it under the floor mat. Compressor works an blows cold air but just wont activate without a little help. Eventually I will replace the CCU but it was a pretty good $250 fix. Most of that being diagnosis and trouble shooting.

If you have any questions, Dave Lomas ("Ketchmi"on Rennlist) can probably explain exactly what he did but all I care is that my AC is cold, which it is.
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Old 08-05-2007, 10:30 AM
  #26  
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Great post.

I lucked out -- a simple recharge is all my AC needed to get 40-degree vent temps. Its last charge had been done in 1996 in Stuttgart.
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Old 08-06-2007, 06:31 PM
  #27  
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I ended up with a large pool of green tracer fluid on the garage floor. No UV light is needed to see that's coming from the evaporator drain. Looks like there will be needed a major job. That's not good news...
http://www.p-car.com/diy/ac/

Regards, Tore
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Old 08-18-2007, 02:39 PM
  #28  
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Default Could use some help with AC, thanks

Brian,

Almost 2 months ago I drove my 96 993 from Phoenix to Dallas and then put it in storage for a little over a month. On the trip down the AC was working but the performance was deteriorating.

Well when I picked it up a week ago the AC was not working and I was going to troubleshoot it once I got time from moving. Well I thought with the gradual deterioration it had to do with low refrigerant but when I went to check I found pressure on both high and low equalized at 25psi.

So here is my question, pressure is obviously low, could this cause the compressor from running? Should I charge the system up to a more nominal level and see what happens? Other thoughts?

Thanks,

Marco
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Old 08-19-2007, 06:14 AM
  #29  
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Lovely job - saved for future use !!
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Old 08-20-2007, 10:08 AM
  #30  
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Hey Marco.
Pressure high and low equalized at 25. Well, equalized means the compressor is not running - or - it means that the compressor isn't compressing anything. On my car, with the compressor OFF, I'm seeing abuot 80 psi equalized pressure. With the compressor on, obviously low goes down and high goes up.

One question... with things equalized, is the compressor running? At 25psi, the 3 level switch would probably not let it engage so I'm guessing it isn't. Sounds like you definitely have a leak to the point where now your compressor won't engage. I think a recharge with the UV dye and a leak light test are in order.
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