Parts Catalog Accessories Catalog Tech Info Tech Forums
 
  Search our site:    
 Cart  | Project List | Order Status | Help    
  We salute a legendary member of the community.
Thank you for selflessly sharing your wisdom with us...
Go Back   Pelican Parts Technical BBS > 1- Porsche Technical Forums > Porsche 924/944/968 Technical Forum

Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Architecture & Porsche's
 
Join Date: Jan 2006
Location: Austin, Texas
Posts: 3,042
AC blows cold, but compressor cycles on/off: help?

Hi all,

I figure since the 951/944 AC systems are pretty similar, I might have a better chance of someone explaining this to me on this forum.

My '88 Turbo S has a great AC system: complete gone-through within 5 years, blows very cold, though the compressor cycles on/off every 10-15 seconds or so continuously: regardless of temperature, time, speed, etc.

Checked the fill & it's correct (R12 as well), but with the cycling I can hear a "click" up by the fuse box & the AC gauges show that it the pressures might be too high, as the gauges rise/fall for 10-15 seconds, then reset & do it all over again.

Thoughts?

Mark
__________________
Ugh, is it 2015 yet?? :/
Old 06-30-2009, 10:27 AM
  Recommend this thread for the PelicanWiki    Reply With Quote #1 (permalink)
Registered User
 
flash968's Avatar
 
Join Date: Jun 2006
Location: O.C. CA
Posts: 3,950
it should cycle on and off as it needs to in order to maintain a specified temperature - that short duration seems excessive though - is it rhythmic? if so, i would suspect a relay
Old 06-30-2009, 10:33 AM
  Recommend this thread for the PelicanWiki    Reply With Quote #2 (permalink)
Architecture & Porsche's
 
Join Date: Jan 2006
Location: Austin, Texas
Posts: 3,042
The AC compressor? Doesn't make sense. Usually the head unit controls mixing to get the temperature you want, otherwise the AC compressor's clutch is always engaging/disengaging, which would lead to premature failure since your rpm's are not low most of the time.

I do hear a relay clicking from time to time & you can feel the drain and vibration on the engine when the alternator takes an increased load due to the fan's kicking in. (yes, new motor mounts)
__________________
Ugh, is it 2015 yet?? :/
Old 06-30-2009, 10:39 AM
  Recommend this thread for the PelicanWiki    Reply With Quote #3 (permalink)
Registered User
 
Razorback1980's Avatar
 
Join Date: Nov 2005
Location: Houston, Texas
Posts: 2,944
Garage
Send a message via AIM to Razorback1980
If the pressures are going too high, then the freon levels probably are not correct or the expansion valve is partially plugged and the high pressure switch is shutting off the compressor clutch. Pressures determine how and when the clutch is engaged. Are both pressures too high or is the high side too high but the low side not? If the expansion valve is partially plugged, the high side will be high but the freon can't get through the expansion valve quick enough so the low side will be low. If both sides are too high, then the system is improperly filled with freon.

As you said, the head unit controls the temperature and the clutch cycling has nothing to do with that.
__________________
Tom

1990 944S2 Cabriolet
2002 Chevy Silverado 2500HD
2003 Maroon Ford F350 dually
Old 06-30-2009, 12:15 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #4 (permalink)
Architecture & Porsche's
 
Join Date: Jan 2006
Location: Austin, Texas
Posts: 3,042
hm, I was suspecting the expansion valve: I have an extra (new), I'll have that installed & try again.

We double-checked the amount of freon (R-12 amounts differ from R134A) in the system (+/- 900 grams-ish if memory serves). I was not confident in the AC expertise of the mechanic after he said the wild fluctuation of the pressures on the gauge were of "normal operation", & packed up & left.

Mark
__________________
Ugh, is it 2015 yet?? :/
Old 06-30-2009, 12:20 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #5 (permalink)
In the Fires of Hell.....
 
kdjones2000's Avatar
 
Join Date: Oct 2007
Location: Phoenix, AZ
Posts: 2,099
Garage
My A/C compressor cycles on and off, even with the head unit controlling temperature. It doesn't cycle every 15sec, more like 3-4 minutes.

Either your high side is going too high, or low side is going to low and tripping a pressure switch. That or some other item not yet discussed.

Good luck, Keith
__________________
'88 951S - with LBE, Guru chips, 3Bar FPR, 1.3mm shimmed WG
3300 lbs, 256 RWHP, 15 psig boost
"Certified National PCA Instructor" Hoooohah!
1987 924S 944 SPEC racecar - '88 pistons/DME, short 5th
Old 06-30-2009, 12:20 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #6 (permalink)
 
Now available:  101 Projects for Your Porsche Boxster!
Registered User
 
Razorback1980's Avatar
 
Join Date: Nov 2005
Location: Houston, Texas
Posts: 2,944
Garage
Send a message via AIM to Razorback1980
Pressures should not fluctuate to any great degree.
__________________
Tom

1990 944S2 Cabriolet
2002 Chevy Silverado 2500HD
2003 Maroon Ford F350 dually
Old 06-30-2009, 12:23 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #7 (permalink)
Registered User
 
Join Date: Mar 2004
Location: Leesburg, Georgia, USA
Posts: 1,477
Garage
Quote:
Originally Posted by MarkRobinson View Post
Hi all,

I figure since the 951/944 AC systems are pretty similar, I might have a better chance of someone explaining this to me on this forum.

My '88 Turbo S has a great AC system: complete gone-through within 5 years, blows very cold, though the compressor cycles on/off every 10-15 seconds or so continuously: regardless of temperature, time, speed, etc.

Checked the fill & it's correct (R12 as well), but with the cycling I can hear a "click" up by the fuse box & the AC gauges show that it the pressures might be too high, as the gauges rise/fall for 10-15 seconds, then reset & do it all over again.

Thoughts?

Mark
Short cycling means it's low on charge. What's the low and high side pressures at 3000 rpm with the thermostat on coldest and evaporator fan on high speed?

Did you evacuate the system, pull vacuum to 30", then charge the right amount by weight as indicated in the A/C decal in the engine compartment?
__________________
1993 964 C2 (100k + miles)
1996 318ti (235k miles)
2011 Silverado for hauling (35k miles)
Old 06-30-2009, 01:11 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #8 (permalink)
Architecture & Porsche's
 
Join Date: Jan 2006
Location: Austin, Texas
Posts: 3,042
they vacuumed it out (dunno to how much vacuum) & then added back the correct amount per this forum (like I said, about 900 grams if memory serves: thsi was 2 months ago).

I dont' recall the pressures, but the high side went up while the low side when down, then after the high side peaked, something would "trip" & the gauges would fall back quickly to their original positions & then the process would repeat: they'd slowly rise over 15 seconds & do it again.
__________________
Ugh, is it 2015 yet?? :/
Old 06-30-2009, 01:20 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #9 (permalink)
Just thinking out loud
 
mattdavis11's Avatar
 
Join Date: Nov 2001
Location: Close by
Posts: 4,447
Sounds like an expansion valve issue. Charge is 950 grams +/- or 2.09 lbs for the later cars.

I'd try to back flush the valve.

Are your condenser fans working properly?
__________________
83 944
91 FJ80
84 Ram Charger

Last edited by mattdavis11; 06-30-2009 at 03:39 PM..
Old 06-30-2009, 03:34 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #10 (permalink)
Architecture & Porsche's
 
Join Date: Jan 2006
Location: Austin, Texas
Posts: 3,042
I think I read like 860 grams of R134-a or 902 grams of R12: something like that.

If you feel 950 is the proper amount, then it possibly this lack of filling is causing the issue, but yes the gauges DO fluctuate oppositely in 15 second increments. I have an extra (new) Expansion valve: I'll have that re-installed & the system vacuumed & recharged with 950.

Yes, fans work ok it appears.

Mark
__________________
Ugh, is it 2015 yet?? :/
Old 07-01-2009, 05:53 AM
  Recommend this thread for the PelicanWiki    Reply With Quote #11 (permalink)
Just thinking out loud
 
mattdavis11's Avatar
 
Join Date: Nov 2001
Location: Close by
Posts: 4,447
The factory service manual says 950 grams, which is 2.09 lbs of R12. The early cars hold more freon, and I guess the 924S does as well. I'm sure the guy cleared the sight glass though.
__________________
83 944
91 FJ80
84 Ram Charger
Old 07-01-2009, 03:10 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #12 (permalink)
 
Now Available for Ordering:   101 Projects For Your BMW 3 Series 1982-2000  [more info]
Registered User
 
Join Date: May 2008
Location: Flanders NJ
Posts: 312
Garage
Quote:
Originally Posted by MarkRobinson View Post
hm, I was suspecting the expansion valve: I have an extra (new), I'll have that installed & try again.

We double-checked the amount of freon (R-12 amounts differ from R134A) in the system (+/- 900 grams-ish if memory serves). I was not confident in the AC expertise of the mechanic after he said the wild fluctuation of the pressures on the gauge were of "normal operation", & packed up & left.

Mark
900 grams is about right. Thats about 2 lbs. Tell us what the pressures are. they should be about 30-40 on the low, and 200-250 on the high. The high side will spike up and the low will go down quite a bit. Some cars cycle just for the fact that the temperature senson in the evaporator satisfies and shuts the compressor to avoid freezing. Its tough to tell your problem, but I would check pressures before taking out the r-12 since you cant get it anymore.

Last edited by my1st911; 07-02-2009 at 04:05 PM..
Old 07-02-2009, 04:01 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #13 (permalink)
Registered User
 
Join Date: Jan 2007
Posts: 1,159
Hi, you might want to inspect the sight glass and see if the freon is clear or if you still have bubbles when unit is turned on. If you still have bubbles you may need to add some more freon. From memory, compressor cycling often means not enough freon.
Old 07-02-2009, 06:04 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #14 (permalink)
Overseas
 
Join Date: Mar 2009
Location: Nuremberg, Germany
Posts: 74
I think all is fine in your system.
My 924S also shuts on and off very frequently: as the freeze sensor in the evaporator box tells it to. And yes, it does engage/disengage the magnet of the compressor all the time: that's the way it is designed And it won't go bad because the magnet allows nice acceleration of the compressor even if you push it hard for the car.
My A/C system (it's the early one) has only one pressure sensor: potection against low (!) pressure. Therefore when it shuts off frequently, that's simply controlled by the freeze sensor.
BTW 950 grams of R12 is the presribed amount.
Hope this helps, I wouldn't worry if I were you
__________________
Tomiki
924S (1986)
Old 09-04-2009, 09:08 AM
  Recommend this thread for the PelicanWiki    Reply With Quote #15 (permalink)
Proprietoristicly Refined
 
John_AZ's Avatar
 
Join Date: Jul 2001
Location: ~Carefree Highway~
Posts: 5,401
tomiki,
On the 924S the freeze sensor is attached to the expansion valve. Can a bad freeze sensor cause the clutch to stay on forever (temp dial is set to max)?. I just converted to R134a and my engine temp now stays a half with the AC on. Previously with R12, maybe 1/3.

Thanks
John_AZ
1988 924S + 1987 924S
Old 09-04-2009, 09:31 AM
  Recommend this thread for the PelicanWiki    Reply With Quote #16 (permalink)
Overseas
 
Join Date: Mar 2009
Location: Nuremberg, Germany
Posts: 74
Hi John,
Well, my car had originally no A/C, but I installed one from an '84 944. In this one the freeze sensor is simply installed with a screw on the inside box and besides the two wires it has no connection to anything. Therefore about its connectivity with the expansion valve I can't really say anything.

On the other hand a faulty freeze sensor can indeed leave the clutch on forever.
And of course the temperature dial does control this greatly. Setting it to higher once the A/C reached its normal conditions, i.e. the inside box is already cold, will make the "clutch off" states shorter and shorter, but the "clutch on" states cannot be controled by anything, as that's done by the freeze sensor.
I hope this answers your question, if not, please ask it again Thanks
__________________
Tomiki
924S (1986)
Old 09-04-2009, 09:40 AM
  Recommend this thread for the PelicanWiki    Reply With Quote #17 (permalink)
Architecture & Porsche's
 
Join Date: Jan 2006
Location: Austin, Texas
Posts: 3,042
well when I'm softly accelerating, there's a noticable bump in power when the Compressor's engaging. I have large, long hills in my area & it'll kick a few times going up these hills, pretty noticable.

Never noticed it in my last 944TS.
__________________
Ugh, is it 2015 yet?? :/
Old 09-04-2009, 10:06 AM
  Recommend this thread for the PelicanWiki    Reply With Quote #18 (permalink)
All Spooled Up
 
Join Date: May 2009
Location: Between NE and Central PA
Posts: 2,516
It blows cold very cold air?

That's the bottom line.

It is working perfectly.

Makes no sense to fock with it!
__________________
>gray 89 951S - K27/8, MAF, 3" intake, 3" exhaust w/separate waste pipe, 55# inj, late cam; >red 87 924S - chip, K&N, punched-out cat&muffler >black 80 924 - (sold) >maroon 77 924 - auto (sold)
Old 09-06-2009, 03:52 AM
  Recommend this thread for the PelicanWiki    Reply With Quote #19 (permalink)
Registered User
 
Crimson Nape Racing's Avatar
 
Join Date: Oct 2006
Location: Mid-South
Posts: 108
Mark - mine was doing the same thing (cycling every 10 seconds or so), and I finally figured out there was too much freon. The high pressure side was 275-300 psi at idle. I bled a small amount of freon back into the canister and mine works like a champ now. I cannot keep the temp all the way down or I will freeze (even in 100 degree Memphis heat). You guys who complain about bad a/c need to drive old 911's for a while
__________________
Bob
96 993 TT
73 911E 3.6 (sold)
89 944S2
Old 09-08-2009, 07:54 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #20 (permalink)
Old 09-08-2009, 07:54 PM
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 03:55 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2011 Pelican Parts - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.