AC blows cold, but compressor cycles on/off: help?

MarkRobinson · 06-30-2009, 10:27 AM

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

flash968 · 06-30-2009, 10:33 AM

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

MarkRobinson · 06-30-2009, 10:39 AM

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)

Razorback1980 · Razorback1980's Garage

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.

MarkRobinson · 06-30-2009, 12:20 PM

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

kdjones2000 · kdjones2000's 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

Razorback1980 · Razorback1980's Garage

Pressures should not fluctuate to any great degree.

bazar01 · bazar01's Garage

Quote:

Originally Posted by MarkRobinson

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?

MarkRobinson · 06-30-2009, 01:20 PM

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.

mattdavis11 · 03:39 PM

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?

MarkRobinson · 07-01-2009, 05:53 AM

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

mattdavis11 · 07-01-2009, 03:10 PM

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.

my1st911 · my1st911's Garage

Quote:

Originally Posted by MarkRobinson

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.

zedsn · 07-02-2009, 06:04 PM

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.

tomiki · 09-04-2009, 09:08 AM

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

John_AZ · 09-04-2009, 09:31 AM

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

tomiki · 09-04-2009, 09:40 AM

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

MarkRobinson · 09-04-2009, 10:06 AM

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.

wild man · 09-06-2009, 03:52 AM

It blows cold very cold air?

That's the bottom line.

It is working perfectly.

Makes no sense to fock with it!

Crimson Nape Racing · 09-08-2009, 07:54 PM

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