Author Topic: My A/C compressor is cycling. . . . .  (Read 206 times)

raueda1

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My A/C compressor is cycling. . . . .
« on: 13 August 2020, 05:15 PM »
 . . . . on and off every so often.  It's as if there's a thermostat or pressure sensor or something going on-and-off every minute or so.  I'm really only aware of it when driving around town at traffic lights.  The airflow remains cold and the system cools extremely well.  There's never any reason to turn the blower above the 1st click regardless of outside temperature, which runs 95-100°F this time of year.  The drier was recently replaced and the system was recharged with.... Freon 12!  (As the makers wished.) But I can't remember if it's normal for the compressor to switch itself on and off in this way.  FWIW, this is the manual Euro A/C and a GM compressor, thank god.  Any comments welcome!  Thanks and cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

UTn_boy

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Re: My A/C compressor is cycling. . . . .
« Reply #1 on: 14 August 2020, 04:39 AM »
If the evaporator temperature sensor (ETR) is sensing that the evaporator is 38 degrees F or lower it'll interrupt current to the compressor......but that shouldn't happen in the middle of a hot day.   I've noticed these ETR's going bad a lot in the last few years.  It's a total pain to get to, but I'd first try disconnecting the wires going to it and bridge them.  If the compressor stays on you'll know the switch is bad.  But hood them back up.  Otherwise the evaporator will ice over as soon as the sun goes down. 
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raueda1

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Re: My A/C compressor is cycling. . . . .
« Reply #2 on: 15 August 2020, 08:16 AM »
Thanks Aaron!  Not sure what that means, A/C isn't my thing at all so will learn more....

In the meantime, more data.  Just drove 600 miles through blazing heat.  When it cycles off the air does get warmer (which obviously makes sense), though not for long enough to really notice unless you're paying close attention.  Still.  Can this mean system is undercharged?  I did have one brief incident of belt squealing.  After stopping for a couple minutes with engine running it started squealing when I got back on the road but stopped after a few moments.   :P
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

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Re: My A/C compressor is cycling. . . . .
« Reply #3 on: 15 August 2020, 04:40 PM »
Dave from my limited experience with this, and this is from ACC perspective, the system has a pressure switch that protects the system in that it prohibits the compressor from kicking in if your system has too low of a pressure.  Might be worth a check.
Daniel
1977 450 SEL 6.9 - Astralsilber

raueda1

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Re: My A/C compressor is cycling. . . . .
« Reply #4 on: 15 August 2020, 05:54 PM »
Dave from my limited experience with this, and this is from ACC perspective, the system has a pressure switch that protects the system in that it prohibits the compressor from kicking in if your system has too low of a pressure. . . . .
. . . . which would be consistent with underfilling the system, no?
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

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Re: My A/C compressor is cycling. . . . .
« Reply #5 on: 16 August 2020, 08:08 AM »
I would think so yes.  You  can try and jumper the pressure switch as a test to see if the compressor keeps running.
Daniel
1977 450 SEL 6.9 - Astralsilber

Jed

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Re: My A/C compressor is cycling. . . . .
« Reply #6 on: 17 August 2020, 08:14 PM »
Dave,

I am having this exact same problem with my porsche 928.  Have not been able to nail it down yet.

Does your new drier have a glass window? If so look and see if there are a lot of bubbles?  Lots of bubbles can mean the system is low on r12.  Proper way to check is with AC manifold gauges.

Does your electric accessory fan come on when the system heats up? this can make a huge difference in passing air through the coil and cooling.  (even more important when your talking about stop and go in town driving)

Definitely  (temporarily) jump your pressure limit switches and see if you can bypass the problem to help troubleshoot.

good luck and let us know...





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raueda1

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Re: My A/C compressor is cycling. . . . .
« Reply #7 on: 18 August 2020, 08:30 AM »
Dave,

I am having this exact same problem with my porsche 928.  Have not been able to nail it down yet.

Does your new drier have a glass window? If so look and see if there are a lot of bubbles?  Lots of bubbles can mean the system is low on r12.  Proper way to check is with AC manifold gauges.

Does your electric accessory fan come on when the system heats up? this can make a huge difference in passing air through the coil and cooling.  (even more important when your talking about stop and go in town driving)

Definitely  (temporarily) jump your pressure limit switches and see if you can bypass the problem to help troubleshoot.

good luck and let us know...
Thanks Jed.  Yes, I have the window.  Will have a look.  Electric fan does seem to come on sometimes.  I've got gauges too but I wasn't 100% confident that I was using them right.  At the moment I'm traveling though, so not a good time to systematically look at this stuff.  When I get home I'll revisit the issue.  At least the system is cooling well.  It's 106F out there.  Brutal!!
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0