weird rev after air con is on

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herrickmky

New Member
Joined
Apr 1, 2015
Messages
7
Car
W124 300CE-24
I notice that the rev keeps bouncing almost every minute after I turn the a/c on. Usually, the 1st 6 to 7 minutes are fine, then the rev goes down and bounces back every 40s to 1 minute. Look like the a/c is on and off. Just wonder if it is normal.

I recorded a video and it is here. In this case, the first 7 minutes were ok, then it happens at 7:36, 9:02, 9:54 and 10:41

https://youtu.be/ZA13aP8Kr14

The car is C124 - 300CE-24
Year of production - 1992

Is it normal? if not any idea what the causes are?

Thanks!
 
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Old air con systems when they switch on and off the revs do drop and rise slightly. It happens on my C124 and I tend to notice it more on the motorway, when it turns on the revs dip slightly then back to normal when it switches off.
It is annoying and modern systems don't do it.
 
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The A/C compressor pulls something like 5-15hp when it engages and the drop in the revs is due to this. At 750rpm idle that is significant on the engine power. The idle control circuit then reacts and brings the idle speed back up - It may do the same when you turn on the headlights or heated rear window to a lesser degree :)
 
Thanks for the replies. I should have asked my question in a more accurate way.

The rev fluctuation is normal when a/c turns on, my question should be if it is normal to happen so frequently, almost every minute
 
There are many causes of the compressor cycling frequently and I'm sure an expert on here will chime in.

On top of my head the pressure switch will cut in to protect the system from it running too high or low. Have you had work done on it? Re-gassed? Replaced the receiver drier?

Climate & humidity plays a part too. High ambient temperature and humidity could cause icing of the evap core, tripping the compressor. Is it cooling the car sufficiently? Try taking a temp. reading from the dash vents. I'm assuming you're from HK.
 
There are many causes of the compressor cycling frequently and I'm sure an expert on here will chime in.

On top of my head the pressure switch will cut in to protect the system from it running too high or low. Have you had work done on it? Re-gassed? Replaced the receiver drier?

Climate & humidity plays a part too. High ambient temperature and humidity could cause icing of the evap core, tripping the compressor. Is it cooling the car sufficiently? Try taking a temp. reading from the dash vents. I'm assuming you're from HK.


Hi, Coalville126, I'm from HK. I've done nothing on the air con system and the system is cool, indeed, I am surprised that it is so cool, no problem at all to cope with the hot weather here. Let me look at the pressure switch you mentioned this weekend

Thanks!
 

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