Amount of expected A/C compressor on/off cycling when in use?

[Jonathan]

As the title says, I'm curious what others experience with their A/C compressor cycling on and off.

I understand the recommended way of determining what is or isn't a normal amount of cycling would be to measure the refrigerant pressures (or volumes present within the system) and the temperatures coming out at the air vents. Knowing that though, I wanted to get a feel for what others see in their own cars in terms of the compressor coming on and off.

Hoping to avoid any confusion in terms, here's what I might consider when thinking of "cycling."

- One complete cycle might be from when the compressor engages on and then off again, up until the point where it is about to come on again.
- Within that cycle you would have the amount of time the compressor is running (clutch engaged, voltage dropped slightly while doing so) and you would also have the amount of time the compressor is not running. Adding those two times together and you have a third time.

- Let's call these times:

ON
OFF
TOTAL

- Where: ON + OFF = TOTAL. Times measured in seconds.

We also know these times change depending on what RPM the engine is at, so a car sitting and idling at 775 RPM would show different times than one out on the highway doing 55 mph. And with that in mind, I would suspect different times depending on what transmission you have, since the automatic will have a higher RPM at higher speeds since it lacks any gears beyond 3rd. That's one of the main reasons I ask, is to see what people have for compressor cycling times in manuals versus automatics. It also depends on what fan speed you have your blower on (1,2,3 or 4) and what A/C mode you are set to (MAX, NORM, etc.)

For simplicity sake, let's say we go with A/C set to MAX on mode switch and report cycle times for four scenarios:

Car parked with engine idling and no throttle, fan speed at 1
Car parked with engine idling and no throttle, fan speed at 4
Car driving at 55 mph, fan speed at 1
Car driving at 55 mph, fan speed at 4

I can already see how there are going to be a ton of variables unaccounted for, like the temperature outside while driving, what gear a manual trans is in, road conditions (flat, smooth, windy, etc.), tire tread wear or condition, tire air pressures, etc. This won't necessarily be scientific, but please humour me with what you find anyhow.

[Bitsyncmaster]

What causes the compressor to switch of is when it draws the low side pressure down to about 30 PSI. If your circulation fan is blowing outside hot air at a high speed, the compressor will never pull down that low.

Most of us use MAX AC which recirculates the cabin air so it usually can pull that low and cycle. A lot has to do with how Sunny it is also. When driving, I can not really tell much if the compressor is cycling.

If you want to see it cycle, turn the AC on and shut your circulation fan off.

[Ron]

Car parked with engine idling and no throttle, fan speed at 1
Car parked with engine idling and no throttle, fan speed at 4
Car driving at 55 mph, fan speed at 1
Car driving at 55 mph, fan speed at 4

Sorry, I can't help with data since I sold my D, but want to suggest that you include ambient temperature, since heat exchanged at condenser is the most significant factor......
Maybe refrigerant type (different type = different pressures at the cycle switch).

[David T]

Sorry, I can't help with data since I sold my D, but want to suggest that you include ambient temperature, since heat exchanged at condenser is the most significant factor......
Maybe refrigerant type (different type = different pressures at the cycle switch).

There are a # of variables that will affect the timing of how often and fast it will cycle so no hard and fast rules apply. Among the variables include:
How much and what kind of refrigerant
If there is any air in the system
Ambient temperature
The setting of the low pressure switch (it has an adjustment screw and can be tweaked some)
Speed of the interior fan
If the car is moving
How well the cooling system of the motor is working

The "proper" way to begin is with a clean, well evacuated system refilled with a measured charge of refrigerant. Otherwise you are just "winging" it. The system was designed to use R-12 so if you make any substitutions you must readjust to compensate. The way the system was designed it MUST cycle or it can freeze the evap coil and overload the electrical system (the cooling fans are not meant to run continuously).