car surges when A/C is turned on

[mrfeh]

Hi folks.

 

I have a '99 Outback that had its heater core replaced a few months ago. I know that as part of that work the A/C system was tweaked, since the refrigerant had to be replaced.

 

Anyway, a couple weeks ago was the first time I used the A/C since that repair (I live in WI). When the A/C is turned on, the car surges; about once a second, the RPMs drop and then recover, resulting in an annoying jerking motion of the car. I can also hear a fairly faint clicking sound from under the dash each time the RPMs drop.

 

Any mechanics out there that could guestimate what's causing this?

 

Thanks.

[BigMattyD]

Consider this:

 

If the a/c system is overcharged with refrigerant the compressor will cycle on and off because of the high pressure limit switch being activated. You will not get a good cooling effect and it will make the car buck, as the compressor clutch keeps cycling on and off.

 

Believe me, it is annoying. It happened to me last year when I added some refrigerant to my system in an attempt to improve the cooling. It turns out it was already full, and didn't need any extra. Because I am not as environment-conscious as some of the others on this board, I vented a bit of refrigerant out of the low pressure side of the system. Everything has been fine since.

 

Maybe the guy that worked on the ac last year overfilled the system. That's the first thing I would check.

 

Matt

[mwatt]

Hi folks.

 

I have a '99 Outback that had its heater core replaced a few months ago. I know that as part of that work the A/C system was tweaked, since the refrigerant had to be replaced.

 

Anyway, a couple weeks ago was the first time I used the A/C since that repair (I live in WI). When the A/C is turned on, the car surges; about once a second, the RPMs drop and then recover, resulting in an annoying jerking motion of the car. I can also hear a fairly faint clicking sound from under the dash each time the RPMs drop.

 

Any mechanics out there that could guestimate what's causing this?

 

Thanks.

 

My wife and I own a 1998 Outback Limited and a '99 Legacy GT---some of what you're feeling and hearing is normal (I've elaborated on this below)---BUT---if the A/C system is not fully charged with refridgerant, the compressor will cycle on and off rapidly, rather than the normal "occasionally". If the A/C system is undercharged, it's designed to "get your attention" by cycling on/off rapidly--about once every second or two.

The A/C compressor is supposed to cycle on and off as necessary to maintain correct evaporator core temperature so it won't freeze up. (the evaporator is the A/C part in the dash that makes the cold air---looks somewhat similar to a heater core). We hear that same faint "click" under the dash of our cars as the compressor cycles on/off---that's the compressor relay just doing its thing.

The engine control computer is designed to bump the engine idle speed up slightly to compensate for the drag of the compressor when it cycles on. So I believe your A/C is under-charged, causing rapid on/off cycling of the compressor, and thus, the normal up and down idle speed adjustment by the computer when it senses "compressor on". You may have a refridgerant leak, or it may not have been fully re-charged. I'll bet once the system is charged correctly (and any leak repaired), the compressor won't cycle rapidly anymore.

[All_talk]

mwatt covered it while I was hunting and pecking.

 

redundant info deleted

 

Gary

[mrfeh]

Thanks for the replies folks. Always good to know what the issue is before taking the car in!

 

My wife and I own a 1998 Outback Limited and a '99 Legacy GT---some of what you're feeling and hearing is normal (I've elaborated on this below)---BUT---if the A/C system is not fully charged with refridgerant, the compressor will cycle on and off rapidly, rather than the normal "occasionally". If the A/C system is undercharged, it's designed to "get your attention" by cycling on/off rapidly--about once every second or two.

The A/C compressor is supposed to cycle on and off as necessary to maintain correct evaporator core temperature so it won't freeze up. (the evaporator is the A/C part in the dash that makes the cold air---looks somewhat similar to a heater core). We hear that same faint "click" under the dash of our cars as the compressor cycles on/off---that's the compressor relay just doing its thing.

The engine control computer is designed to bump the engine idle speed up slightly to compensate for the drag of the compressor when it cycles on. So I believe your A/C is under-charged, causing rapid on/off cycling of the compressor, and thus, the normal up and down idle speed adjustment by the computer when it senses "compressor on". You may have a refridgerant leak, or it may not have been fully re-charged. I'll bet once the system is charged correctly (and any leak repaired), the compressor won't cycle rapidly anymore.

[BigMattyD]

Read this article about the A/C system:

 

 

http://endwrench.com/images/pdfs/ACMechanism.pdf

 

I know that the compressor will cycle on and off about once a second when it is over filled. I did it myself.

 

I don't know if the compressor will cycle quickly when it is under-charged.

 

From my understanding of this document, if the system is under-charged, the compressor will fail to run at all or will run constantly with minimal cooling effect. I have seen this happen on other vehicles.

 

If the system is over-charged by an amount that is just under the high limit, then the compressor will start briefly, but shut off once the high side pressure reaches the limit. Because no cooling is taking place, the temperature sensor in the car is telling the compressor to run, but the limit switch is telling it the pressure is too high, so stop.

 

Therefore, you get the rapid on and off cycling.

 

That is what happened to my vehicle. When I released some of the refrigerant, it returned to normal operation. That was a year ago, and the problem hasn't returned.

 

Matt D