AC cycling?

boeing 717 · 03-24-2017, 07:21 AM

I recharged my 968 system. I know that I got in 28 oz of 134. I know there are 5.5 oz of pag oil in the system not counting an unknown amount in evaporator. I charged with new gauges and referred to Porsche manual. Car has a new compressor, condenser and drier.
I was seeing 20-22 on the low side and 200-205 on the high side. Driving car the ac blows cold but the compressor cycles on 7 seconds and off 18 seconds. I've read where it should be about a minute.
Does this sound normal to you guys? It seems it's cycling too much and the compressor has a faint wurrrr noise when it's engaged.

Thanks

Pauld_94S2 · 03-24-2017, 11:06 AM

Need ambient temperatures to assess properly.
Generally you do not have enough refrigerant to satisfy the system demand when the compressor short cycles.

boeing 717 · 03-24-2017, 11:41 AM

It was 26C or 79F. Book calls for 30oz 134, I weighed cans and put in very close to +-28 oz 134.

mfiazzo · mfiazzo's Garage

Do you see any of the refrigerant lines freezing up? Did you check the pressure switch to see if it was operating properly.

Di you properly evacuate the system before you charged it? Could be air in your system as well.

boeing 717 · 12:39 PM

No freezing noted. Evacuated for at least 3 hours to 30 inches vacuum. I did not check pressure switch but the gauges show it cycling when pressure is reached on the high side, 200-205 psi at idle, the compressor kicks off, high pressure drops, low pressure increases and then it repeats.

If the pressure switch was bad what would be an indication? Isn't there a low and a high pressure switch?

mattdavis11 · 03-24-2017, 03:09 PM

I would leave it alone and wait to fine tune it when it gets to 90-95F ambient. You are in Hot-lanta, and the charge right now might yield the perfect pressures later, optimizing the efficiency when it really needs to be spot on. Summer.

Every car is different too, which affects performance of the ac. The cleanliness of the condenser, radiator and evaporator (air flow), the compressor health, oil charge,.....

Just wait for the ambient to come up.

boeing 717 · 03-24-2017, 03:55 PM

Quote:

Originally Posted by mattdavis11

I would leave it alone and wait to fine tune it when it gets to 90-95F ambient. You are in Hot-lanta, and the charge right now might yield the perfect pressures later, optimizing the efficiency when it really needs to be spot on. Summer.

Every car is different too, which affects performance of the ac. The cleanliness of the condenser, radiator and evaporator (air flow), the compressor health, oil charge,.....

Just wait for the ambient to come up.

I'm not really worried about the pressures, they appear normal on the gauges and it was a brand new gauge set. it's the AC cycling on and off so much that I'm wondering is abnormal as well as as faint compressor noise.

Pauld_94S2 · 03-24-2017, 05:00 PM

Try adding 1-2 oz of R-134a to see if the compressor cycling gets longer.
I think your inlet pressure is still a little too low.

boeing 717 · 01:45 AM

.......

boeing 717 · 03-24-2017, 05:59 PM

Would the difference between 28ish and 30.3 really make a difference like that?
I will try it though. Thanks Paul.

Bradical · 03-25-2017, 03:49 AM

Quote:

Originally Posted by boeing 717

Would the difference between 28ish and 30.3 really make a difference like that?
I will try it though. Thanks Paul.

Yes it will.

mattdavis11 · 03-25-2017, 05:27 AM

He forgot about the charge hose loss.

Boeing, you are charging to a desired vent temperature and system pressure now. A rise in ambient will bring the pressures up. Get it right when it gets hot.

boeing 717 · 11:12 AM

Quote:

Originally Posted by mattdavis11

He forgot about the charge hose loss.

Boeing, you are charging to a desired vent temperature and system pressure now. A rise in ambient will bring the pressures up. Get it right when it gets hot.

I don't really get what your telling me Matt. Are you saying wait till it gets hot to check the pressures again?

How much is charge hose loss on 60 inch hoses?

Bradical · 03-25-2017, 01:51 PM

Quote:

Originally Posted by boeing 717

I don't really get what your telling me Matt. Are you saying wait till it gets hot to check the pressures again?

How much is charge hose loss on 60 inch hoses?

On a standard three hose manifold- 3 to 6oz of refrigerant easily.

boeing 717 · 03-25-2017, 02:21 PM

Quote:

Originally Posted by Bradical

On a standard three hose manifold- 3 to 6oz of refrigerant easily.

Ah ok, I wondered about that. So that's how much remains in the hoses and manifold after I disconnect?

Bradical · 03-25-2017, 02:39 PM

Quote:

Originally Posted by boeing 717

Ah ok, I wondered about that. So that's how much remains in the hoses and manifold after I disconnect?

Yes, exactly. What I do to try to combat that is to remove the high side and then open the valves to pull the refrigerant back into the suction before I remove the low side. That'll suck the lions share of the refrigerant back in and leave, Im guessing, less than an ounce in the manifold.

mattdavis11 · 03-26-2017, 05:15 AM

Boeing, what I am trying to tell you (not very well), is that you are very close, and dialing it in when it is hot is best. You knew you put 28oz in. How?

You want to get the correct charge amount, what I want you to do is get the vent temperatures as low as it can get when it is hot. Quit messing with it until that day arrives.

boeing 717 · 04:31 AM

Quote:

Originally Posted by mattdavis11

Boeing, what I am trying to tell you (not very well), is that you are very close, and dialing it in when it is hot is best. You knew you put 28oz in. How?

You want to get the correct charge amount, what I want you to do is get the vent temperatures as low as it can get when it is hot. Quit messing with it until that day arrives.

Thanks Matt
I put 2, 12 oz cans in and then put the third can on a scale and when it went down 4 oz I stopped. I didn't go full 30 because I was already on the upper end of the allowed high pressures for that ambient temp and was worried about overcharging.

I did notice a white bubble stream flowing through drier sight glass.

boeing 717 · 03-26-2017, 06:32 AM

....

Pauld_94S2 · 03-26-2017, 06:47 PM

Unfortunately, the white "bubble stream" in the sight glass is largely irrelevant when using R-134a, because it was intended to show the volume of R12 in the system. Since you always charge an R-12 system with less weight of R-134a refrigerant, it will not give you an accurate indication of refrigerant volume, which it did for R-12.

If you find the current vent temperature within an acceptable range for you to operate the AC system, follow mattdavis11's advice and wait until the weather gets warmer, perhaps into the 90 +/- 5 degree range.