Tony K

I'm just wondering what the normal cycling time of the compressor is. I have 134a in a stock system.

On really hot days, at *idle*, my compressor will run a few minutes straight without ever clicking off - I don't let it stay like that to see just how long it will run before it clicks off because I am afraid of damaging the compressor from running too long.

On really hot days, on the *freeway just under 3k RPM, steady*, it will range from 14 seconds on/14 seconds off (fan **** turned to 1st position) to 21 seconds on/9 seconds off (fan **** in 2nd position). It's plenty cold, so I never need to put it to the third position! (I listen to the relay to get the times.) Is my compressor running for too long intervals/not enough off, or am I okay? AC is nice and cold; *I'm just fussing with the details!* I am concerned with burning up/wearing out the compressor.

Also, I filled the system with about 8 ounces of esther oil and 25-26 ounces of 134a. (total is around the 33.5 called for on the label). Are these amounts right, or do I need to add/remove? Once again, air is nice and cold - feels just like a late model/new car system.

I don't have access to gauges right now and I am about to take a long trip; sorry I can't tell what the pressures are. When I filled it several weeks ago, it was 250psi on the high side at 90F outside temperature. But there is a possibility of a slow leak (even though vacuum test did not reveal)

Many thanks,

MichelleJD

That sounds about normal to me. When I lived in FL there were times that the compressor barely kicked off!

Charlotte944

A/C Compressor cycling is a function of engine RPM, ambient (outside) air temperature, and the amount of thermal load on the system.

For a given RPM, the cycle time will be longer for a "hot" interiror. For the later cars like yours, the climate control module monitors the cabin air temperature via a sampling hose in the dash, compares that temperature to the temperature control setting, and then cycles the comperssor/shifts mixing dampers as appropriate.

Additionaly, the climate control module monitors the temperature of the air leaving the evaporator and will cycle the compressor to prevent the formation of ice on the evaporator.

You system is working fine, so there is no need to manually cycle the compressor.

Tony K

Michelle - thanks - that is reassuring! Did you ever have any problems with your AC?

Cliff - I am not interested in manually cycling the compressor; I am concerned if refrigerant might have seeped out since I charged it and the compressor is having to run longer to produce the necessary pressure/temperature. I am concerned that the compressor running too long may be bad for it. And yes, I understand the factors that go into AC compressor cycling; you left one out - amount of refrigerant present in the system. A certain amount of pressure needs to build for the expansion valve to work; if the system is low, the compressor has to turn more and/or faster to build the required pressure. So two questions:

1) is it bad for the compressor to stay running for too long? i.e., will parts of it heat up and be ruined? If so, what is the longest that a compressor should be turning (at a given speed) that won't risk damaging it.

2) So for an extended interval of just under 3000 engine RPM with 90 degrees outside and the sun shining on a white car and the standard 944 greenhouse interior with no window tint, and the fan **** on the second speed thus drawing more air out of the dashboard, does 21 seconds on/9 off sound about right, or do you think I might need to add more refrigerant?

Thanks,

Mike1982

Hey Tony, I wouldn't be as worried about it if the air coming out of the vents is COLD when it is 90 outside. When it gets to about 90 here (same place as you) my car won't get ice COLD just cool but I know everything is working fine after that $800 bill I just paid Eric for a complete rebuild. I think I might even have a small leak or not enough in my system.

Charlotte944

Tony:

Trying to assign a number to the compressor cycling time is (IMHO) an excersize in futility for the simple fact that there are just too many variables.

Secondly, there is no guide line or specification that I am aware of regarding compressor cycle times, and all of the system performence data that I have seen were taken AFTER the system stabilized, not before. There fore, compressor run time prior to system stabilization is useless information.

The best way to determine how well the system is functioning is to connect gages to the system, record the ambient temperature, and monitor the vent temperature at the center vent.

Although R-134 has different pressure/temperature charactarists, you can use R-12 pressures and temperatures to get a 'ball-park" feel for how the system is running.

I have three cars (my two 944s and the wife's Forester) that use R-134, and all three system pressures are within 5 or so psi of each other. Both of my 944s have stock A/C components. I have converted 2 944 A/C systems to run R-134 (my current '87, and a previous '84), and my 951 was running R-134 when I bought it this past March.

Personally, I get the feeling that your are over reacting. If the system is/was leaking it won't be long before the system pressure gets to the point where either the compressor cycles on and off very rapidly ("short cycles"), or the compressor clutch will not engage at all.

However, as long as the system is properly charged, running the compressor will do no damage, no matter how long the compressor runs before the system stabilizes.

If you are really paranoid about doing damage to the compressor, I highly recommend having a certified A/C tech check the system for you. NOTE OF CAUTION: A/C shops like to sell stuff and once they find out this system was converted to R-134, stand by to get pummeled with suggestions as to why you MUST change this or that part, and typically these "must have" items will be at a premium price because they are for a Porsche.

MichelleJD

Oh yeah. Lost it twice in my n/a, and just fixed it in the turbo. Being an ex-FL girl, I go nowhere w/o A/C

Tony K

Thanks Cliff - I'll breathe easier. I always fuss with things a bit too much. I'll just enjoy my cold air for a while.

Yeah, I've heard all of the warnings and "must buys" before! Funny, according to their horror stories, the sky should have fallen by now for how long I have used 134a in old systems! With the exception of one Porsche shop that I know, I would never take my car to a shop - just because somebody does something every day does not mean that they can think. I am batting close to 1.000 for experiences where preferred, recommended, well-reputed shops (of all kinds of service and for all cars) turned out to be just a bunch of monkeys that did more harm than good.

83na944

Tony,

There's a TSB on the conversion to R134a. Check the FRWilk site for a copy.

sm

I think that's about double the factory recommended amount (~4oz) of oil, but if your A/C is working, than leave it be for now.

I have about 5oz of oil in mine and am thinking of adding more oil to it b/c the compressor is still not 100% smooth. However, I heard adding too much oil screws up the pressures.