Youcef

Could anyone please tell me how to test the A/C pressure switch with my Volt meter.

Thanks

Youcef

Adrian

Dear Youcef,
You need a multimeter because the pressure switch, switches an earth. Actually it breaks an earth line.
It is very hard to check properly unless you know the pressure in the system. The switch is best tested by using the correct aircon test equipment rig. This means you need an experienced mechanic.
You also need the wiring diagram for the switch.
Technically if your system is in running order the switch should be closed at all times. Inotherwards if there is 12V on the compressor clutch wire (clutch engages) the pressure switch is working.
If you suspect the pressure switch jumper it (bypass it) and if the clutch engages you have found the problem.
Ciao,
Adrian
964C4

Youcef

Adrian thanks for the help.

The issue I have is that when my A/C is set to 4 the compressor runs all the time, if I set the fan speed at 1 or 2 the compressor cycles on and off. I hooked up my test manifold and pressure at idle on low valve is 25 psi and at high valve is 250. If I raise revs up at 2000/4000 rpm the low side drops to 10/15 psi and the high side only gets up around 300psi.

Any thoughts?

Youcef

DaveK

Quite a while ago, I posted that my A/C compressor cycled on and off - and a number of people said theirs did to, and it was normal. Someone (Randall I believe, since he is the resident A/C expert) also posted a link to an A/C article which suggested that there were two types of system - those that had to cycle, and those that didn't.

It is odd that yours doesn't cycle on setting 4 though (I assume you mean the fan)? I can't try mine now since my evaporator leaks and I don't have enough pressure for mine to work at all.

Randall G.

I can only guess that a fan setting of 4 is putting more load on the system, and keeping the evaporator from getting too cold, which is keeping the compressor from cycling.

Speaking of compressors cycling--or not. When my A/C was working great, and cooling to the low 40's (Fahrenheit) at center vent, the compressor cycled regularly. My system was last charged (converted to R-134a) in July of '02. As this past summer (2003) wore on, I suspected the system was cooling just a tad less effectively. I kept a pencil thermometer in my car, and actually tracked center-vent temperature as it increased from the mid-40's, to the low 50's, to the mid 50's and--by December--not cooling noticeably below ambient. Interestingly, with the system providing no cooling, the compressor no longer cycles--it runs continuously. So, I believe there's enough gas to keep the compressor from tripping off on low pressure. But, not enough to cool.

I suspect if I wait long enough, the compressor will quit running altogether (like Dave's). But, I don't plan on waiting that long. I plan on getting a (low-cost) R-134a charge in May, which should (hopefully!) last until next summer.

Youcef

Hmmm, the hunt goes on. At what pressure does the pressure switch close the connection to the compressor clutch to release it? I have looked in my Bentley Without Guesswork and I dont see where it is mentioned. It concerns me that the compressors cycles especially if you consider the clutch engaging at 4000 rpm.

Dave, I also was told my evaporator was leaking but after a long search found it was the crimp on the high pressure hose at the end of the rubber section behind the engine blower motor. Cost me zero dollars to have a new section crimped in. I discovered this finally from the oil residues on the foam padding under the clamp that holds the hoses to the upper engine bay. Problem solved until now!

Youcef

Adrian

Dear Youcef,
I have never seen pressure rating written anywhere for the switch I am afraid.
The fan speed should have nothing at all to do with the air con system. That fan speed is purely for the left and right blower fans. As has been suggested there could be a low speed problem with the fan(s) which impacts the control unit which then maybe causes the condenser fan operation to be impeded resulting in increased heat and then the pressure switch activating seemingly cycling the compressor. I would hate to guess but my bet would be the control unit not the 964 electrical system. Why? Because the system works properly at fan speed 4 which puts the highest load on the electrical system.
Ciao,
Adrian
964C4

Youcef

Adrian,

I agree with you and also cannot understand how the aircon fan speed can have anything to do with the compressor cycling. Do I understand correctly that the thermostat in the cabin mixes the desired air temp and does not shut down the compressor to increase the interior temperature?

I must admit though on my new Jeep Cherokee the A/C cycles on and off also, my Honda CRX cycled on and off so it is a bit hard to understand why this was designed not to do so.

I checked the condensor fan and works fine at all times, had the ballast resistor replaced about 3 months back. Could the A/C relay be playing up? Should I plug back in my noisy little temp sensor fan at the back of the CCU?

Thanks for all the advice guys, I will kepp trying.

Youcef

Randall G.

Hey Yousef,

Besides searching the 'net (and posting the link Dave alluded to above), I asked Griffiths ( http://www.griffiths.com/porsche/ ) about the compressor cycling on a 964. Charles Griffith's response copied below:

"There are usually 3 devices that will turn on and off the compressor:
A) if the interior cockpit thermostat is not satisfied, meaning you have
the temperature set at 65f and cockpit is still at 75f then the compressor will continue to run, however once the temperature in the cockpit reaches 65f then the compressor should stop running, so you can say that the thermostat can turn on and off the compressor
B) the evaporator will probably have a "freeze out" switch or
thermocouple, this device will shut off the compressor if the temperature of the evaporator gets too cold, this is to prevent the evaporator from icing up, so you can say the device can turn on and off the compressor
C) the system usually has a "high-low" pressure switch, if system
pressures get too low or too high the switch will turn off the compressor, pre 90's 911's (non c2 or c4) did not have this device
D) optional, I'm not that familiar with the 90+ ac system, however there
may be a "cycling" switch such as found on the 944 (in the 944 it is located above the gas peddle), this device can be adjusted to control cycling."

Besides what Charles has written, imagine what could happen to your evaporator if the compressor just continued to run with low system load (lots of cooling, little heat removed from the evaporator). Of course, it would ice up. But, this is only going to happen if the system has a proper charge. Otherwise the system never gets cold enough to chill the evaporator.

Oh, and I also posted before that the A/C compressor on both of my other cars cycle. Both blow very cold, one car is almost brand new. Finally, think about how many times you've been sitting at a traffic light, and hear a clicking coming from the engine of the shiny new car next to you--the A/C compressor cycling.

The low-pressure set-point for the switch is 2.5 bars, the high-pressure set-point is 27 bars.

Good luck. Though it sounds to me like you don't have a problem, if your system is blowing nice cold air.

Youcef

Hey Randall,

Thank you for such great information. I had surfed the Griffiths site when I was told I had a evaporator leak. The A/C blows really nice cool air, I infact have to reduce the fan speed because I get too cold.

Yep, I have heard it so many times the click of the A/C clutch and the revs going dip for a moment and it is something you hear at traffic lights on so many cars.

I am a little confused though, you mention that the low pressure switch is set a 2.5bar around 36psi. I thought and other rennlisters have posted that 25 psi was normal low pressure port readings. Maybe I need to add charge to the system!

Am going out now to check the guages again! But must admit that I am beginning to think it is working normally! Thanks again, appreciate it.

Youcef

Randall G.

>I am a little confused though, you mention that the low pressure switch is set a 2.5bar around 36psi. I thought and other rennlisters have posted that 25 psi was normal low pressure port readings. Maybe I need to add charge to the system!

Oops, sorry. That's the low-pressure set-point on the high-pressure side of the system.

Youcef

Well I dont think I am going to do any more investigation on this one. I am convinced that the A/C system I have is meant to cycle.

I went out to check again my readings, I used another test manifold at a local shop this time and confirmed running low port pressure as 29 psi. I bought a pencil thermometer also and recorded a low temp of 7.0C after 10 minutes running(with all recommended precaution taken), the outside temp tonite at 8.30pm is 29.0 C.

From what I can gather from the information I have at hand this is quite good for a R12 system, thanks to all appreciate all the time you have given me.

Youcef

Randall G.

Yup, 7.0C at 29C ambient is very good.