Mizuno

Hi folks,

Have a 70k miles, '98 993 with a manual a/c that seems to be working fine. However when I engage the a/c MAX switch, I experience no changes. Fan speed remains as is even if below 4. I tried to press/depress the manual a/c switch but no impact. Any idea on what may be the issue? Thanks!!

ToreB

The A/C max button is a shortcut for A/C compressor enable, max front fan speed, minimum temperature, mixer servos closed, fresh air servo closed, recirculation flap open, and no air to foot well and defrost nozzles (maximum cool air to the manually controlled air nozzles)
If you experience air flow problems even at full fan you probably have a problem with one or more air vent servo. Have a look on my HVAC DIY website for finding the fault and how to mend it.
Cheers,
Tore

Lorenfb

"However when I engage the a/c MAX switch, I experience no changes."

You may have a bad switch. A simple test is to put the temp **** on
full heat. If you depress the regular A/C switch, the compressor clutch
will not engage. Now if you depress the A/C max switch, the compressor
should engage and override the temp setting.

ToreB

Good point Loren, I've seen some cases of corroded switches on the CCU front panel.
However, I disagree with you on the A/C compressor engage. As far as I know, it does not depend on the temperature setting. The clutch shall engage when pressing any of the two A/C switches. There may be version(sw) differences in the CCU's, but I have not seen the behaviour you describe.
Cheers,
Tore

Lorenfb

I said:

"A simple test is to put the temp **** on
full heat. If you depress the regular A/C switch, the compressor clutch
will not engage. Now if you depress the A/C max switch, the compressor
should engage and override the temp setting."

You said:

"As far as I know, it does not depend on the temperature setting. The clutch shall engage when pressing any of the two A/C switches."

The engagement of the compressor clutch when the normal A/C switch is
depressed is dependent on the temp setting. This is the case for ALL
CCU functions on most ALL vehicles, i.e. it's to minimize the use of the
compressor - engine load & fuel economy. When the A/C max switch is
used, it overrides the temp setting to engage the compressor. This has
always been the case for all the USA 993s I've worked on in the last
10 years. The key point is that the compressor cycles on/off in the normal
mode (lower A/C switch) dependent on temp. Overriding the temp and
engaging the compressor clutch is the major factor of A/C max in most
all well designed A/C systems.

Mizuno

Thank you, folks. I will try it out, and read about the servos. Will let you know what I uncover!

ToreB

Loren, I did not give up on this and have done some thorough investigations on our discussion topic. The A/C compressor clutch engaging action in the 964 and 993 depend on at least five factors: (there may be more)
1: If the A/C button(s) are pressed
2: The engine has to run (Term X cycle detected)
3: If the fan speed is 1 or above (993 only)
4: The temperature reading from the evaporator (freeze protection)
5: The high/low part of the three-level pressure switch (too low or high refrigerant pressure)

The normal cycling action of the compressor is due to the evap temperature cycling, and, to my surprise, not the mixer chamber sensor readings. The compressor clutch state does not change depending on the temperature setting on the CCU. The above have been measured in my 993, and verified in my CCU simulator/test bench setup.

Q.E.D.

:-)
Cheers from Tore

Lorenfb

"The compressor clutch state does not change depending on the temperature setting on the CCU."

Sounds like you may have a problem with your A/C system. In the many
years working on both 964s and 993s, all have the A/C compressor
cycling with the temp setting. That's not the case when the max switch
is used on the 993, though. Besides, a standard design for any modern A/C
system is to have the compressor cycle based on the cabin temp, and Porsche
wasn't any different in designing the 964/993 A/C system. You need
to test some other 993s and also test 964s.

mdude

I have a related but slightly different issue w/ my AC. The cooling function works fine in normal AC. But when switching to max AC, it stops blowing cold air, just normal air flow.

What gives?

Thanks for any advice you guys and provide...

ToreB

Your problem looks like a rear fan/resistor/fuse failure. Have you seen my DIY website?
Cheers,
Tore

Pr William

Lorenfb, today I noticed the AC Max switch malfunctioned as described by this thread's initiator. I followed your simple test and pretty much verified that I may have a bad switch. I was about to ask 'now what', do I have to change out the CCU to fix the bad switch? However, before shutting the car off I gave it 1 more try and bingo, everything worked as it should. So now I have an intermittent bad switch?

x50type

The bad news is the switch is NLA the last time i spoke to my favourite Porsche parts person.!

techman1

I am a novice with the ac, but if the ac does not engage at the warm temp setting, how would it be able to dehumidify the air? In theory, the scenario Tore presents allows the ac to cycle, even though the asked for temp is above ambient.

ToreB

If you set the A/C **** ON, the A/C compressor will be activated at all temperature settings. It will cool and dehumidify the fresh air from the front air intake, not the hot air from the heat exchangers. The opening of the front fresh air intake depends on the temperature setting, cabin temperature and ambient temperature.

Non-op or intermittent switches, (A/C or the others) mostly is due to corrosion on the copper switch parts. It is a fairly simple job to fix using a high-quality electronics contact cleaner and some work on the contact pads/rollers.

Cheers,
Tore

techman1

Matches might thoughts, thanks!