I did some more diagnostic work on my A/C. I originally thought the compressor was not turning on at all, although that is not the case.

Here are my observations:

1. The system holds about 24psi of pressure
2. Upon turning on the A/C the compressor turns on spins for less than 10 seconds and turns off. It appears that it spins until it gets down to 22psi and then shuts off.
3. In the passenger compartment cold air is blowing but at a very slow rate. For example, if i have the fan at position 4 without A/C on, there is a good volume of air blowing through the vents. Once I turn on A/C then it drastically cuts back on the volume of air to a minimal trickle of air.

So any ideas of what is wrong with this picture?

Cheers,
Boris

 

Hey Boris,

Sounds like the dreaded vacuum system induced air cut-out. Somehow or another, the 993's ventilation system has a vacuum function associated with the recirc function (added in '95--964 doesn't have it). When you turn on the A/C, it automatically places the system in recirculation. If your vacuum system isn't right, the air flow goes to nothing.

Wish I could tell you more, but I'm still waiting for someone that can explain exactly how the vacuum system interacts with the HVAC on a 993. If you don't have the shop manual, I can post the sections that discuss vacuum controls and the HVAC.

Curious. What happens when you press the recirculation button, while the fan is set at 4? Does your air flow remain normal?

 

From your description of the compressor cutting out in 10 seconds my guess is that is not caused by a vacuum leak, but by a refrigerant system issue (still low or you have a contaminant).

How soon after switching to AC does the air volume drop? Is there any difference with recirc flow on or off? If you blip the gas, as engine returns to idle does the air volume increase (if have a small vacuum leak I think it might, due to higher vacuum avaiable as you release the gas and engine returns to idle). Might also expect a drop in air flow with system in recirc vs. normal.

Far as I know from watching my system, vacuum and associated switches are used to switch between various air exits (dash, defroster, heater) and to control the air mixing valves (heated vs. conditioned air based on temperature).

Unsure about Randall's description of system automatically switching to recirc on startup; no explanation in shop manual nor 993 intro book. It does say that the compressor and the condenser blower aren't started til 10 seconds after ignition is activated.

 

Mine does the same thing until I turn the temp control to full cold. Then it blows like crazy.

 

Hey Ray,

>Far as I know from watching my system, vacuum and associated switches are used to switch between various air exits (dash, defroster, heater) and to control the air mixing valves (heated vs. conditioned air based on temperature).

The vacuum is only used for the "bypass air flap" (whatever the hell that is). Everything else is servo motor controlled. You can also see this on Sheet 3 of the electrical diagrams, left hand side. Somehow or another, the bypass air flap is tied into the recirc. function. Don't know more, 'cause the 964 doesn't have one.





>Unsure about Randall's description of system automatically switching to recirc on startup; no explanation in shop manual nor 993 intro book. It does say that the compressor and the condenser blower aren't started til 10 seconds after ignition is activated.

You can test this on your own car. With the A/C off, not in recirc. mode, feel/listen for air flow at the fresh air intake, base of the windshield. Turn on the A/C. Feel/listen to see if the air-flow stops. Try the same for swapping in and out of recirc. mode, A/C off. On my car, the original CCU didn't automatically switch to recirc. when the A/C is turned on. It does with my updated (993 prefix) controller. This feature may be described somewhere (at work, don't have references), but--regardless--I'm making the assumption that any 911 with a post-964 revision 0 controller switches to recirc. automatically when the A/C is turned on.

In the above paragraph, I'm speaking of the normal A/C button. Of course, the MAX A/C button switches the system to recirc. automatically, and this function is clearly described in the owner's manual.