tbennett017

I need a little help wrapping my head around this situation... 993

Recent Evaporator replacement. I did it myself - and everything went fine as far as I can tell. Now I get a strange behavior in the vents which is messing up my cold air.

This thread is awesome, but I may need to get more sleep as I can't quite tell where this information is pointing me.. perhaps you can help...
https://rennlist.com/forums/993-foru...diagnosis.html

If I start the car with the fan on 4 and the temp setting at cold, I get about 10 seconds of good airflow from the center dash vents. Shortly though it shifts, and almost stops entirely, with the fans still blowing, but no air happening - just noise. When I adjust the temp control to HI, the air flow returns to full force, (but of course warming). When I go back to cold I get cold air for that same 10 seconds or so and it goes back to being restricted. Per the PDT999, I can get the mixing flaps to work, and the fresh air motor responds to input. I can check the vacuum on the recirc, but that appears to work when I actuate the button...

I know that the clues are in this thread, but does anyone have a specific answer?

Thanks!

pp000830

Thoughts:

• When you replaced the evaporator coil did you plug the three servomotors under the cowl into their correct plugs? I am assuming all three have the same shape electrical plug.
• Did you run the OBD Codes on the HVAC as it will tell you if the servomotors are working properly? I use Durametric for this. There is a kit available to rebuild the servomotors.
• Is cold air from the evaporator being blown directly on a temperature sensor causing the CCU to believe the cabin is frigid and cutting off the cooling?

All educated guesses.

ToreB

You have a non-op resirculation flap. When you set to minimum temperature, (with A/C on) the CCU will close the fresh air intake and open the resirc flap to get to the desired temperature as fast as possible.
It takes some seconds to close the fresh air intake, and when it is closed fully, the fans have nowhere to draw air from, thus the reduced air flow.

Check the vacuum line coming from the engine to the solenoid valve, as well as the hose from the valve to the flap mechanism. Maybe you have a mechanical problem with the flap actuator.
See this page for details, chapter Vacuum Operated Resirculation Flap.
Cheers,
Tore

tbennett017

Thanks @ToreB. Perfect diagnosis as always. I can pull a vacuum with a hand pump on the white line (goes to internal recirculation vacuum actuator) and the airflow returns. Looking through the CCU hole I can see the vacuum actuator work and the flaps open - and airflow returns to normal operation. Pulling a vacuum on the engine side is inconclusive yet. I need to look into that more. For the short term I might opt to tie-wrap that actuator open until I can take more time with a diagnosis. For some reason my car has two black vacuum lines coming to the vacuum relay location. I am not sure why. I might have re-connected the wrong one? Not sure what would have caused two to be in that place at all.

One observation, the plastic pivot points in the mechanical linkage for that flap actuator are pretty flimsy... they tend to fall out or simply disconnect based on their design. That's something for others to bear in mind when diagnosing.

chsu74

Is the fresh air servo part number 993 572 905 00 as well or is this the servo part number for all?

ToreB

Fresh air servo 964 624 901 00
Defrost 964 624 902 00
Foot well 993 572 905 00
2 pcs Mixer 993 624 325 00
Cheers,
Tore

chsu74

Thank you Tore. Further investigation of my nonworking vent is:
No air normally open or closed
I do get AC at max though.
Will have to figure out which servos to replace.