I have a quick question before I digging deeper:
I noticed that the fresh air blower switches off when I push in the AC button. The button lights up as usual, and the compressor kicks in. You just don't feel the cold air until you switch the AC off and the blower kicks in again. The AC button light is fed from the same 12v (connection post 8) as the blower, and so is the AC itself. Both work, so 12v is available all the time.

It becomes even more strange: when push AC on, and I turn the blower to the max position, basically bypassing the resistor pack, it runs at full speed, and I have max cold air. No, it is not the resister pack as all works fine as long as I keep the AC off. There is no direct link to the blower and the AC button.

I studied the diagram and did not come up with anything obvious. I'm going to check the connection point 8 (12v power post) and the fresh air blower relay. Maybe there is a high contact resistance, but neither is likely to cause this.

So.... has anyone ever seen this?

1992 928gts Midnight Blue
1988 928s4 Cherry Red
The Netherlands

Got it.
Guess what happens when the car is pretty hot, stood in the sunshine, and the resistor pack gets heated above the thermo switch trigger point. The early S4 go to full mode (magic blower syndrome) and the later ones go into shutdown. But when the temperature stays high they remain in shutdown mode until things cool down. When you turn the blower switch to max, you get a full 12v on the blower, but even that did not cool it down quickly enough.... so I was caught with a switched off fresh air blower. Per design :)
When you drive you have some airflow so that it seems as if the fan runs, but it doesn't. My mistake.

When you push the AC button, the center mixing flap changes position (I think) and causes even less air to enter through the grid. So it shows the lack of cold airflow generated by the blower. As if the blower is shut down.

How to solve? Well, just wait and let it cool down in the garage. Took over an hour.
Maybe the resister thermo switch is a bit sensitive and caused it to stay open quite long. There are no specs about this I think.

regards and thanks to all who replied,

Any of you ever heard of this before? The A/C ventilation system in my 1988 S4 appeared to be working and then suddenly, it will now only let the vent fan come on at full speed and only in the defrost/defog position. Any ideas about this?
Greg Landwehr

Replace Relay X.

You can test by swapping Relay X with another "53" relay, such as the defrost or horn.

Wally Plumley
928 Specialists

The 1988 928s4 gets power from the fresh air blower relay via fuse #17 and bus 30.
The defrost relay picks up the 12v power from behind the same #17 fuse but feeds power via a different route directly to the blower motor. So if the defrost works, the fuse and the blower are ok.

I'd have a look at the fresh air relay (swap it or connect terminals 30-87 to put 12v to the resister block). If that does not solve the problem, and there is no blower activity when you turn speed for the blower to max (pos IV), I'd say you have a loose wire. If the blower switch only activates it at pos IV, it is in the resister block.

If the blower works when you bypass it, check plug at G14, and you'd have to dig in deeper, as the relay is apparently not activated. Check the HVAC console to see if the plugs are seated correctly.

Hope that helps....

1992 Porsche 928 GTS Midnight blue
The Netherlands

Flaky relay X. Went through 2 before I found one that would stay on. I had just replaced all of the relays critical for running but had not touched the others.
Thanks to all who replied