WallyP

The blower problem sounds like a loose plug on the HVAC control unit.

I would suggest actually finding out what the temperature problem is before starting to replace parts. A faulty heater valve will not normally just open on its own and then close on its own.

Air that randomly goes from very cold to very hot is likely to be a very different problem from air that randomly goes from pretty cool to somewhat warm.

If the air is truly going from ice cold to very hot, the first thing to check is the temperature control system.
Remove the left console side panel. There will be two or three small Phillips screws on the bottom of the panel. After those are removed, slide the panel and slip it out. Find the temp control actuator - it is a black plastic box mounted vertically on the side of the main HVAC box.
With the engine running, place the condition lever (lower air distribution lever) on anything other than the off or defrost position. While watching the arm sticking out of the top of the temp control box, move the temp control lever (upper) to full cold, wait a few seconds, then move it to full hot. You should see the arm move up and down as it moves the temperature control flap to route air thru the AC evaporator and the heater core. Note which arm position is for cold, and which is for hot.

Leave the side panel off, and drive the car. When the air temp suddenly goes to hot, look at the arm on the temp control box. Is it now in the hot position, or is it still in the full cold position? The answer will immediately tell you whether the problem is the temp control system or not.

If you go to our web site, and go to tips and links, down thru Wally's World to HVAC, you will find a paper that explains how the HVAC system works. Reading it will give you a better understanding of how to find the problem.

Homer43

So if it isn't the valve then it must be the relay inside the head unit? I will check the valve today to see if its the culprit. I will update the post with my findings. Thanks!

Ad0911

A problem for the owner but an opportunity for the driver..

WallyP

"So if it isn't the valve then it must be the relay inside the head unit? I will check the valve today to see if its the culprit. I will update the post with my findings. Thanks! "

No, there are also numerous other possibilities. Both the valve and the relay are common problems, but neither will give exactly the sypmptom that you described.

Homer43

Thanks wally, you are talking about removing the panal on the side console on the driver side correct? I had it off before and didnt see anythiong like you described, but I will pull it off again and look. If this unit is bad is there a repair or does it need replaced? Also where can I get one? Thanks for your help!

WallyP

Mike - Stop thinking about replacing parts! You simply can't afford to try fixing 928 problems by throwing parts at them.

Find out what the problem is first...

Homer43

Yes I hear you. I will attempt to diagnose ther issue today and go from there. I am new to 928 ownership and have a lot to learn. These forums have been a big help. Thanks.

Homer43

I discovered the hot water valve under the airbox isn't fuctioning. I disconnected the plunger from the rod and wired it. It now blows cold air all the time. I am going to order an aftermarket valve from nappa. If it fails to work I will try an electric valve and simply operate it via a switch. Thanks for all of your replies and posts. I am a newby when it comes to Porsches and it is definely a learning experience. I will say though, I am enjoying the hell out of my new 928!