Youcef

My A/C blower fan speed has a mind of its own, you set it on three and it will jump to 4 or higher! There does not seem to be any reason, the road is not rough and no bumps when the speed changes. The other day it mad itself so upset it blew its fuse, after sweating my way home I changed the fuse and it was fine again....meaning back to its erratic self!

Any ideas on what this could be caused by?

Temp sensor faulty?

Randall G.

Hey Youcef,

Logically, this would be your final stage regulator that's acting up. It's a glorified, CCU controlled, rheostat that adjusts the resistance between the fan motors and ground. In doing so, it adjusts the current passing through the fan motor, and--thus--its speed.

That said, you may also have a short to ground that's causing the problem. The short lowering the resistance between the fan motor power supply and ground, causing the current and fan speed to spike. If the short is "good enough" (low resistance), the fuse will blow.

You could also try electrically disconnecting each of the interior fans in turn, to see if this can help you zero in on the offending fan.

Good luck!

Youcef

Randall,

Thanks for the advise I will have a look into what you have mentioned, I hope it is as simple as the fix you guys gave me my last ignition switch problem.

Where do I find the final stage regulator? I will probably need to show it to the shop or they will remove the body of the car trying to find it!

Adrian

Okay firstly there is not one fan there are two. The blown fuse No 1 supplies a lot of equipment. The final blower stage amplifier is mounted in the rear of the luggage compartment. The failure you have is most likely one of the blower fans. Left or right. Which one you can only determine by disconnecting a fan (I recommend the right first) and seeing if the problem stops. Then you do the other fan and check again.
There is a little known function in the final stage amplifier. It has a built in temp sensor (no it is not checkable nor replaceable). This sensor monitors the temperature of the final stage itself. If the temperature of the amplifier increases past a certain point a blower fan (normally the onenot causing the problem) will increase in speed to provide additional cooling. If the temperature continues to rise it cuts out the fans. The most common cause of amplifier overheating is one of the fans.
My guess is you have a bad fan.
Ciao,
Adrian
964C4

Randall G.

Adrian...very interesting what you've written about the final stage temperature sensor. Curious. How does increasing interior fan(s) speed cool the final stage regulator? It's not in a place where air from the fan(s) flows across it. I thought maybe you were speaking of cooling the regulator by decreasing the power consumed by the regulator, but that would mean slowing fan speed >> raise internal resistance of the regulator, reduce current (I), reduce the product of I-squared and R. Increasing fan speed means lowering internal resistance, thus increasing the produce of I-squared and R. Thanks....

Youcef...I'll get you a picture of the final stage regulator tonight. I have a bad one given to me by Arnie--his was passing current with the ignition off (not a good thing for the battery ).

Edit: With a series circuit (e.g., the interior fan motor and regulator), it is possible to decrease resistance of (for example) the regulator, raising the circuit's current (fan speed), while reducing I-squared-R across the regulator. This would occur if the regulator's resistance is lower than the fan motor's.

Adrian

Dear Randall,
You will have to go argue that one with Porsche Engineering. This is the way they designed it and described it in the description of operation of the circuit. Fan speed is voltage controlled by the control unit. There is airflow in the luggage compartment rear section of that I can assure you. I see the evidence of this in winter.
Ciao,
Adrian
964C4

Randall G.

Hey Youcef,

Pics of the regulator. What you'll see installed in the car will have this general shape. But, most likely, the only part you'll be able to see of what's shown in the pics is the connector (e.g., you won't see the circuit board).





Good luck, and let us know what you find.

Youcef

Gents,

Thank you for the detailed explanation, I will most definitely get back to you with what I find, however my explanation may not be quite so technical as you have outlined in this thread...I was sweating when I read it!

There is one issue that I did notice yesterday while on our club drive, one of the fans developed a high pitched squeal and actually sounded like the bearing was catching and every so often the fan would jam for a second or two. There was a very noticeable change in the operating volume of the fans when this happened, I did recall I noticed this at the time my fuse blew and I was was thinking that blower seemed very quiet....might be the fan ( L or R) was jammed from start up and contributed to the problem I mention.

Do I get at the fans from the luggage compartment also?

Thanks for the help.

mauro6569

Hello Randall, You can post some detailed photographs of blower amplifier circuit?
I have to restore some of slopes burned.
Thanks