Toon

Hi,

my defroster sometimes behaves strangely.
When I put the defroster on, the blower speed seems very erratic. Sometimes full power and then suddenly it slowes down and comes back on etc.

It does that also (but to a lesser degree) with nowmal blower operations.
What could be the culprit here?

Adrian

Dear Toon,
I would recommend you check the rear blower fan fuse in the engine bay fuse and relay panel.
You also need to check the operation of the rear blower fan because if it is running up to speed and then slowing down it will cause the front blower fans (2 fans not 1) to do the same and eventually will damage the control unit.
Ciao,
Adrian

Toon

Adrian,

what am I specifically looking for then?

Fuse: I would think this is blown or not causing the fan to work or not at all and not causing erratic fan speeds.
Relay: How can I check if the relay is causing these problems?

Adrian

Dear Toon,
Everything is contained in my book on this subject. How the blown fuse of one piece of equipment in this case the rear blower fans affects the operations of others in this case the front blower fans.
Ciao,
Adrian.

DarrylH

My erratic rear blower fan (hence my erratic front blowers as well) was because of the rear blower relay - subbed in the A/C fan relay as a temporary measure to check, and things returned to normal. YMMV...

Toon

Adrian - you mention blowe fan resistor in your book but could not find info on where exactly to find it ....could you tell me?

Darryl - will try. I assume both are the same?

DarrylH

is in the fan output tube, on the left side of the tube. There's also a temp sensor in the same area, closer to the fan body, and easier to see.
Yes, there are three of these relays in an A/C equipped car - rear blower, A/C condensor fan, and oil cooler fan all use the same one. My rear blower relay had some burned traces visible when apart. Try a search within this forum, these fans go awol on a regular basis and virtually every problem's been experienced by somebody at some point...

Richard Curtis

Adrian: Are you saying that the rear blower fan somehow controls the defroster fan? Are the front blowers the same ones that defrost the windshield? (I thought the defroster fan was a different one.) I'm a bit confused, but then again, that's usually the case. (BTW, I have your book)

Adrian

Dear Richard,
Yes and no. It was not designed to do that but it does it because everything is controlled through the control unit.
The control unit uses current draw to determine if a fan is working or not.
Somehow due to the mysteries of electronic designs of the 80s the control unit designed for the 964 reacts badly (strangely) to a failed rear blower fan.
I cannot answer for you exactly why, just that it does and that whenever you have fans playing around up front you have check that the rear blower fan is operating correctly.
As old Ripley used to say "Believe it or not".
The right and left front blower fans are the air con fans, heater fans and defrost fans.
No there is no special defrost fan.
What changes is the position of the myriad of differential pressure operated flapper valves in the air distribution system. This gives the illusion of other fans operating because it sounds different.
Ciao,
Adrian.

Richard Curtis

Adrian: thanks for the prompt response. I think I understand now. But another question: All my fans are working but when I engage the defrost, it comes on like a hurricane. Is this normal? Or should I start searching for a cause. I'm a Porsche newbie, so please excuse my ignorance.

Adrian

Dear Richard,
Hurricane is good. Both your blower fans are switched to max speed and a hurricane means all the flapper and servo valves are in the right place, both fans are in very good condition and you have no obstructions in the plumbing.
Ciao,
Adrian.

PS: Porsche did fix this control unit eventually with the second edition of the 993 system in model year 1995. No they are not installable into a 964. Only the model year 1994 versions.

Toon

Hi,

I replaced the realy and all's back to normal operations!
Adrian thanks for the info

SNOWBIRD

I am 'electrically challenged' therefore I have not attempted to work on this same problem on my car.
" Yes, there are three of these relays in an A/C equipped car - rear blower, A/C condensor fan, and oil cooler fan all use the same one "
I think I will just go ahead and order 2 of these before I even open the compartment. Everybody seems to feel it's likely that the relay(s) are the problem... Everyone agree? Part#141-951-253B correct?
Once again the Rennlist forum comes thru!