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Rear blower fan

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A soon as I switch on the ignition from cold the rear blower fan comes on full...since there is apparently nothing to cool down this seems wrong. It does not stay on full all the time the engine is actually running.
Can anyone shed any light on this ?.. I have looked at the rear ballast resisitor which is very different from those serving the air con and oil cooler under the front wings. Don't really know what I'm looking for but it seemed ok.
 
Thanks for the input. It seems that the system is probably very similar to the 964 because the ballast resistor (on which I burnt my thumb!) is a 964 part number. I have eliminated the relay as a possible cause by swapping with a known good one from the front fuse board. Do you know what the resistor does ? I just replaced the front two which had both failed.
Thanks to Leo Sayer (and I never thought I would say that) I have found that turning the temp. control on the CCU right down to the blue dot will stop it blowing, but is that good or bad?
Steve
 
Steve,

Turning the temp control to the blue dot SHOULD shut off the rear blower - that's how it's supposed to work. The main purpose of the rear blower is to push hot air up towards the cabin via the mixing chamber ahead of the dash. If no heat is called for (i.e. temp selector on the blue dot) the CCU will shut off the rear blower.

The ballast resistor for the rear blower performs exactly the same function as those up front for the oil cooler and aircon condenser - when it's switched in to the circuit via the relay it drops the voltage to the blower and reduces the speed of the blower. When the CCU wants the blower to run at full (i.e. high) speed the resistor is switched out, for slow speed it's switched in. The reason the resistor is placed as it is in the blower plenum is to keep it cool As you've found out already - it runs VERY hot. There have been cases reported when the resistor has come adrift from it's mount in the plenum and caught light. Any spilt or leaked oil in that area could easily give rise to a serious fire. The later -02 resistors have a bi-metal cut-out to prevent them overheating.

Hope that helps.

Dave
 
Comprehensive and comforting thank you Dave. The "king of Porsche " book says that the resistor has a "lock out" mechanism that can be re set. Do you know what this means ? Meanwhile, I think I will have to go and find another part to fret about!

Steve
 
Steve,

The "lock out mechanism" is the bi-metal strip cut-out that I referred to earlier. If it cuts out you have to press the contact to reset it - that's all. The early (-01) version of the resistor didn't have this cut-out and had the problem of over-heating and causing the fire. The new (-02) part introduced the safety cut-out to eliminate that risk.

Dave
 
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