993 engine bay heater fan always on?

[Irisblue95]

Hi all,
i'm new on here, have just (last week) purchased a 95' 993 cab, have been running an 85' cab daily for last 6 years and decided to buy something a little newer with less miles, less stone chips and less rust !
The 993 really is as good as everyone says, is a superb car, i'm still getting used to the gear ratios in the 6 speed box, all is going well except that i noticed today that even with the climate control (non aircon model) set to the coldest setting and the cool air front blower fan set to zero i noticed the rear engine bay heater fan seems to run continously when the ignition is turned on and also while the engine is running, its on all the time?
it definately wasnt like this last week, i would have thought this fan should only start when heat is being called for via the climate control unit temp dial/knob)
(the fan stops just like the wipers/lights and other stuff when cranking the engine over )
When the ignition is turned OFF the fan stops ok, so its not draining the battery when the car is not in use but it is obviously on unneccesarily/wearing itself out (over time) and having replaced two units on my earlier cars i know they aint cheap..
In both the SC and 3.2 both heater fan motors were activated by levers/switches, this one running continously (although no heat is felt up front when the controls are set to cold) it has to be wrong,

My thoughts were either a faulty sensor in the ducting in front of the fan unit, ive removed it wiped it clean but no difference,is there an OHM reading i could take to verify the unit is ok?
With the ignition on & the climate control set to cold/no heat being asked for and engine bay fan is running, if i remove the sensor plug socket the fan stops then starts after a few seconds after what sounds like a click from a relay activating within the blower housing is this where the relay for heater fan is?
My fuse box does not seem to show a rear heater fan relay so i assume its located elsewhere any one know where it is?,
Logic says it has to be either :
The relay stuck on,
The temp sensor in the ducting faulty,
or the expensive climate control unit gone faulty, (although every other function on it works fine and it delivers heat when required)...
has any body on here had the same problem and what was the offending item?
if i place a jumper wire across the temp sensor socket, what should happen? can i prove anything by doing this,
any advice or observations most welcome,
i was for some time reluctant to buy a newer 911 due to the amount of electrical stuff in them, seems i might have been right.. i dont want to fall out of love with this one this early on..
thanks in advance.

[cjoenck]

Welcome to the community. The blower in the back does not just deliver heat to the cabin, it also acts as additional engine cooling when temperatures in the back get too high, i.e. in stop and go traffic. The blower should also come on after a spirited drive in hot weather - maybe an impossibility in your part of the world - after you switched off the ignition, again to cool the engine.

First check should be the temperature sensor just behind the blower in the big black duct. Do you have access to a code scanner like a Bosch Hammer or the Durametric tool? If not the sensor should read between 9 and 11 kOhm at 25C and 3.1 to 4 kOhm at 50C - just use a hair dryer to heat it up.

Do not short the sensor plug. This is a NTC sensor, i.e. the higher the temp, the lower the resistance. If you short the plug, the CCU will "thing" the engine compartment is very hot and will keep the blower running....

If the temp sensor is OK, you might have a bad CCU.

Hope this helps

[Irisblue95]

Thanks for your quick reply,
Actually we have had a really hot day today, almost like summer, probably the only one we will get this month..
i went out again in it this evening, the ambient temperature had dropped and the car was cold, and no fan running when ignition on /engine on, as you said if it supplies additional cooling it would fit that under the conditions earlier today that the fan would definately have been required to run on,

thanks again for the reply, reading it after my drive this evening it makes perfect sense, as it wasnt running when i got in just now (midnight and quite cool) so there isnt anything wrong, think i'm maybe a bit paranoid, maybe even a bit frightened of about things going wrong and not being able to fix it, my sc was just nuts and bolts compared tho this, but i am enjoying this one lots, cheers again.