Adrian

Dear John,
Speed selection is via the blower relay. This is a dual relay unit. You have two inputs into the fan. One direct and one via the resistor. The relays are controlled by the control unit. The NTC sensor supplies the air duct temp to the control unit. After shut down, the dumping of air is obviously done at low speed. Flatten the battery otherwise. High speed operation is when the engine is running.
If you have a blower fan running after shut down you either have a very hot engine (air temp in the ducts is very high) or power is getting to the fan when it shouldn´t. This system remains active for around 20 minutes after shut down.
Ciao,
Adrian
911C4

johnfm

Adrian
just checked fan with ignition on (engine cold - its been parked for 3 hours). With temp selector on blue DOT fan came on full (High Speed)when I turned the key. I'm sure it was high speed, as it sounded a lot louder than when it was staying on after switching off the engine. the key.

However, it stopped after about 4.5 secs momentarily then started again.(Is this the relay 'checking' things? The fan went off completely when I turned ignition off. Could I have a dodgy relay?

Adrian

Dear John,
At this point it is hard to say. The controller is actually doing the work. It receives an input from the NTC Temp sensor. Maybe the sensor is shorted. In the Blue dot position only a hot engine can turn the fan to high speed. Either the controller is getting a bad input or it is giving a bad uncommanded output. Both are possible.
Relay could be stuck of course. I would check all the components on the engine carrier plate.
Does the fan work at low speed with heat selected. This is very important to know. Is the temp controller **** nice and tight on the shaft,
Ciao,
Adrian
911C4

johnfm

The tem contro; inside works fin. When I'm idling at the lights, I turn the fan to 0. I can hear the small fan. If I turn temp control to blue DOt it stops. If I turn temp control to , say 27, I get hot air. If I pres demister button, footwell vents close & hot air goes to screen etc.

The bimetallic switch is 'open' at the moment, ie with engine cold. Is this correct, or have I been a bit rough while cleaning it?

I think it should be open as a default (ie fan off)and close whn the air in the duct gets hot. The hot air closes the switch, which makes the resitor coil hopt as current passes throuigh it. I'm just concerned that the fan won't come on at all when I switch off, unless the engine is supper hot.

I'll go for a blast and test in.

Adrian

Dear John,
It was my understanding that the switch should be closed when cold and open when overheated. Acting like a circuit breaker.
This is the explanation I have,
Ciao,
Adrian
911C4

johnfm

Is the bi-metallic switch just a sefety device to prevent the resitor coil glowing red hot & causing a fire? If so, the default should be closed as you say.

However, I have noticed that if you physically close the switch by hand, the fan comes on, so this switch not only supplies/interupts current to the coil but also to the fan. If the default was closed (ie fan on), the fan would be on when the engine was cold.

I humbly acknowledge your knowledge of 964 systems, but this is puzzling me. If the default was closed, the fan would stay on for 15 minutes (or whatever the relay timing is)whenever the ignition is turned on, wouldn't it, or would the signal from the temp sensor (if temp less than 95 deg)instruct the relay to shut down the fan?

I may try fiddling switch again to default closed and see if temp sensor switches fan off.

The other thing to bear in mind I supposes, is tha in the UK the ambient outside temp is about 7-8 degrees C. If my engine runs to normal operating temp and it is 8 degrees outside, maybe the fan doesn't need to come on??

Adrian

Dear John,
The resistor is only a current limiter. The problem is not that the switch should be open or closed (helps to know but this is the end of the story), the problem ist you have power applied to the resistor in the first place. You do not want power to this resistor until you need heating or engine cooling.
This power path is provided by the speed control relay which is controlled by the controller which is supplied data from the NTC sensor.
So either the relay is stuck or the contacts welded together or
the sensor is shorted or
the controller is knackered.
The power to the resistor is switchable. On or Off. You have power there all the time. With full cold selected there should be no power. With engine off and everything cold, there should be no power.
Ciao,
Adrian
911C4

johnfm

With engine off & everything cold, If I hold the switch closed the fan runs on full & the coli heats up & I burn my fingers !

Which relay is the problem?? Is the replacement a Porsche part on will a relay of specific type from any good auto shop do??

Adrian

Dear John,
The relay is on the engine carrier plate. It will be marked which one it is. Find it and remove it. This should remove power from the resistor.
Can you email me privately. I can send you something which might help you.
This work really needs the books. You need to find out what is holding the power on.
Hope you can take large attachments. These relay packages are best sticking with Porsche ones. The current ratings of others can be a problem.
Ciao,
Adrian
911C4

Randall G.

Hello,

I can help a little bit here. The rear blower relay is part #811 951 253 (written on the relay). It's the same relay used for the A/C and oil cooler blower fans. You might try a swap with one of those relays. The AC/oil cooler blowers relays are in the luggage compartment fuse box.

Here in the US, you can get a new 811 951 253 relay for approximately $28 from a “discount” Porsche dealer. Roy E. has said in the past the relay is “generic,” and can be purchased from Pelican Parts for $15.80.

Best of luck!

johnfm

PROBLEM SOLEVED!!!

Check temp sensor with multimeter - got no reading, ie, infinite resistance. The meant the fan controller was being sent an 'air in duct is really hot' signal. Bought new part for £18.60 + VAT - measured resistance was 13k Ohm. As soon as I plugged in new temp sensor the fan switched off.


Hoorah and thanks to everyone, esp Adrian, for their help and advice.

Now, a question on my oil leaks.......