Alan

This just goes to show that Porsche guys can be morons sometimes - and make major mistakes in their documents...

It really is a Blower Resistor Pack/Unit

In the wiring diagrams the name varies over the years and generally got less descriptive over time - it was never called a switch - just not what it does
77-78 Resistor unit for fresh air blower
79-83 Fresh air blower Resistor unit
84 Resistor Group
85-89 Resistance Group

The function over this whole period was basically the same.

Alan

The Fixer

Easy big fella (Alan) but i decided to actually LOOK at this device and
it looks to be a bi-metallic switch to me. Maybe I'm wrong or your GTS
uses a more modern device. It is a resistor but it looks as though the
point contact makes contact via a bi-metallic process.

Useful Note Alert:
Cleaning the point contact as well as the male plug contacts (see how cruddy) made a world of difference
in the operation of the control head. Pics are before cleaning.

Mrmerlin

Matt what you see is the emergency supplementary cooling circuit incorporated into the resistor pack

NOTE with the resistor plugged in and the system running ,
the blower will be supplying cool air to the resistor unit thats why its inside the airduct.


Now if the resistor unit gets too hot IE not enough cooling air,
the contacts you have pictured will close and rerout the power to the highest blower setting,

thus increasing the blower fan speed ,
this increases the airflow in the duct to cool the resistor pack.

Once the higher flow is enabled the extra airflow will then cause the spring contact to open,
and thus the lower power setting will resume.

Thus the fan speed will return to the lower selected setting.

This is the part that is responsible for MBS =magic blower syndrome.

The usual cause is a degraded blower motor thats got bearings that are sticking / loss of lubrication,or corroded connections,
when this happens then the blower motor isnt going to be able to supply enough air to cool the resisitor pack,
thus the contacts close and send the blower motor more current

The Fixer

Very straight forward part/design.


Now that you've added this info, when looking at the coils or windings you can see that they create resistance and
would heat up considerably due to this.

So if they heat too much the contact arm will bend (bi-metallic process) make contact
and kick the blower into high automatically to cool the resistor windings back down
to an acceptable temperature.

Thank you for adding clarity Stan i feel i completely understand the function of this part now.

Alan

You completely miss my point - I never said there wasn't a switch - I said don't call the WHOLE device a switch.

Unfortunately you are now more confused...

The bimetallic switch is only a backup safety system. In normal operation it should NEVER operate - if it does you have a significant problem with the blower motor or the airflow. If this should ever start activating and you do nothing the system will fail permanently quite quickly - see posts on magic blower symptoms. Ergo - The whole device isn't accurately called a switch its a resistor pack comprising 4 resistors to control blower speed with a backup bi-metallic safety switch (in some models).

This switch has no normal operational function at all. In normal operation only the resistors do anything. So should we call the whole device a bi-metallic switch or a resistor pack? We don't call the window motors thermal switches either - would you?

Alan

Mrmerlin

Sorry I should have added what Alan did,
as he is correct about this switch not being in use unless other parts dont work as they should.

I fixed my original post to reflect this would be and emergency supplemental part of the unit

The Fixer

No don't be sorry Stan i understand and understood completely.

The bi-metallic switch is not part of normal operation. Got it.

As i said above if the resistors where to get too hot it would trigger the BI METALLIC switch to kick the fan to high and cool
the resistor pack down. This is why it is located where it is...the coils are open to air flow with in the duct because they heat up with use.(fan On)

What else would there be to cool down in there other than the resistor pack itself? I am not confused.

It would make little sense that it (B-M switch) would kick the fan on HIGH if there was a problem with air flow or the blower motor
was failing. That would cause additional load on the blower. Not buying that one.

It must be there in the rare event that the resistor pack over heats. It would then be cooled down and resume
normal function. I have not read about magic fan syndrome? though. I would guess that their resistor packs are bad though and
need replacement like any resistor that goes bad with use. They don't last forever, I am surprised mine still works.
But it is made very well.

Alan:

As i said, I don't have a dog in this fight to call this device a Bi-Metallic Switch,

It was called that by my local Porsche Dealer Parts Guy and is listed as such on his micro fiche.

Matt

Mrmerlin

I have fixed a few of these resistor packs with some silver solder.

NOTE the wire usually snaps at the crimp,
its pretty easy to torch the parts to get them hot enough to accept silver solder

Alan

Matt - the blower motor going bad does trigger this - because it then takes more current and causes the resistors - actually counter intuitively usually the higher value ones in speeds 0 or 1, to consume too much power and get too hot. The motor can just have bad bearings or simply too little airflow due to blockages - commonly leaves packed around the intake. Obviously if its just a bearing issue then high speed will then cause it to fail faster, if the air intake is blocked running faster will also load the motor and airflow will be lower than expected - but since no resistors are active they will still cool down...

Usually after a few tens of operations the bi-metalic strip failed to work and the resistor pack safety mechanism was lost...

When it was redesigned the bi-metalic switch was removed and a series (not parallel) simple NC thermal switch was installed - if the pack gets too hot the blower fully shuts down. This has a major advantage - when it happens people tend to take it in to get it fixed rather more quickly.

Frankly you are right that the original design never worked too well, it was an interesting idea - but in practice not very useful - it's a solution to a problem that didn't really exist.

I can say this - if you experience magic Blower syndrome - you have a real problem - it will not go away on its own. Best case you need to clean out the leaves, more commonly you will need a new blower motor and likely also a new resistor pack. The one with the bi-metallic switch is no longer available - pretty easy to imagine why...

Like I said - Porsche makes mistakes... especially on the secondary systems & their documentation. Best to just accept that it happens.
Sure is a lot of fight in that dog you don't have

Alan