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U'mmm No! The contacts are open until the resistor pack over heats, then the the bimetallic strip bends and closes the contacts creating a circuit that by-passes the resistor pack making the fan run at full speed. This allows the restore pack to cool from air flow and from not being utilized to reduce the fan speed.
Such a suggestion on your part demonstrates a fundamental lack of understanding of how the item in question actually functions. Which is no surprise to me.
Ok, I'm putting this conversation in Magic Blower, to cool the rhetoric. (High Speed to blow away the heated conversation)
Many good points made. And the resistor pack have performed well, while the wiring age/corrosion is a real issue. And the thermal cycling of the resistor coils, in the pack, tends to lead to long time operating failures. Greg's point is correct, clean resistor coils last longer.
Some of you already know why PWM, works more efficiently. For those who do not; The secret to PWM, is that the transistors are ON and OFF, very fast, to minimize heat load. With the average on time yielding various Speeds. As, EPLEBNISTA, correctly stated the minimum speed must be chosen to prevent noise.
The point of the POST, is to look at resistor pack alternatives. ICSAMERICA comment for MB is a good suggestion. What is current MB fan speed control solution? Plug compatible/modifiable to our cars?
What started this post, was my Tina was driving the 81, Risky Business car, and she reported smoke in the cabin. Obviously, from a resistor speed not used for a long time. But she was very concerned the car was on fire. So I get a panicked call the car is on fire. She loves the car, and I don't want her to be afraid to drive it. If these cars had a cabin/OA filter, less lint to land on resistors, but we have no filters, currently. On my 944's I use a blue shop towel over the air inlets to provide limited dust collection.
Last edited by Alan 91 C2; 12-01-2022 at 08:50 AM.
Based on my observations,
the failure chain for this system is,
usually the blower motor shaft bearings get dirty,
along with worn slip rings and grooved brushes,
This then causes drag on the shaft.
This then increases the load on the motor.
Now the drag on the fan shaft begins to slow down the fan rotation.
then less air with the combination of dirty air being fed across the resistor pack,
(The dirt fouls the contacts over time, as does the dirty contacts create resistance when they spark closed)
finally causes the emergency system to go to fail safe condition and go full power to increase the fan speed ,
this will usually turn off the fan after a few mins of operation.
NOTE with really dirty combination of dirty and corroded contacts they can weld closed due to poor connectivity.
SO the solution is to replace the fan motor with a new part and clean and adjust the contacts in the Resistor pack,
this should restore the system to full service
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
Just to confirm - all the Resistor packs apart from the 94/95 pack are NLA.
A modern replacement would be awesome.
Alan - can 928 616 101 02 be adapted to be backward compatible as you found out on 928 616 101 00.
I have one here if you want to experiment - happy to send it to you.
@ROG100 Roger - are they physically compatible? will a 94/95 pack fit in the plenum hole? It is not fitted there in those year cars but if it will fit physically and has the same connector then by pin mapping it can be made to work - though without a true zero speed of course. If it physically matches the plenum hole - can you send me a picture of the base with the pins - Or confirm where the 4 pins (1-4) are Vs. the 7 (1-2 & 4-8) pins on the older versions.
All that considered it might also be a good idea to check the ground and change the 12v supply wire from the Central electrical panel to the fan and have a look at the fuse socket. Both of these items can age fail and stack resistance which drops voltage and increases current draw.
Sorry it just doesn't work like that. Any resistance in the circuit reduces both voltage and current, there is no way around this (hint - this is how the resistor pack works). The build up of electrical resistance anywhere in the circuit is not likely a cause of magic blower syndrome though it may be detrimental to normal operation in many other ways. Blocking of the airflow anywhere in the line or a poorly operating blower fan (mechanical resistance via corrosion or poor lubrication, damaged bearings) are far more likely causes.
As previously noted once the bimetallic strip switch is activated a few times it starts to deteriorate - so its best to resolve these issues relatively quickly to avoid long term damage that may make it less predictable. Note that not all cars have this kind of thermal switch, later cars ('89-‘93) have a (replaceable) series thermal switch that just shuts the blower off if the resistor pack gets too hot, and '94/95 cars have no thermal switch (but also no 0 speed - blower is actually off - this is the most problematic speed).
What started this post, was my Tina was driving the 81, Risky Business car, and she reported smoke in the cabin. Obviously, from a resistor speed not used for a long time. But she was very concerned the car was on fire. So I get a panicked call the car is on fire. She loves the car, and I don't want her to be afraid to drive it. If these cars had a cabin/OA filter, less lint to land on resistors, but we have no filters, currently. On my 944's I use a blue shop towel over the air inlets to provide limited dust collection.
You can relatively easily add filters to the 944 using this retrofit kit from Porsche 94457290500.
I wish 928s had a similar kit for the back of the HVAC box to replace it with the later version to add the filter.
You can relatively easily add filters to the 944 using this retrofit kit from Porsche 94457290500.
I wish 928s had a similar kit for the back of the HVAC box to replace it with the later version to add the filter.
On a 928 that is a lot of work - you need to cut a slot in the top of the plenum and fabricate a removable cover. The 1994/1995 GTS cars do have a cabin air filter - but the HVAC plenum is an entirely different version and retro fitting this is more trouble than modifying the earlier version. Another possible but very difficult item (at least suitably sized filters do exist).
I used to have the infamous magic blower syndrome. Possibly caused by a worn blower motor. I replaced the squealing motor for a functioning used one. Worked fine for a while.
Now I'm experiencing the all-or-nothing treatment from my blower. I want to replace both the blower and risistor pack, but like Roger mentioned the resistor pack is no longer available.
Hopefully, eventually, someone will come up with a solution to this dilemma.
Hey Santa, I've been fairly good this year... you know what I want?
U'mmm No! The contacts are open until the resistor pack over heats, then the the bimetallic strip bends and closes the contacts creating a circuit that by-passes the resistor pack making the fan run at full speed. This allows the restore pack to cool from air flow and from not being utilized to reduce the fan speed.
Such a suggestion on your part demonstrates a fundamental lack of understanding of how the item in question actually functions. Which is no surprise to me.
This is the 94 to 95 Resistor Pack - 928 616 101 02.
Even if it could be made to work in the earlier cars there are only 15 left in Germany - not a sustainable supply.
At list of $224 not commercially viable.
__________________
Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission? George Layton March 2014
928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
Roger that is interesting. It clearly would not fit into the plenum, nor is the attachment method even the same. It has more pins than I expected - the wiring diagrams show only 4 pins, while it has 5 main pins and 2 extra side pins. I believe the side pins are the (T49) connector for the blower motor (not shown as integrated on the WDs). This suggests that the extra main pin on the resistor pack is just the Ground for the blower motor and feeds only to the side connector pin. The '89 - '93 resistor pack loom has 5 connections already so repurposing one of them for the ground makes some reuse sense here. This is not at all what the wiring diagrams show though.
So a bust both for compatibility and availability/price
Anybody made a schematic of a working one? Got plenny EE mojo laying around. Worn blower motor theory probably good. There are three things that work against the supplied voltage: the resistor pack (I think)* current, the resistance of the motor winding* current, and the back EMF of the motor. The back EMF is linear with motor speed, so if the motor speed goes down due to worn bearings, the back EMF goes down and more of the voltage is being scrubbed off in the resistor pack.
12-01-2022, 01:19 AM
George Layton March 2014