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This has happened a few times but the blower fan will suddenly jump to max speed and I smell an electrical burning smell. When I shut it off and turn back on the blower motor, everything is fine and it goes at the speed I select. I'm not sure if this is caused by the blower resistor or the fan controls? This is my 85 928.
Well known problem. Look up the Magic Blower Syndrome on the 928. Check your resistor pack in the HVAC evaporator housing. You might be able to fix it by adjusting the metal tabs to add 1mm spacing per WSM, otherwise replace.
Well known problem. Look up the Magic Blower Syndrome on the 928. Check your resistor pack in the HVAC evaporator housing. You might be able to fix it by adjusting the metal tabs to add 1mm spacing per WSM, otherwise replace.
Usually in most cars if the resistor fails it only allows the fan to work at max, not suddenly go max when the fan is set to low. It is one of the few things I haven't taken apart yet on this car, so I will look into it. Thanks for the info.
It's possible your blower fan has failed in such a manner that it's drawing too much current. This overheats the resistor and no amount of fiddling with the resistor will remedy that.
It's possible your blower fan has failed in such a manner that it's drawing too much current. This overheats the resistor and no amount of fiddling with the resistor will remedy that.
It might and it might not. Someone could have placed a higher amp fuse in the blow motor location. Lots of things can happen in the course of 38 years. In this case and even with the correct fuse it's possible to draw just enough current to ever heat the resistor pack and not enough to blow the when the fans runs full blast.
Typically as the fan ages, the shaft bearings begin to drag and the worn away brush material doesn't allow the fan to operate as efficiently as designed so it moves less air and draws more current. Both of these conditions lead to an overheated fan motor speed resistor pack and the closing of the fail-safe circuitry. Porsche clearly planned on this eventuality by designing in the bi-metalic contact that cuts out the resistor and operates the blower at full speed. If the motor was fully seized and dead shorted then it would pop the fuse. It's my understanding that Porsche updated this functionality in later models to turn the fan completely off when the resistor pack overheats.
All that considered, a good look at the whole system might be a good idea, ensure air can flow by opening closed vents, checking the integrity of the rubber boot that connects the blower to the rest of the system, and ensuring the air inlets are not clogged with debris such as leaves. If all that check out, the 38 year old blower motor might need replacing.
It might and it might not. Someone could have placed a higher amp fuse in the blow motor location. Lots of things can happen in the course of 38 years. In this case and even with the correct fuse it's possible to draw just enough current to ever heat the resistor pack and not enough to blow the when the fans runs full blast.
Typically as the fan ages, the shaft bearings begin to drag and the worn away brush material doesn't allow the fan to operate as efficiently as designed so it moves less air and draws more current. Both of these conditions lead to an overheated fan motor speed resistor pack and the closing of the fail-safe circuitry. Porsche clearly planned on this eventuality by designing in the bi-metalic contact that cuts out the resistor and operates the blower at full speed. If the motor was fully seized and dead shorted then it would pop the fuse. It's my understanding that Porsche updated this functionality in later models to turn the fan completely off when the resistor pack overheats.
All that considered, a good look at the whole system might be a good idea, ensure air can flow by opening closed vents, checking the integrity of the rubber boot that connects the blower to the rest of the system, and ensuring the air inlets are not clogged with debris such as leaves. If all that check out, the 38 year old blower motor might need replacing.
The blower motor is one of the things I took apart due to a noise it made. I found a lot of rats nest in there and gave the whole thing a good cleaning plus a lot of lithium grease for the shaft to operate noise free. It's worth looking into the resistor first. If that looks good then a new blower motor is certainly next. Someone made a video that shows what I need to look for.
Roger sells new blower motors with new fans it takes a bit of doing but fotment is not too difficult make sure to clean out the rest of the ductwork including inspection of the resistor pack
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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."
I'd start by cleaning the plenum, evaporator and resistor pack and also the intake area around the blower - amazing how much junk there can be in those places clogging up the airflow (esp. the evaporator). You will likely have to remove the resistor pack to evaluate and clean the evaporator.
The resistor pack overheats either because of too high a current load, lack of cooling airflow or damage (corrosion etc.) to the resistor elements and/or the bimetallic thermal switch (repeatedly triggering the switch damages it - its only a failsafe mechanism)
Took it off and found some cotton like stuff in the resistor, probably left over rats nest. That might cause the problem. So I cleaned it out and polished up the copper because I love cleaning copper, and since then this hasn't happened since.
Tha cotton stuff looks like spider nests. Are you breeding spiders in your car?
It sat for 15 years so who knows? So far no problems but I did cover it in lithium grease and that was a mistake because this thing gets really hot and burns it off. It's a simple design in that each coil is a fan speed. The more coils it uses the slower the fan spins but the hotter it gets. There's this metal leaf spring in the center that I assume if it gets hot enough will make contact and turn the fan speed to 100% to help cool it. That explains what was happening to me. The crap in the middle wasn't allowing air flow to keep it cool. I like simple designs I can understand.
11-25-2022, 07:04 PM
Blower fan goes full speed
George Layton March 2014
