ths2005

Magic Blower Syndrome or not?

My 1980 Euro 928S has suddenly developed a strange behavior, and i do not think it is the resistor pack/Magic Blower Syndrome thing. I have searched the forums and looked at the posts but nobody has this behaviour.

Regardless of wich mode i set it in, Auto, Eco, Defrost etc. It blows at full speed. Whatever position i set the fan regulator to, it is on full. The only way to stop it is to slide the lower slider **** to off(left).

Sometimes when i accelerate hard or drive over a bump it switches back to normal, and everything works fine for a while. The fan regulator works like it should with all 4 speeds and switches off when set to 0. Also every slider position is like normal and all the speeds are like they should.

Is this my HVAC control unit that is messing with me, a relay or is it the resistor pack after all?

FLYVMO

Funny, I had a very similar symptom on my 86.5
It turned out to be a loose wire on one of the connectors on the control motor (heater control motor on left side on HVAC housing, in the cabin). Kind of hard to get to but take a look and see if you can find a loose wire. Mine was towards the center of the connecter, and I vaguely remember it being red with a stripe on it.

Cheers!
Carl

dr bob

On your car there's a defroster relay that's usually not in use, engages only when max defrost is selected. Try unplugging that relay (XXII in your 1980 car) and see if normal operation of the fan resumes. If that restores normal operation with the defrost relay out, we'll go seaching fromthee for the cause

After that, start with the easy stuff. A voltmeter/ohm meter check of the fan resistor pack is easier than ripping through wiring inside the car. Magic Blower Syndeome is usually caused by the thermo contacts in the resistor pack closing when there isn't actually a thermo problem. Or maybe it's closing due to real heat buildup in the duct thanks to partially plugged AC or heater exchangers. You can diagnose with the ohm meter at the terminals for the resistor. The thermo contact is between terminal points 6 and 8; unplug the harness and use the ohm meter to check continuity between those points on the resistor pack terminals, the male pins point out. If there is continuity (zero or very close to zero ohms) the thermo contact is the problem. If there is no continuity, we get to look more closely at the problem. Still at that connector though.

Note that relays are a chore to get out. I use a pair of paint can openers that have been modified slightly by bending the end tabs around to a full 90º. Put the bent ends under either side of the subject relay, and pull. Good idea to lift the battery ground lead for this, by the way, since the metal tools can easily connect relay terminals to each other as you wrestle them into position. Beware that pulling on the metal case of the relay will often net you the metal relay case in hand and leave the relay ad base still in place.

ths2005

Hi.

Sorry for the late update, but i finnaly got the time to try and solve the problem.

Thank you for your tips and advice. I unplugged the XXII defrost relay, but no difference at all.
Then i checked the HVAC unit itself, as i was fitting a stereo and was in there anyway. I cleaned all the contacts and also checked and resoldered any suspicious soldering points. No difference. Also checked for loose wires and contacts in the dash, nothing wrong as far as i could gather.

Then i checked the infamous resistor pack. The connector was extremely tight and almost impossible to get loose, so it might be the original from 1980.... I cleaned the contacts and tested with the Ohm meter between 6 and 8 as suggested. Zero ohms. So i guess this is a resistor pack problem after all.

When the resistor pack is disconnected the fan stops, and will only run when i the fan speed is set to 4 .

So should i just get a new resistor pack? Or is there something else i can do?

Wich route is the best way to go? Buy a new one from Porsche, or is there maybe some third party or updated part that is recomended?

Thank you so much for your help so far!

Bill Ball

Some people have succss with removing the cover and cleaning and adjusting the resistor pack contacts. I didn't. Still, I think it's worth a try. Unless the recent PET is wrong, the pack price has gone way up ($126). I thought it was expensive at $75when I bought one a few years ago. Also, I'm not sure the "early" pack is still available new. The later (89+) one fits but requires a wire swap in the plug to wrok with pre-89 cars. 928 Intl may have used early packs. Since these tend to go out at some point, I'm not sure I'd buy used, but it is an option that will save you some money.

ths2005

Fixed it

thank you for the tips. Basically it was the heat override circuit that was closed. My resistor pack was a bit rusty, and i think gravity and a vibrations made it bend slightly over time. I just bent it a bit backwards as suggested in other posts and job done. I also bent the resistor wires back into shape and cleaned it a bit.

After bending, picture before cleaning:


It took less than 30 minuttes to fix. I used a thin wire and made a "U" at the end and put it through the open pin at the contact. Just insert it and twist it. Very easy and no problems at all to get the pack out and then back in place after i fixed it. Offcourse you could just save yourself the trouble and just remove pin eight like others have, but then you don't know the condition of your resistors.

$126 saved, it will hopefully last some years now