R.Pires

Hi,

I’ve been having some strange symptoms in my HVAC, and searching the Forum I found some stuff about the Mystery Blower Syndrome, but I think my problems don’t apply to that, anyway I would like to know the experts opinion!
In position 0 and with both ***** totally to the right I can hear the fan working, very slowly, and I believe that’s normal.
In positions 1, 2 or 3 sometimes it works, sometimes it doesn’t, randomly, but when it starts working it blows in the right speed, not in the maximum speed as I think happens in the MBS.
Sometimes everything works ok for 10, 15 minutes or more, I can change the fan speed and changes ok, sometimes doesn’t work, I keep the **** in position 1 or 2 or 3 (doesn’t matter) and suddenly it starts working in the correct speed!
What do you think I should start searching?

TIA for your help!

Regards

dprantl

I was seeing something similar when the blower motor was dying in my old '86.

Dan
'91 928GT S/C 475hp/460lb.ft

Jim M.

Put the HVAC control lever in the defrost position. If the fan goes to high and doesn't work in 1, 2, or 3 then it's the MBS. In the defrost position the resistor pack is bypassed and the fan goes full speed. In all other positions the resistor pack controls the speed of the fan by adding resistance to the circuit. You can try to clean the contacts, but my recommendation is just call Roger for a new one and be done with it.
Usually when the fan is failing, it's most often the bearings and the failure mode is a screetching fan motor. Thats not to say you can't have an intermittent open in the fan motor wiring.

R.Pires

Hi,

Thanks for your answers.
I guess I wouldn't call my problem the MBS, because WHEN it works it works ok at any speed!
It seems that one of this days I'll give my mechanic another lovely time removing and replacing the blower motor (hoping that the replacement will solve the issue...)
Thanks.

Regards

Alan

I think you need to provide more info if you want a diagnosis... you tell us only a little bit... find out for sure exactly what is happening & when - if there is intermittent operation - what causes is it to cut in & out? (time? bumps? slider/switch posistions).

To your knowledge has the resistor pack been changed? the stock resistor pack for your year is succeptible to MBS - if it was replaced with a later model resistor pack (due to stock NLA) then the failure mode is different (motor just stops in low speed modes).

Alan

dprantl

Very true. If your fan always works on defrost (fan speed 4), then it is probably not the blower motor. When mine stopped working intermittently, it would not work on any fan speed or defrost setting. Going over a bump in the road and voila, it would start working again. When it stops running, set it to defrost and see if it runs.

Dan
'91 928GT S/C 475hp/460lb.ft

M. Requin

Another cause of these symptoms is the slider switch in the control unit. It's not sealed, and the slider contacts get filthy (which is why I am working on a different design) and so make very poor and intermittent contact. Just remove the control head for a visual inspection and clean 'em up, if that's the case.

Alan

But the slider only affects the DEF mode for the blower.

Alan

LT Texan

search "Magic Blower Syndrome"

R.Pires

Hi,

Thanks for your answers. Alan, I understand that the more details we give about the problem the easier it is to find what's wrong. My problem is that, as I refer in my first post, this fan issue happens absolutely randomly and I can't find any relation with anything I do.
I usually drive my car every two days and since this started to happen, the last 10/12 times I drive it the first thing I do after starting the engine is to see if the blower works. I then keep the fan in the highest speed and both sliders in the defroster position, to know when and if it starts. In exactly the same situation, sometimes it works, sometimes it doesn't! When it doesn't sometimes it starts after 5, 10, 15 minutes or doesn't start at all! When it works it works fine in any speed or temperature!
For example, today I was in the motorway for about 15 minutes when suddenly it starts working. My wife was with me and in the next 30 minutes in the motorway she tried every possible combination of speed and temperature and everything works fine. Even put everything off for a few minutes and when re-started it still works ok.
Later, arriving at my destination, engine off, removed ignition key and them I though, just in case, let me try now, key back on ignition, engine on and no working blower! As you can see, there is no special reason (at least I can't think of one) why the blower starts and stops!
As I'm writing this, I'm thinking, probably wrong if it could be in any way related with ignition or ignition switch.
This is the most detailed explanation I can do on my mystery blower!
Thanks for your help!

Regards

M. Requin

And whether the blower is on or off. As you suggested, "find out for sure exactly what is happening & when - if there is intermittent operation - what causes is it to cut in & out? (time? bumps? slider/switch posistions [my emphasis]." If the slider has poor connections, I think it affects the blower's operation.

Again, I suggest you look at the slider contacts. Easy enough.

Alan

You are correct for this year (and others) - you reminded me - although this isn't true for the later models where the blower is always enabled... there is no blower "off" mode for these (incl. GTSs.)

Alan