Porsche Repair: Power window failure, 1978 911 SC, amp fuse, haynes manual

Question
Dave,
I've seen your answers to 911 window problems and hope you might steer me toward solving mine.  I have 1978 911SC Targa whose power windows both failed recently (simultaneously as nearly as I can tell--passenger side full up and drivers side halfway).  Neither motor can now be heard when the window switches are activated (when the switches are activated on either door).  Both windows moved slowly prior to failure.  I suspect a common fuse or relay is failed (I doubt both motors failed independently at the same time), but cannot find a burned fuse in the trunk fuse box and moving the relays (located atop the fuse box) does not reveal a common failure (all seem to function).  Is there another common fuse or relay that I need to locate?  The wiring diagram in my Haynes manual does not provide much clarity.  Thanks in advance, JT Russell

Answer
Hi JT,

You're on the right track, since both motors wouldn't die simultaneously...they just wouldn't dare!  So...the culprit must be in some common item.  Like you, the Haynes manual isn't giving me what I want...in fact I don't even see power windows for the '77 911 they show.  

Right now I'm looking at the diagram on page 300, which should be reflective of your situation.  That 25 amp fuse number 21, or your 1978 equivalent, is the first place to look, which you've done.  I'd like to have you re-check that fuse, or even replace it just because you can.  Notice on the diagram how fuse 13 is also involved in powering the window relay.  All your fuses are perfect, right?

Since there is only one relay, and I assume when you say you moved the relays around you mean you plugged the window relay in elsewhere and it did what it was supposed to do in its new location, right?  Otherwise there is a fairly easy test to check the relay.  Sounds like you covered all the basics here.  Good job.

What I'd suggest next involves using your multimeter (cheap is fine...all you want to be able to do is test for 12v here and there as well as continuity...the buzzer buzzes when the two probes are touched together and when the two probes are touching the two ends of the same wire.  Short of this, you'll need a couple scraps of wire long enough to span the distance from the fuse to the battery.  

And... if you had both a multimeter and those scrap wires; and, you told me you could easily get access to the underside of the window switches...that would be perfect.  I don't want you tearing your interior apart, but often the switches are held in place by friction more than anything.

Let me know when you're ready for the experiment, OK?

Dave