928 S4: Electronics run amok

  • Hello everyone,

    after a long time in reading mode, I'm actively back in the forum :).
    I still have my 1987 928 S4 and so far it has been great fun season after season.

    However, a bug has crept into the electronics and I am at the end of my tether and need your valued help. :hilferuf:

    I have used the last few days to give the car a bit of a rest. After a few weeks in the workshop for the MOT, oil change and inspection, it came back quite dirty and was washed, the leather upholstery was reapplied with Protector leather milk, the D&W air filter was cleaned and the air filter box was properly fixed (the rubber-metal bearing was torn).
    Status up to that point: the fat one runs like a 1.

    Then I straightened the crooked steering wheel, but disconnected the battery for the work to spare the nerves of the noise-sensitive neighbors. Once the steering wheel was straight again, I wanted to take it to the rented garage after a short drive around town. When I drove into the garage, I noticed the following:

    • When I press the brake pedal:
      • Folding headlights do not go out
      • Windows do not go up
      • Lights in clock and instruments go out
      • Brake lights without function, remain off despite the brake being pressed
    • When I release the brake pedal, everything works as usual, even the headlights go out.
    • I have extended the headlights and pressed the brake pedal again:
      • All headlights go out
      • Power windows no longer work
      • Brake lights work but now
      • Folding headlights can no longer be retracted until I take my foot off the brake.

    My first suspicion was that I had messed something up when dismantling the steering wheel, e.g. something to do with the horn. We then pulled the relay and the horn fuse, but the problem persisted as described above.

    I then disconnected the battery again, removed the baffle plate and temporarily insulated the terminal to the horn pot with shrink tubing to rule out the possibility that it was one of the switches in the steering wheel for the horn --> problem persisted as described above.

    The whole thing somehow feels like a ground fault, because it feels like the entire electronics are going crazy.

    I would rule out the horn itself. Could it be the cable for the negative terminal of the battery? I have disconnected and reconnected it several times.

    I would be very happy to receive tips from you to help me get to the bottom of the issue. At the moment, I wouldn't want to drive it to the garage about 30 km away, because I'm afraid that the car might go up in flames or the electronics might burn to nothing.

    Many thanks and best regards

    Matthias

    This post has been automatically translated.

    Porsche 928 S4 - schwarz
    Porsche 968 - kobaltblau

  • Hello Mathias,

    I think it's a faulty relay. I once had this on an S2 that wouldn't turn off when the ventilation was on. As soon as the ventilation was switched off, the engine also switched off.

    Greetings

    Ludger

    This post has been automatically translated.

    Gruß Ludger

    Rettet die Grotten, weg mit den Schrottpressen

    928 GTS Kompressor

    (928 S4 Bj 89 Automatik) vekauft
    928 S2 Bj 85 Automatik Umbau DP - Motorsport
    928 S2 bj 85 Schaltgetriebe

  • Hi Ludger,

    You're really quick, thank you very much!
    A relay is a hot lead, because I can really hear something "clicking" or switching when the fuse box is open.
    Do you have any idea which relay this could be or what is the best way to find the defective relay?

    Thank you and best regards

    Matthias

    This post has been automatically translated.

    Porsche 928 S4 - schwarz
    Porsche 968 - kobaltblau

  • Hi Mathias,

    I can't tell you which relay it is, but just try pulling the relays one by one if the condition is like this. If something changes then you should be on the right track.

    Greetings

    Ludger

    This post has been automatically translated.

    Gruß Ludger

    Rettet die Grotten, weg mit den Schrottpressen

    928 GTS Kompressor

    (928 S4 Bj 89 Automatik) vekauft
    928 S2 Bj 85 Automatik Umbau DP - Motorsport
    928 S2 bj 85 Schaltgetriebe

  • Hi Ludger,

    All right, I'll try that out tomorrow. I can't break anything with it, can I?
    Thanks a million for the quick reply, I'll keep you up to date :)

    Regards

    Matthias

    This post has been automatically translated.

    Porsche 928 S4 - schwarz
    Porsche 968 - kobaltblau

  • Hi Matthias,

    What is always very good is to inspect ALL (9 ?) earth points carefully, and the 2 at the fuse board, unscrew the bolt and polish all eyes clean, dry fit new (M6) bolts, then cover with vaseline. (only the last ones are often corroded, which you cannot see)

    also pull out all fuses, polish/renew them and put them back.

    Perhaps replace the ground cable on the engine.

    Afterwards you can be sure that the earth problem is most probably no longer there.

    You have to isolate complex problems first, and start by eliminating the simplest one.

    Suddenly many things do it again,... when that was not the case before.

    This is also how you get to know your car.

    This is always a satisfying passage of time, as it leads to results.

    only then continue looking for solutions to problems

    greetings Remco

    This post has been automatically translated.

    stemhamer

    100% schmierung gibt 0% verschleiss. :old:

    porsche 928S4 felsengruen automatic 1986

    leider nicht mehr mit seine speziale 2stufen-LPG-Gaz-anlage ,

  • Hello,
    I recommend using a fiberglass eraser [ad] to clean the contacts.

    It also works very well in hard-to-reach places.

    Greetings Stefan

    This post has been automatically translated.

    alte Porsches:
    911S Ex US '76 mit Umbau auf 3,2l 207PS (Motor 930/21 mit Getriebe 915/68) - Silbermetallic, Schiebedach

    911SC Targa Ex US '81 3,0l 179PS (Motor 930/16 mit Getriebe 915/63) - Weinrotmetallic

    944/1 Targa '84 2,5l 163PS - Platinmetallic (bis 03/2017)

  • Hi Remco and Stefan,

    thanks for your tips.
    A problem with the mass was the first idea that came into our heads, because the electronics escalated completely from one moment to the next.
    We also noticed yesterday that it felt like it had less power at low revs than before, which would also point to a mass problem.

    We took the car to an independent Porsche workshop yesterday in the hope that the ODB interface would be able to pinpoint the problem even further - we'll see what comes out of that. I also don't have a stage at home so that we can check the earth points around the engine & co. properly and clean or replace them if necessary.

    The tip with the fiberglass eraser is really good! THANK YOU!

    As soon as I have info from the workshop, I'll let you know.

    Best regards

    Matthias

    This post has been automatically translated.

    Porsche 928 S4 - schwarz
    Porsche 968 - kobaltblau