Complete Electrical failure - am I the only one?

993paul · Jul 20, 2008

Driving to Classic Le Mans I suffered a progressive collapse of my electrics on my 993 (RS). It was in heavy rainfall, with full lighting on and commenced with the all instruments ceasing to work, although everything else seemed OK at first. Then after some manouevring at the Port the battery charge light flickered on and off (the clock seemed to be working), and by the time I arrived at St Malo the car started but then stopped completely 200yds off the ferry! Alarms and Monobloc went haywire and I had to be towed back on to return home [

]. as there were no Porsche Agenst able to help for 5 days,.

Later, I found the alternator was not charging (11.8 volts with engine running - after charging overnight). I then found the Fuse no 7 had blown (this was not obvious as it had not blown at the top of the fuse), and having replaced it - the alternator worked fine (14.0 volts) [:-] and the instruments sprung back to life. Now everything seems to be working again so my questions are:

1. Why does the alternator only work if the instruments are working? Is there a 'Teaser' electrical feed to the alternator coils via the instrument panel? If so why?

2. Why did the instruments blow in the first place? Are they prone to shorting out in rain?

3. Any other ideas welcome as I dont want this to happen again - especially in France!

The only good news was that I organised the Historic Lotus display which won the Concours Cup at le Mans [

] . However all the 50's and 60's Lotus drove there without incident, so I was given unending stick about PORSCHE being an acronym of Positively "˜Orrible' Revolting Schisse Hateful Electriknotworken has worn a bit thin now, BUT I'm left wondering why I sold my1958 Lotus 11 to buy a "reliable" RS??!![8|]

colt · Jul 21, 2008

your thoughts are along the right lines.
the feed to the altenator is via he bulb in the clock (battery sign) its done this way to provide the warning function in addition to the basic function of allowing the alternator to charge. the positive feed goes to the battery lamp bulb and then on to the alternator exciter wire. it needs this feed to start the charging cycle but not continue it once its begun.
when the alternator is not charging the exciter wire is allowed to run to earth making the warnig lamp illuminate (positive on one side of the bulb, negative on the other)
when it starts to charge this earth route ceases to exist and so the bulb goes out and you know its working!

if the power feed to the instruments stops the feed to the alternator stops.(although if the failure happened whilst going along the alternator would in theory be charging so would continue to do so untill you stopped the engine?)

a high resistance at any point in the circuit will produce heat which can melt or burn the point at which the resistance occurs. so if this were a dirty fuse contact it would eventually fail altogether once the resistance got too high.the main body/ chassis /engine earths need checking. faulty battery master switch, any faulty wireing or electrical consumer assigned to that fuse number, such as but not limited to any of the instuments(the clock one is well known to fail in various ways)
incorrectly rated fuse installed?(would get hot and melt/blow)
excessive current load on the fuse by badly fitted non std accessories(tracker/ice etc, wired in to circuits they shouldn't be)

does car require the warranty engine wireing loom replacement and if so is it fitted correctly?

this would be a difficult fault to find in a workshop. if its ok now ,chances are there in no fault to find at this moment but i would inspect the fuse box connections where that fuse fits although i thought it was a low rated fuse 7.5a??? for the instruments, so burnt contacts seems unlikley their.
the alternator needs a good clear resistance free view to the battery via the big main red cable and the earth straps ,otherwise a electrical voltage /current spike could occur and "pop" a fuse giving your experience.
a failing alternator voltage regulator(inside the unit itself ,sometimes replaceable as an item on its own) is another possibility.
I know of a 993 which kept blowing the instruments fuse(causing the same problems as you experienced) and i put an amp meter in place of the fuse,occasionally it showed a dead short circuit. i spent my time unplugging electrical consumers on the circuit trying to isolate it down to an area but mindful that to leave that short on for too long would burn out what ever was causeing it or the weakest link in between (usually the fuse for good reasons!!!but in this case the entire wireing loom)
needless to say after some time pulling at connections wires and components the short disappeared without realy knowing what changed,otherwise i'd tell you to go and look at it!. And the owner has not reported that its failed since.Having said that a few spare fuses may be needed whilst the problem worsens to the point where you can get the meters on the circuit when its actually occuring

I hope this is of some use to you
regards C

993paul · Jul 21, 2008

Colt, thanks a great deal for this, most helpful [

] - I will take a look at the obvious connections and stock up with 7.5 amp fuses! I am wondering if the heavy rainfall might give a clue - in the engine bay? rather than under the dash or in the fuse box as this all seemed bone dry, or may be wiring under / close to the wheel arches . . . . . ?

You have given me clue on non standard items too, one of which was rewired recently, although not via the instrument circuit I believe.

Appreciate you sharing your knowledge[8|]

colt · Jul 21, 2008

hi again guys just to update you, Their are no wires under the wheel arches to get wet .All wires are inside the shell except where they terminate to the engine bay or specific components like a rear lamp unit.
just about every electrical problem ive ever dealt with has not been the result of heavy rain.
Constant dampness/moisture maybe inside the front compartment on 3.2 and older models fuurs up exposed fuses and fuse box(well known).A Recent water related electrical problem(leaking cabrio roof ) rotted the wireing of a bad tracker install on floorpan and burnt part of factory loom causing very strange problems!!!!(dont worry it was a 996)

I think its more likley that the weather caused you to put the whole electrical system under full load by having everything switched on at once (shouldn't normally be a problem, even for your bare bones racer!).Which may point to nothing more sinister than a "green" battery post or the like. might be worth inspecting the engine loom connection in the rear fuse box as this carries several dozen wires.but we are looking for more than the exciter feed failing as this may stop the charging ,depending on when it occurs,but doesn't explain the blown fuse.
you mention the fuse was not blown in the way you expected, Exactly how was it blown /faulty ,unususal?

PPs thanks for the accolades maurice but i pay my mortgage fixing porsches,so i ought to be able to talk about these subjects with some authority otherwise i wouldn't be worth visiting.
think of me as alesser known peter tognola/ gregg kinear (you know, owns a porsche / enthusiastic about them and backs it up with forty seven weeks a year constant experience
by the way maurice you and i have met and chatted on the various merrits of the finest evoloution of the air cooled experience!!
see you all at the classic this weekend

best regards Colt

993paul · Jul 22, 2008

Thanks Guys - proves the value of a forum like this.

I will check the loom - I have a Warranty recall in 2003 in my service book W0001 /W 63113 - engine wiring harness improvement. Can I tell the difference between an original and replacement?

Which neck of the woods are you in Colt?