Porsche Repair: 911SC 83 Brake LightsNot Working, brake warning light, electrical gremlins

Question
Hi Dave

I have had problems for some months with the brake lights not working or working very intermitantly.  I have had the brake switches (both) replaced 3 times and this appears to temporarily fix the problem.  The brakes work fine; there is sufficient fluid; the brake warning light is functioning but does not warn that the brake lights do not work.  It appears that the switches are simply not recognising that the brakes are applied.  But as these switches have been so often replaced I am suspecting an electrical rather than mechanical issue.  Any thoughts?

Kind regards
Sean

Answer
Hi Sean,

I have looked at an '84 wiring diagram...as my references do not have anything for an '83...and it does indeed show your two switches...but there's no clue why you have two.

Anyway, the diagram also indicates the brake light housing itself is grounded to the chassis (meaning the body/frame combo) by friction.  Friction means that the light assembly is mounted in such a way so that it JUST HAS TO BE touching the chassis is some way.  This means that the electrical current flows from the battery positve terminal to the light switch and the fuse, to the light itself, through the housing onto the chassis and back to the battery negative terminal.  

Many other cars have an actual ground wire leading from the housing to the chassis to ensure the ground is more permanent.  

Just last week after many days of searching for electrical gremlins of my own, my son reminded me that "intermittent problems are always caused by bad grounds!"  He was right, I'll have to admit...and he may be right in your case as well.  If the light housing is grounded by friction when it wants to be..the brake light will work fine...but when the housing moves a tad, the brake lights will not work...whether you have one, two or eight brake switches.

If it was my car I'd just run a couple ground wires from the housing ( as close as possible to the bulb's holder) to a known good ground.  The other way to fix the problem would be to remove the housing and clean all the points where the housing touches the body, ensuring each point is free of paint and grime.  After reinstalling the light housing test the continuity of the housing to the chassis, meaning that if current is moving through one it has to move through the other because they are now joined as virtually one piece.  If so, you should be good to go again...til vibration loosens the housing for the next owner many years from now.

I just hate it when my son out-does the old man!

Dave