The brake lights in my 1979 Porsche 928 failed. As part of my work, I prepared a detailed description of the wiring/diagnostics and repairs for the 1978 - 1983 Porsche 928 Brake Light system. I'm pleased to share it with you. Send me a note at electronikrepair@earthlink.net and I'll send the PDF document.
The 1984 - 1995 use a switch enabled by the brake pedal rod, so these diagnostics steps don't apply to newer models.
The 1978 - 1983 Porsche 928 Brake Light system uses two brake fluid pressure switches to turn-on the brake lights. These sensors also are used to illuminate the brake pressure warning light if one of the two brake circuits experiences a pressure loss.
There is a Brake Lamp controller box located just above the fuse panel that interfaces with the lamps and the diagnostic illumination system. It also can fail, so the diagnostics description includes this box. If it fails, the 1986 box is reverse compatible to early versions.
The two brake pressure switches failed on my 1979 928, always replace both. These three pronged switches, part number 113 945 515 G , were used in many European vehicles including VW though the mid-1990s. So there isn't a need to purchase the switch as a Porsche part, they less expensive though other channels.
A bit of trivia, a two prong version of this switch was installed in the VW Beetle starting in 1948.
Rich
Membership Chairman
Charter Member
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast
