Brake light switch help please

[wareaglescott]

'79 SC

I did the mod to add the extra brake light switch to the front of the master cylinder because I noticed I needed a lot of brake pedal deflection to activate the brake lights. Followed the directions and used the switch I found referenced in other threads. The new switch works great and the brake lights activate at first pedal deflection.

However since I have done this I now can not get the dash parking brake warning light to go out when I release the parking brake. It stays on all the time and I have verified proper function of the brake lights regardless of parking brake handle position.

I wired my switch into the forward most switch by tapping into the blue/green and black/yellow wires. (the car was repainted red in the past so the little red you see on the wire is paint overspray)


Here is the schematic, the two plugs and the wiring if any of this helps.






Trying to troubleshoot this I have completely unwired the new switch and returned it to how it was before I started. The parking brake warning light is still illuminated.

I will say before I did this mod the warning light worked normally. I am stumped now that I have returned to original wiring and I still have the problem.


Any thoughts or suggestions of steps I can take to troubleshoot or things I can try would be appreciated. Electrical is not my strong point but I thought this one should be easy enough!

Thanks

[RDM]

The parking brake light is not just the parking brake light. It's also the brake system warning light. If there is a pressure difference (shown as a voltage difference) between the two brake pressure sensors attached to the master cylinder, the light will light.

That said, are you sure your wiring to the two brake pressure sensors is as it was before? Do you have before/after pictures of that?

[Jonny H]

You also need to disconnect the battery for a few seconds to reset the brake warning light

[wareaglescott]

Originally Posted by RDM

The parking brake light is not just the parking brake light. It's also the brake system warning light. If there is a pressure difference (shown as a voltage difference) between the two brake pressure sensors attached to the master cylinder, the light will light.

That said, are you sure your wiring to the two brake pressure sensors is as it was before? Do you have before/after pictures of that?

Yes it is the same as it was before. I simply removed what I added.

[wareaglescott]

Originally Posted by Jonny H

You also need to disconnect the battery for a few seconds to reset the brake warning light

I disconnected the negative for about 5 minutes with no change after.

[walt]

When I made this mod I simply disconnected one of the pressure switches and led those wires to the new mechanical switch, thinking I didn't really need 3 brake light switches. That said I don't really know why your brake light switch is on.

[wareaglescott]

Originally Posted by walt

When I made this mod I simply disconnected one of the pressure switches and led those wires to the new mechanical switch, thinking I didn't really need 3 brake light switches. That said I don't really know why your brake light switch is on.

Interesting thought.

[Jdub]

Now that you have reset the light via battery, take it for a spin, apply brakes lightly then with a few hard stops - nothing crazy. Bring it home and shut it off, then check again. If somehow there is disparity in the pressure between the switches right now, it might be causing the issue. Good luck!

[wareaglescott]

Originally Posted by Jdub

Now that you have reset the light via battery, take it for a spin, apply brakes lightly then with a few hard stops - nothing crazy. Bring it home and shut it off, then check again. If somehow there is disparity in the pressure between the switches right now, it might be causing the issue. Good luck!

The light did not reset after I disconnected the battery. It is still illuminated. I did not consider test driving it. Are you suggesting if I drive it that the light may go out?

[Marc Bixen]

We always replaces both switches. Problem solved. 2 wire switches get replaced with 2 wire switches, 3 wire switches get replaced with 3 wire switches. Replace them both. No need to re-wire, and re-invent the wheel. To "walt" congratulations, now you no longer have a brake pressure failure warning system. Porsche updated earlier designs to this design in 1977 or a reason, might as well back date it to an inferior set up????????

[old man neri]

Brake fluid low by any chance?

[wareaglescott]

Originally Posted by old man neri

Brake fluid low by any chance?

No I just rebuilt the brakes calipers and changed out the flex lines and rebleed with new fluid about 200 miles before doing this. No leaks and fluid is right where I filled it to.

[emcon5]

Originally Posted by old man neri

Brake fluid low by any chance?

SC doesn't have a sensor for that.

[wareaglescott]

Well I took the new switch completely out of the system and reset the battery again.
That cleared the dash warning light.

Then I tried putting the new switch back in and the warning light came back even after disconnecting the battery.

Then I disconnected both the original switches from the master cylinder and left the new switch wired in and reset the battery. This resulted in no dash warning light and also normal brake light pedal activation but just because the new switch is doing its job and that does not give me any backup switch or warning as designed so I do not like that.

SO whatever the problem is it is not liking having the new switch wired into the existing system.

I wired the new switch into the blue/green and black/yellow. I noticed on the schematic the black/yellow was going to the brake lights so I decided to try unhooking the blue/green splice and just running that to a grounding point. That did not work.

Any further thoughts on what to try?

[wareaglescott]

A buddy theroized I might need a diode in the system to make this work properly. He thought the way I have it wired the new switch may be closing the circuit first and allowing the voltage back through the still open switch 2 and fires off the parking brake warning light.

He sent his schematic. Any thoughts on this please?