Figured it out. I'm not sure which years have the three-prong BLS, but on my '81 there is a discrepancy logic tied into the "BRAKE" warning light. Essentially, if one of the pressure switches closes to energize the brake lights and the other doesn't, you get +12V to the BRAKE warning relay which opens or closes it. This lets you know one side of the split system doesn't have pressure.

Apparently it's a latching relay and you have to reset it by disconnecting the battery. But, if the problem isn't fixed, it's just going to happen again. With the mechanical switch mod, daisy-chained off one of the OEM connectors this fault will continue to repeat. One solution I think is to unplug the OEM connectors from both switches and just run the new mechanical as a stand-alone. But I like the idea of an idiot light. Because, well... :rolleyes:

SO, after scratching my head and reviewing the schematic I came up with this. I hope it works like I think it will. I jumpered the two pressure switches, so they will still do their job of comparing halves of the system. And I left them in the brake light circuit so if the mechanical switch fails, they should continue to function. But I isolated them from the mechanical switch with a 6A blocking diode. This way current through the mechanical switch won't activate the BRAKE warning relay when if flows through even though they're still in their normally open position (because they suck!)

I also added a relay (not diagramed) to relieve the Sonata BLS I'm using, because I get the impression that it's probably just intended to close a relay, not run several amps continuously while sitting at a stop light, etc... It's all plug and play (no cutting/splicing required.) It all just plugs in between the wiring harness and the OEM pressure switches.

I figure what's the worst that can happen? My brake lights fail and I start my car on fire. No biggie. ;)




http://forums.pelicanparts.com/uploa...1398789435.jpg

Finally got it all set up and a new pair of pressure switches to boot. Wanted to follow up.

Works like a charm. I've got the standard two-switch system functioning in concert with a mechanical switch and the diode works to prevent any false indication of a system failure. Very happy.

Thanks for your great design. What if my 85 car has the 2 prong housing? Only (green/blue) and (black/yellow) cables. The system should work fine with out the final modification you do with the diode, just plugging the Hyundai sonata switch to the OEM (green/blue) and (black/yellow) cables, omitting one of the hydraulic switches the one nearest to the driver???

Thanks for your time!

Where you purchase the Hyundai switch??

Some one enlighten me about the guts of the MC. When you press the brake pedal what moves in what direction (in or out) inside the MC? Does some thing bug out which then presses the installed switch inward activating the connection? Yeah I know it's a stupid-a$$ question but that is me.

No, Rick, it goes the other way. Press the pedal and the linkage pushes into the MC. The switches used for this are "normally closed" switches. You install them so that they do not complete the circuit (pin depressed) when the pedal is released. When the pedal is pushed, the switch pin is released and the circuit is completed.

Got it. Thanks

Arne's got it. When I installed this switch, I screwed it in until the button on the switch was pressed in, then I screwed the locking screw in to hold the position. When you push the brake pedal, there is a see-saw like mechanism that pushes the MC back towards the driver, pushing the plunger in, and allowing the switch's button to release and trigger the light.

Here's a shot of the back of the MC housing. It's difficult to see the "elbow" that's just inside the larger hole, but that's what the switch presses against until you press the brake pedal:

http://forums.pelicanparts.com/uploa...1417284266.jpg

I'm not sure why the others had any problems with the cabin brake light coming on. I really didn't overthink this install, I literally just pushed the blades into one of the two hydraulic switches, essentially making my mechanical switch a jumper. Simple as that, and it's worked like a charm.

I did also wrap the wires to clean up the look. Here are some of my final pics:
http://forums.pelicanparts.com/uploa...1417283865.jpg
http://forums.pelicanparts.com/uploa...1417284006.jpg
http://forums.pelicanparts.com/uploa...1417284064.jpg
http://forums.pelicanparts.com/uploa...1417284105.jpg
http://forums.pelicanparts.com/uploa...1417284138.jpg

Great shots. Thanks a ton for posting.

Ok, need some help since I am mentally challenged. Have the Hyundai switch, grommet, and wiring for my 82. If I completely remove the stock 3 wire plug from the MC the brake lights still work without it plugged in. Tested it when plugged back in and found one always hot connection and one which turns hot when the brake pedal is depressed. Avoiding the always hot connection on the plug, no combination with the aftermarket switch makes any difference with the brake pedal partially pressed where the stock switch does not complete. I have tried the aftermarket with the plunger pressed in and not, no difference. I do have the key in accessory mode if that makes any difference but the brake lights do come on when the pedal is pressed half way down. It seems like since the brake lights come on independent of the plug with the wires being plugged in, adding the aftermarket switch to that does nothing. There may be a second wired plug behind the first, which is behind some air ducting, but that is next to impossible to reach. Any suggestions beside give up and start drinking heavily?

Hey RSBob, I'm a little confused. I never unplugged any of my stock wiring. I literally plugged my new switch into the front hydraulic actuator switch by pushing the two blade ends of my mechanical switch wiring, into the hydraulic actuator switch.

Here's a dark picture of the front hydraulic switch (theres another aft of this one that I didn't use).

http://forums.pelicanparts.com/uploa...1418537670.jpg

Yeah I followed your lead and plugged the spade ends into the end of what you are calling a switch. To me the switch is probably housed in the silver assembly the plastic wire plug/receiver is plugged into. Putting the spade ends into the plastic plug did nothing, but I have three wires terminating in mine 1 always hot, one switched on hot when the pedal is pressed and the other the ground. Which wire colors did you plug into? Because I was having no luck, I pulled the plastic plug and was surprised the brake lights still worked. I am guessing they are being activated by the second switch in the back.

Found the answers to my wiring questions here: http://forums.pelicanparts.com/porsche-911-technical-forum/543914-sc-mechanical-brake-light-switch-project.html

Put in the recommended jumpers to the wires with the color stripes and another to connect the plug to the ground, 3 total, and EDIT - now the Brake Warning light is on full time. Will have to try CraigD's method again but may have to solder the connections since I couldn't get the spade ends to stick. The good news is that just a slight touch of the brake pedal and BAM they are on. What a great mod.

If that doesn't work, either get out the black tape or follow G150drivers mods.

That's great to read! I apologize for missing your previous post, and am relieved to see that you got it working!

Cheers!

- Craig_D

Craig, appreciate all the help. Cheers

Follow-up: brake warning light is on in cabin and cruise control will not engage. Looks like I will need to use g150driver's mod.

Update: Just did the mod and the brake warning light is now off and with my new LED brake lights are now instant ON! Just a touch of the brake pedal, like a modern car, and they are on Bam!

Now just need to see if cruise control works.

Having never seen or studied the schematic of an '85, it's hard for me to say. But I can tell you that Auto Zone has the brake light switch, for less than $20.

So this is strange.. I've installed one of these switches just as you all and very happily now have instant brake lights.

The weird thing is the lights go out around half press, then come back on at full press. Is the lever inside the housing coming back to press the switch again?

That's bizarre. The lever inside the master cylinder housing, pushes away from the button/trigger, releasing it and triggering the brake lights. I don't know how it could press the button in again.

It's been awhile, but the issue when adjusting the mechanical brake switch on early 911s is - there is no obvious height adjustment of the switch.

The pin that transfers pedal motion to the master cylinder can be adjusted for overall length. This can provide a limited brake pedal engagement adjustment range for e.g. easier heel-and-toe actions.

However, the large washer that bears against the switch lever arm is normally clamped against the pedal end of the pin. Changing the pin length also affects when the switch contacts close relative to brake pedal movement.
http://forums.pelicanparts.com/uploa...1475263803.jpg

If increasing the arm length for higher pedal engagement, this results in delaying switch activation (i.e. contacts close - brake lights ON) until further downward movement of the pedal stroke.

A simple solution is to install washers under the switch mount so the lever is in a better position relative to the large washer.

Another "solution" is to install spacers on the driver side of the large washer (spacer-washer-nut). This allows the large washer to be placed along the short threaded section of the pin so it can bear against (and release) the switch arm at a more desirable pedal height.

Sorry. Don't have a pic of the suggested spacer-washer-nut arrangement, but hopefully the description suffices.

Sherwood