Engineered solution for slow brake lights

[vulcan300]

The problem

The solution Porsche engineers chose for brake light activation in many older 911s is a pair of switches that are activated by brake fluid pressure as the brake pedal is depressed. These switches tend to wear out quickly as the current passing through the switches arcs and causes degradation of the contacts (according to one of the best theories I've read). Replacing these switches can solve the problem for a period of time but can't guarantee that the problem won't reoccur.

Some members of the forum have come up with creative solutions to solve the problem but all of these solutions have involved some form of modification to the Brake Bearing Bracket. A picture of the unmodified bracket:



In the picture below you can see the screws that have been tapped into the piece of metal that is used to secure a brake light switch to the Bearing Bracket.



I wanted a solution that requires no cutting or drilling, provides a solid base for the switch, and could be used by other Pelicans that want to try this fix.

The solution

I started by measuring the Bearing Bracket and the locations of the mounting studs and mocked up an initial prototype in some very high tech cardboard composite :-)



I tweaked the design after test fitting and created a template that I planned to use for plasma cutting of stainless steel which would then be folded to shape. The challenge with this approach was that accuracy of the plasma cutting wasn't precise enough and the folds were unlikely to be crisp enough to turn out a decent product. It'd also be hard for people to find a local shop to make the piece so I decided to go back to the drawing board. And with 3d printing in mind, came up with this:



Printing this out of stainless steel would have cost about $500 USD (about $1B Canadian dollars!) so I decided to break it down into a plastic upper assembly and two small stainless steel lower pieces which would allow the bolts holding the bracket to be properly tightened. The first printed prototype turned out nicely and with a few small pop rivets I've produced this for around $100 USD:



The Next Step

Test fitting and wiring into the brake light switches comes next. I'll report back later. I'll upload more pics once it is installed and working.

[93nav]

Nice work!

[ganun]

Nice to have access to a printer!

[merlinfe]

Good Work.

[OsoMoore]

I've got to try this with my wife: "Honey, if I get a 3D printer think how much money I will save!"

It looks fantastic for prototyping, and not too expensive when you actually have the initial expenses overwith.

[vulcan300]

Originally Posted by OsoMoore

I've got to try this with my wife: "Honey, if I get a 3D printer think how much money I will save!"

It looks fantastic for prototyping, and not too expensive when you actually have the initial expenses overwith.

I wish I had my own printer! But I think today's home printers can't achieve the level of precision I wanted. And forget about printing Stainless Steel at home. But don't let this stop you from talking your wife into letting you buy (invest) in a 3D printer for home

I actually used Shapeways.com to do the printing for me. I found it to be super easy and their on-line tools helped me identify potential problems with the parts before printing. I was able to do a few quick design iterations to deal with the issues and then order the parts.

I'm still not 100% satisfied with the plastic component. One half warped a bit during the printing process. But the part is still usable as a prototype. I plan to modify the design of the upper portion to make it a little more robust and then print it again.

I'll post the link to my Shapeways store once I'm happy with the design. That way people can order their own parts, rivet them together (the $10 rivet gun from Harbour Freight is perfect for this) and then DIY the electrical bits.

If you have a printer at home and want to do the whole thing in plastic I'd be happy to post the 3D printer files.

[RSBob]

I added one of the jury-rigged brake switches and after battling the brake warning light issue and winning after adding a capacitor and some fancy wiring I learned that the solution knocked out my cruise control. It wasn't receiving the expected current and stayed disengaged. The solution was throwing the switch, fancy wiring and capacitor away and installing two new brake switches.


But your solution is very elegant.

[Arne2]

That's odd, Rick. I added a switch (drilled holes, unfortunately ) to mine and the cruise still works fine. No cap, no added wires, just the switch in series with the originals.

[brighton911]

I added a mechanical switch a few years back with no impact on the cruise function. I left the original hydraulic switches in as backup ( they still worked) but my wiring was in parallel. Series would mean the hydraulic switches would have to be in the closed position to pass current to the mechanical switch.

[gsxrken]

Few years ago I bought a home-brewed relay setup from a Pelican. So the brake lights weren't carrying much current and theoretically will last much longer.

[Tippy]

I see you went all out, but why not just come off pedal assy?

[Pazuzu]

To help you on your journey...
I found this brake switch, works exactly the same but the wires feed from the side, so it's not as long and not as likely that the wires get damaged by something moving around in the frunk

http://www.amazon.com/Beck-Arnley-201-1818-Brake-Switch/dp/B002E53RMC

Oh, even cheaper!
http://www.amazon.com/Standard-Motor-Products-SLS-344-Stoplight/dp/B001KRY7BA

Yes, they use the standard type 2 blade plug that we have all over our cars already.

[canamfan]

Good work! Though for me, personally, I'm on the hunt for a cross referenced pressure brake switch (say... an older BMW, Volvo, Benz or ??) with a lower pressure setting to activate lights earlier. But you sir have ingenuity! Good luck.

[RSBob]

Originally Posted by brighton911

I added a mechanical switch a few years back with no impact on the cruise function. I left the original hydraulic switches in as backup ( they still worked) but my wiring was in parallel. Series would mean the hydraulic switches would have to be in the closed position to pass current to the mechanical switch.

This is the wiring path I followed. Since I am electrically challenged I am guessing the solution was in series: Mechanical Brake Light Switch Modification - '78 SC

It worked great for instant on lights though.

[vulcan300]

I came up with the wiring diagram below, but I'm not sure it will work without the Diode. I suppose it depends if my car has two or three prong connections. My car is currently in the shop so I can't check it at the moment. It is a 86 Targa and I'm assuming the connections are two prong.

The wiring will be:

- Mechanical and Hydraulic switches will all be wired to an existing ground point.
- Mechanical switch will be wired to Pole 86 on both relays
- Hydraulic Switch 1 will be wired to Pole 86 on Relay 1
- Hydraulic Switch 2 will be wired to Pole 86 on Relay 2
- 12V from Connector 1 will be wired to Poles 30 and 85 on Relay 1
- 12V from Connector 2 will be wired to Poles 30 and 85 on Relay 2
- Ground from Connector 1 will be wired to Pole 87 on Relay 1
- Ground from Connector 2 will be wired to Pole 87 on Relay 2

Current will flow from Pole 30 to Pole 87 within each relay when the mechanical switch or either hydraulic switch connects the path to ground. This will keep high current out of the switching portion of the circuit. I plan to add a pair of fresh hydraulic switches during the next brake fluid change so this fix should last for the life of the car and I end up with extra redundancy when it comes to brake light activation.

[uwanna]

Originally Posted by targamaniac

I came up with the wiring diagram below, but I'm not sure it will work without the Diode. I suppose it depends if my car has two or three prong connections. My car is currently in the shop so I can't check it at the moment. It is a 86 Targa and I'm assuming the connections are two prong.

The wiring will be:

- Mechanical and Hydraulic switches will all be wired to an existing ground point.
- Mechanical switch will be wired to Pole 86 on both relays
- Hydraulic Switch 1 will be wired to Pole 86 on Relay 1
- Hydraulic Switch 2 will be wired to Pole 86 on Relay 2
- 12V from Connector 1 will be wired to Poles 30 and 85 on Relay 1
- 12V from Connector 2 will be wired to Poles 30 and 85 on Relay 2
- Ground from Connector 1 will be wired to Pole 87 on Relay 1
- Ground from Connector 2 will be wired to Pole 87 on Relay 2

Current will flow from Pole 30 to Pole 87 within each relay when the mechanical switch or either hydraulic switch connects the path to ground. This will keep high current out of the switching portion of the circuit. I plan to add a pair of fresh hydraulic switches during the next brake fluid change so this fix should last for the life of the car and I end up with extra redundancy when it comes to brake light activation.

Targamaniac,
Really like the idea of the relay solution to manage the current to the brake light.
I have found one "nit" in your schematic and description though.
You state to connect 12v to pole 30 of the relay, and ground to pole 87. When the relay "picks" you would then have 12v to ground which equals FIRE and melted wires!! I don't think that's what you really meant.
I think more correctly, the wire you hook to pole 87 is really the positive wire that runs to the brake light bulb POS side. The ground is on the other side of the bulb filament, and lights the bulb when the relay "picks"
Had to peruse your schematic for a little while till it made sense what you were trying to accomplish, but it all came clear!
No criticism meant, but just a clarification for the folks who might take 12v to ground literally.

[Jonny H]

I'm not sure about the relays. I'm pretty certain there are laws in some countries about having to 'hard wire' brake lights.

Also, be careful as your schematic suggests you are unwittingly removing the fuse from each brake light feed. A standard automotive relay will happily carry 40A which will be enough to light up your 911 loom if the feed wire gets short circuited.

I have suffered the poor quality brake light switches but they failed in so much as they leaked brake fluid. I replaced with genuine Porsche parts which are visually much higher quality than the VW or aftermarket ones.

[uwanna]

Originally Posted by Jonny H

Also, be careful as your schematic suggests you are unwittingly removing the fuse from each brake light feed. A standard automotive relay will happily carry 40A which will be enough to light up your 911 loom if the feed wire gets short circuited.

Extremely good and NECESSARY point! (wish I'd have thought of it, I looked right past it) We already have too many unfused circuits in these cars as they come from the factory!

[RSBob]

Now you need a fused relayed solution for 3 pole switches.

[uwanna]

Originally Posted by Jonny H

Also, be careful as your schematic suggests you are unwittingly removing the fuse from each brake light feed. A standard automotive relay will happily carry 40A which will be enough to light up your 911 loom if the feed wire gets short circuited.

On further study of the wiring diagrams, if the 12v wire to pole 30 of the relays is the same wire that originally provided 12v thru the ignition switch to the brake pressure switches, then the circuit is indeed still protected by a 15A fuse! I humbly recant my previous statement about no fuse protection!