Carerra / SC mechanical brake light switch install.

[andyt11]

My car suffers from the same problem a lot of 80’s 911’s do with late showing brake lights. It’s often on my mind and I find myself stopping harder than necessary sometimes just to make sure the lights show. After reading all the threads about the problem, it seems the problem is the hydraulic switches simply require too much pressure to activate, either by design or because the contacts get toasted by the current going through them.
I decided to fix it once and for all on my car and put a mechanical switch inline with the hydraulic switches.

Another thread on here showed a mechanical switch mounted on the brake booster pedestal, ( SC mechanical brake light switch project )......being operated by the push rod that comes up from the pedal box. Seemed like a neat fix and safer than mounting something on the pedal itself where it can be disturbed, so I decided to do the same. The rod moves away from the switch with the pedal is pressed so you need a switch that is “push to break” if you know what I mean.

Here is the pedestal before the install. The rod is in the top hole.



The most robust switch I could find was marked as being good for 15amps at 125vAC or just 0.5amp at 125vDC. (ignore the box in the photo - thats a different switch for a different project). I’m not an expert at all, but by my math that equates to safely handling 5.2 amps at 12v DC.

I’m assuming my car has three 21w bulbs for brake lights, which is 63watts, or 5.25amps at 12v. It would probably have been fine, but I decided to use a 30amp relay to actually do the switching and just use the micro switch to control the relay. I mounted it all on a piece of bent plate that I had kicking around.





Like the link above, I used some self drilling screws to attach to the soft alu pedestal but before I drove them all the way in I backed them out and cut them short so as not to interfere with the linkage inside. You don’t want to strip these, so hitting something on the inside would be a problem, not to mention the fact that I don’t want anything catching on the brake linkage!

To hook up the electrics I soldered some twin wire to the top of the spades that are in the plug that goes onto the existing brake switch. You can release the spades from the plastic housing for this purpose. Below is a crude wiring diagram.
You can see that the relay is just in parallel with the existing switch (which is fused) and that power for the actual winding inside the relay was also taken from here. The micro switch completes the circuit and operates the relay winding. The only current the switch sees is the current needed to operate the winding.



And here are a couple of photos of the whole assembly mounted. (I did vacuum up the alu shavings)







I think it will be rather robust, obviously its under the trunk carpet so there is potential for it to get a knock but I'm reasonably confident it will be fine.

It is very precise and the lights come on within the first 1/4" or so of pedal movement. I will have to check on it before and after driving for the next few months but I'm pretty happy with it so far.

I hope I drew the diagram correctly and I hope this helps someone else solve the problem.

[andyt11]

couldnt add this to the original post, but here is the info on the switch used....

[Quicksilver]

There are 2 alterations I would do to this project:
- I would wire the original switches in parallel with the new switch so any of them could trigger the relay.
- I would mount the switch in a way that didn't permanently alter the aluminum "bearing bracket". A quick suggestion would be to run a strap from 2 of the brake booster mounting studs (on diagonal corners) over the hole you are using to actuate the switch. Then the system can be removed, returning it to a stock appearance in a couple minutes.

---
Couldn't Porsche come up with a better name for the big aluminum piece then "Bearing Bracket"?

[andyt11]

It is operating in parallel. If the new switch or relay fail, the old hydraulic switches will eventually operate the brake lights.

And yes, I hear what you are saying about the big alu piece. I like mods to be completely reversible but I had to make a concession here in the interests of reliability. I was originally going to mount something on or under the floor board but couldn't satisfy myself it would stay adjusted correctly.

Still working after a couple of months. I'm pretty pleased with it.

[lupin..the..3rd]

that's a neat solution. but why didn't you also replace the hydraulic switches while you were in there? i had the "late brake lights" problem on my '87 Carrera and simply replaced the two hydraulic switches and all was well again. I like the mechanical switch as a supplement to properly working hydraulic switches, but not as a replacement for them.

[andyt11]

From what I read, replacing the hydraulic switches was only going to be a temporary cure at best anyway. The hydraulic switches seem to degrade quite quickly. Of course I don't know that for sure because I didn't bother.

[DaveMcKenz]

Hi Andy,
I want to thank you for this helpful idea. I am usually pretty good with relays, but I was hitting a mental block. Your wiring diagram showed me the way.
Also thanks to Arne, for the switch source that I used.
In case anyone thinks the relay would slow down the reaction time, I ceertainly can't detect any delay. This came about when I noticed my new LED brake lights were coming on kind of gradually.
Thanks again,
Dave

Originally Posted by andyt11

My car suffers from the same problem a lot of 80’s 911’s do with late showing brake lights. It’s often on my mind and I find myself stopping harder than necessary sometimes just to make sure the lights show. After reading all the threads about the problem, it seems the problem is the hydraulic switches simply require too much pressure to activate, either by design or because the contacts get toasted by the current going through them.
I decided to fix it once and for all on my car and put a mechanical switch inline with the hydraulic switches.

Another thread on here showed a mechanical switch mounted on the brake booster pedestal, ( SC mechanical brake light switch project )......being operated by the push rod that comes up from the pedal box. Seemed like a neat fix and safer than mounting something on the pedal itself where it can be disturbed, so I decided to do the same. The rod moves away from the switch with the pedal is pressed so you need a switch that is “push to break” if you know what I mean.

Here is the pedestal before the install. The rod is in the top hole.



The most robust switch I could find was marked as being good for 15amps at 125vAC or just 0.5amp at 125vDC. (ignore the box in the photo - thats a different switch for a different project). I’m not an expert at all, but by my math that equates to safely handling 5.2 amps at 12v DC.

I’m assuming my car has three 21w bulbs for brake lights, which is 63watts, or 5.25amps at 12v. It would probably have been fine, but I decided to use a 30amp relay to actually do the switching and just use the micro switch to control the relay. I mounted it all on a piece of bent plate that I had kicking around.





Like the link above, I used some self drilling screws to attach to the soft alu pedestal but before I drove them all the way in I backed them out and cut them short so as not to interfere with the linkage inside. You don’t want to strip these, so hitting something on the inside would be a problem, not to mention the fact that I don’t want anything catching on the brake linkage!

To hook up the electrics I soldered some twin wire to the top of the spades that are in the plug that goes onto the existing brake switch. You can release the spades from the plastic housing for this purpose. Below is a crude wiring diagram.
You can see that the relay is just in parallel with the existing switch (which is fused) and that power for the actual winding inside the relay was also taken from here. The micro switch completes the circuit and operates the relay winding. The only current the switch sees is the current needed to operate the winding.



And here are a couple of photos of the whole assembly mounted. (I did vacuum up the alu shavings)







I think it will be rather robust, obviously its under the trunk carpet so there is potential for it to get a knock but I'm reasonably confident it will be fine.

It is very precise and the lights come on within the first 1/4" or so of pedal movement. I will have to check on it before and after driving for the next few months but I'm pretty happy with it so far.

I hope I drew the diagram correctly and I hope this helps someone else solve the problem.

[m_dudey]

Which relay did you use?

[DaveMcKenz]

Just a standard Bosch style black cube type. I mounted in near the new brake switch on the MC mounting structure in the trunk. It's not seen, and easily replaceable.
Dave

[A horse with no name]

Hi...Has anyone worked out how a switch would work on the brake pedal assembly?
If so, would you happen to have any info on how to do it, including photo's?

Thanks in advance

[SCadaddle]

My 82SC (built July of 81) recently required a firm press on the brake pedal to make the brake lights come on and then finally they just quit working all together. Replaced the 2 brake light switches and everything is fine. Noted that the old switches were both stamped 3 81 which tells me they were the original switches.

I think you'll find that after perusing many many threads here on the forum regarding the brake light switches that when the brake light switches fail and illuminate the brake warning light in the dash that the cruise control is also a part of that warning light circuit. So if you've got the cruise control option might be something to think about before just going to a mechanical switch.

[SCadaddle]

[A horse with no name]

Originally Posted by SCadaddle

Hi, thanks for the info. I have 'Spoke's' brake LED's with a resistor built into them and the CC does not cause any more problems. Prior to the resistor, due to the CC, the LED's would remain on, even though very dim.

My issue is the same as yours were; the lights do not go on until the brake pedal is pressed hard to actually brake. I have no way of letting the car behind me know that I am slowing down unless I am braking. Are you saying that by replacing the two switches that this will solve the issue? If so that would be great as I do have two new switches but I have never replaced the original ones because they do work when braking.

[A horse with no name]

Hi Andy...Once the top bung is removed from the master cylinder, was a thread already there for the switch to screw into or did you run a thread die?

Thanks in advance!

[SCadaddle]

Originally Posted by A horse with no name

My issue is the same as yours were; the lights do not go on until the brake pedal is pressed hard to actually brake. I have no way of letting the car behind me know that I am slowing down unless I am braking. Are you saying that by replacing the two switches that this will solve the issue? If so that would be great as I do have two new switches but I have never replaced the original ones because they do work when braking.

I'm suggesting that if you replace your brake light switches you just might find that the effort of your foot on the brake pedal may be considerably less to make the switches complete the electrical circuit to your brake lights.
In my instance I had a neighbor hold his foot on the brake pedal pushing it down until he felt resistance. Then I unscrewed one of the switches and screwed the new one back in while the neighbor kept his foot on the pedal without pushing it further. Repeat for the other switch. When I was done and upon starting the engine, the brake pedal went closer to the floor for about 4 or 5 pushes of the pedal and then it was back to the usual height and usual pressure. I didn't have to bleed the brakes as perhaps only a couple of drops of brake fluid exited the master cylinder while replacing the switches as described.

Plenty of "searchable" threads on this forum describing changing the switches. I know I read a bunch of them before the job.

[A horse with no name]

Originally Posted by SCadaddle

I'm suggesting that if you replace your brake light switches you just might find that the effort of your foot on the brake pedal may be considerably less to make the switches complete the electrical circuit to your brake lights.
In my instance I had a neighbor hold his foot on the brake pedal pushing it down until he felt resistance. Then I unscrewed one of the switches and screwed the new one back in while the neighbor kept his foot on the pedal without pushing it further. Repeat for the other switch. When I was done and upon starting the engine, the brake pedal went closer to the floor for about 4 or 5 pushes of the pedal and then it was back to the usual height and usual pressure. I didn't have to bleed the brakes as perhaps only a couple of drops of brake fluid exited the master cylinder while replacing the switches as described.

Plenty of "searchable" threads on this forum describing changing the switches. I know I read a bunch of them before the job.

Thanks Again... Good idea. I'll try this first.

[A horse with no name]

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