Installing mechanical switch for brake lights, with pics

[Won]

Related to: Mechanical brake light switch installed! by pmajka

I meant to do this for a long time; it just doesn't make sense NOT to have the brake lights come on when you put the slightest pressure on the brake pedal. I think part of the problem is that when people change the pressure switches on the master cylinder, they don't fill the little holes with brake fluid prior to installation, thus creating air pockets that will never get bled out. Anyhow, I didn't really want to pull out those switches, and they are known to go bad from time to time so I decided to just add a mechanical switch. Originally I tried to use one of the door light switches or the trunk light switch, since none of them worked on my car, but I found a generic brake switch at parts store for about $10 and managed to keep the car intact... for the switch part, anyway.

I pretty much took the same approach as pmajka and mounted the switch on the actuating rod and the brace. I made a little "mount" for the switch out of mahogany. Why mahogany? Because it smells like crap and I like my car smelling like crap. Working on the pedal area always makes your back ache and this was no exception. Here is a picture of the end product:


Another one showing the hose clamps and the L bracket:


I tapped the wires off the pressure switch that is closer to the front of the car, as I only had to take off the cardboard panel and cruise control brain to get to it. I first used the male-female-male "add-on" connectors between the pressure switch and the wiring socket, effectivly creating a 3-switch parallel circuit, but the car didn't like it and the brake warning light kept coming on. My car had 3 prong switches so I left the "off" ground connection from the wiring to the pressure switch and connected the other two wires to the mechanical switch. 10 seconds later with the positive battery terminal disconnected, the light was reset and never came back on.

I adjusted the switch so that as soon as all the "slacks" in the linkage was taken up, the light would come on. That corresponded to about half an inch of pedal travel before the light came on, and well before the actual braking. With the on-off delay the bulbs have, that was just about the right time.

I had to trim the opening on the pedal board for the brake pedal because one of the hose clamps was slightly binding. I could have repositioned the clamp, but by then the frustration from working in that area had gotten me and I just went for cutting.

Come to think of it, I may still need to take out the pressure switches and fill them up to get rid of this last bit of spongy-ness on the brake pedal. Meanwhile, I can now drive with less fear of getting run over by soccer-moms (hockey-moms?) in their minivans, SUVs and now, Cross-overs. But only slightly less fear. I still fear them. I really do. Do this mod today, and you can feel slightly safer too!

PS. As a side-project for this minor project, I painted the black ring for 3rd brake light. Newer cars came with this baked on the glass. Mine didn't. The PO was sensible enought to get the 3rd light mounted high up. The light came with a white double-sided tape that help up for 10+ years. I didn't like the look of the white tape. So I took it off and tried many other stuff with no success. In the summer when the interior gets hot, everything failed including epoxy (which turned yellow first, ugly!) and JB Weld. I heard about using black chalking, then I realized I had this stuff from before:


It was white, but then it occured to me that I could easily draw the black ring with touch-up paint. I masked off the glass and put a thick layer of the paint. Here is the end result:


Not too bad, eh? A nice OEM look, and now I could use any adhesive in any fugly color to install the light, as long as it held up in the heat. So far so good, through some intense Interior BC heat.

Search keyword: brake switch pressure hydraulic mechanical soccermom Aztec Rendezvous Caravan 3rd third light adhesive tape

[jester911]

Nice write up. I have thought about this project myself. Never
really understood the pressure switches. Sometimes I think those wacky engineers over do it.

[Johnb911]

Won,

I'm a 12 Volt Audio guy but relays ans switches are not my fortea.
Do you have a digram for the wiring?

John

[ZOA NOM]

oops. never mind. I see you didn't want to use the MC switches.

[87coupe]

Having the third brake light at the top must be terrible for visibility out the rear view mirror. Don't think I could live with it there.

[safe]

I was going to ask:
"Why not use the stock pre -78 switch?"

Then I looked it up, $78 !!!!! I hope its made of gold!!!

[Won]

John, here is the diagram, now connections at 82a and 81 on one of the original pressure switches are disconected but I just left 81a there, I think it's common ground. Your 79 may have a 2 prong switch, without the 81a connection.


There are many holes on the firewall around the ignition switch. I found one with just a plastic plug, so I made a hole and used it as a grommet.

Brandon, Targas have pretty nice rearward visibility where you need it (C-pillar area). Besides, whatever's in my rear view mirror's clearly losing, so I don't care much Seriously, from outside the light's still hanging lower than those on many other cars on the road. I am always concerned about being seen by the SUVs and pickup trucks with drivers sitting 20 feet in the air, so I don't mind.

The factory moved around the mounting points for the light, I remember there was a thread about it where people pitched in with their cars' build dates to establish a timeline... Sorta like the 83 SC sump-or-no-sump talk that I'm currently engaged in

Magnus, the contacts on the switch look like gold plated I'm pretty sure the same kinda switch is used for the clutch on/off switch for cruise control; mine was bent out of adjustment last time I saw it inside the center tunnel.

[ZOA NOM]

See thread for my setup using magnetic reed switches for throttle, WOT, and brake lights that I mounted behind the ashtray for in-car video review. The magnetic switches eliminate the physical contact required for switch operation.

[Johnb911]

HEY
THANKS FOR THE HELP EVERYONE.

This is the first forum I have actually participated in a forum.
I read the Audi World ones for interest but, I don't ever have questions because the A4 is too new I guess.

Once agian appreciated.

John