Da Groj

Okay, so my brake light switch failed. This is apparently a common problem with MG's of this era. THIS is where you can maybe blame Lucas... but, they were just building the part that MG asked for, at the price they were willing to pay. So, blame MG.

The switch is a "Normally Closed" plunger type switch. Mounted on the car, the brake pedal at rest depresses the plunger. So, "in circuit", it acts as a "Normally Open" switch. When you push the pedal, it closes the circuit. Almost every car works this way, nothing new there.

The problem is that it's a cheap switch, and it has to carry the full 4-5 amps of the brake lights. EVERY TIME you press and release the brake pedal, it's arcing the contacts inside that switch. Other cars probably work this way, too... but, this is a particularly cheap switch.

Anyway, the contacts end up looking like this over time. And with that much crud on the contact... it quits working.
And, of course, nothing is ever as easy as it should be. The first thing I wanted to try was to adjust that switch, since we just bled the brakes, and the pedal is a LOT higher than it was, and very firm. The switch is very accessible on the front of the pedal box under the hood, just in front of the brake MC. It's got a lock nut on it that you can loosen. But, you CAN'T rotate the switch more than about 1/6 of a turn! Why? Because the shape of the body of the switch is an extended oval, about .5 x 1.25". There's not enough room on the bottom side for the fat side of the switch to rotate! It's impossible to adjust the brake switch without disassembling something else. Ditto replacing it... you have to at least loosen the pedal box (6 bolts and two Phillips head screws) to lift the pedal box up far enough to gain clearance.

As soon as I saw that, I said, "there's no way I'm putting that same switch back in there, there's got to be a replacement that fits better than this!" And so, I set out to researching that. Pretty quickly found that it's been discussed on the MG forums. And the first solution that popped up said that I guy used a '69 Corvette brake switch. Cool! That should be readily available at the corner parts store. Wanting to get the car back to operational status ASAP, I checked to see that it was in stock, purchased online, and sprinted to the parts store to get it.

Got back home. Used the new switch to verify that the switch is, indeed, bad. (new one works just fine) Then proceeded to break the body off of the old switch to make it easier to remove.

Upon removing it, I realized that the threaded part of the new switch is very much too large! Not even close! The guy who used this switch must have drilled out the hole and tapped it to the 1/2" size of this switch... he didn't mention that, of course.

Further research (LOTS of it) indicates that there ARE at least a couple compatible switches of the correct size that are a direct fit. And, of course, EVERY one that gets mentioned, they never include a part number or source. "Honda" was mentioned, but I'm pretty sure the newer Hondas use 10mm, and some people did mention rethreading the hole, or the switch, to make it fit. (again, no reference as to WHICH Honda) Another common mention was Isuzu, and eventually, I found the Standard brand SLS-105, which is common to a lot of 70's era Isuzu AND Honda vehicles. It looks to be the right size and thread, just looking at the photos. I went ahead and ordered one. Worse case, it'll be 10mm, and I can re-thread it to 3/8-24.

But, that's not all!

As-mentioned, 4-5 Amps goes through this switch and causes arcing. The OE Lucas switches are actually GOOD compared to other options. I mean, most of them did last 30+ years. The one I have could very well be original. A NOS Lucas is available on eBay for $32!!! (the SLS-105 is $9) Most/all of the reproductions are known to be junk. Many of them fail in a matter of WEEKS!

I'm trying to avoid the OE form factor, anyway. But, odds are any replacement switch probably won't be any better, and could fail in a matter of weeks or months. Ugh.

The solution that the MG folks have found is to use a relay for the brake lights. Use the cheap brake switch to flip the relay, nothing more. Folks who have done so report that it was the end of their string of brake switch failures. Of course, there are those who gripe about adding more connections AND a relay into the system, more points of failure. But, consider that on a modern car, absolutely EVERYTHING is controlled by the ECU (or body control module, or whatever), including the brake lights.

I'm going to be mounting a relay board in the car eventually that will have 6 relays... a couple more than I needed. So, I'll have a relay available to support this. But, for now, I'm just going to one-off hack it in with a stand-alone relay.

I'm still on the fence about whether I want to incorporate the design work that I did on a decel-based brake light control using an Arduino. I'll probably leave that on the back burner.

Right now, I just want to get the brake lights working... reliably... so that I can drive the car.

3 brake light switches later, and a complete electrical system upgrade... the brake lights are now working properly!

Switch #1, mentioned above. 1969-82 Corvette. Nah, that's not it.
Switch #2, SLS-105, fits various 70's era Hondas and Isuzus. Nah, still not it. But, at least it's a steel-shaft, and it was 10mm, so I could re-tap it to 3/8-24. I did that. Still didn't work because it's too long. I got it to fit... just. But, it was putting pressure on the brake pedal and applying the brakes! No bueno.
Switch #3, 51590, fits Triumph TR6 and various other Triumphs and British cars. Still not it! 1/2" diameter, I'm looking for 3/8". This one was plastic, which would be easy to re-thread... but, the plunger itself is almost 3/8", so impossible to rethread that small!

All three of those switches people on the MG forum had said that they successfully used as a replacement for the original switch! Be damned if I know how.

After the 3rd switch, I decided to go ahead and shorten the shaft (and plunger) of the 2nd switch by a half inch. Did that, cleaned it up, chased the threads again, and installed it. Was able to properly adjust it, and it works fine! And with the relay now in the brake light circuit, it should last forever.

So, at least this job is done!