Tuesday, April 25, 2017

MGB fuse box

While waiting for the box of parts to arrive from Moss, I wanted to look into the brake lights. They worked initially, but then stopped working after the test drive.

Check Your Fuses First
Since both brake lights went out at the same time, it wasn't a bulb, but it could be electrical. I started where I should always start: looking at the electrical diagram to see what the circuit does. If you thought the fuse box for the VW bus was simple with 12 fuses, the MGB fuse box will blow your mind. There are 4 fuses.

I'll pause while that sinks in.

4 fuses.

pic swiped from MGB forum
To be fair, there are 2 additional in-line fuses floating around near the box, but the box only has 4. The brake lights run through the same fuse as most of the rest of the car (the top, #1, fuse). This fuse also runs the wipers, lights, turn signals, etc. Some of those systems worked, but all of them were temperamental. I checked connectivity with my multi-meter, and the #1 fuse wasn't reading consistently through the box. One of the fuse-holders was flaky.

New Box
Swapping out a fuse box for a modern car is a huge undertaking. Even for a bus it could be a little tricky. Doing a 4-fuse box, however, was ridiculously easy. Circuit by circuit, I simply moved the old connections to the new box. the box sits on the passenger-side wheel arch and is held on with a couple sheet metal screws. A replacement is $8. Seriously. I paid many times that much for a used fusebox for the bus. While this did fix all the other systems, the brake lights still didn't work.

Brake Switch
how to test brakes on a roadster
After getting the fuse and fuse box to pass electrical signal, the brake lights still wouldn't light up. After checking the electrical diagram, I traced the electrical to the mechanical switch in the pedal box. The switch, like so many of the systems in the MGB, is extremely simple: a depress switch with 2 wires on it. It's basically an open door switch in your modern car: when the door is open, the little spring-loaded shaft extends, and the electrical connection is made / unmade. With the brake switch, it follows a similar principle, but unlike a door switch, the trigger sensitivity can be set. Around the outside of the trigger shaft is a threaded housing that is held in place with a thin nut. By varying the depth of the nut on the shaft, you can control how early or late in the travel of the pedal the brake light comes on. Neat. Seeing how this is a little car driving around in a world of really large SUV's and luxury sedans, having those lights fire as early as possible is the best idea. At this point, of course, I just want the to fire at all. So, I threaded the nut down so that moving the pedal less than an inch will fire the lights.

As you can see from the picture on the right here, I verified my various solutions by depressing the brake pedal with a roller pole. In the picture, you can see that I got the brake lights to work.

That's it for today. There continues to be a lot going on with the project. Multiple efforts are mid-progress, so there could be a delay in  posts while I finish one of them. As always, thanks for following along.

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