The 80-20 Rule

I'm sure you've heard the phrase 80/20. If not, its a theory that the last 20% of the project takes 80% of your time. No where is that more true than working on old cars. In one of my last posts about the MG, I detailed all of the various things I had done, and how close it was to road-worthiness. Since that fateful drive, I have spent many many hours trying to diagnose the small electrical issues that remain (See Ug-letrical). Today runs through some more of that fun.

Seeing Green

The MGB has 4 major circuits running through 4 fuses. The brown circuit is "always hot". It basically runs from the Alternator to the Battery, providing juice to the headlight switch and power to the hazard relay. Red is for the courtesy lights: side marker, running lights, and license plate. Purple runs the horns, the clock, cigarette lighter and "dome" light. There are 2 lesser circuits: blue-stripped between the headlights and the headlight switch and red-stripped for illuminating the gauges. Everything else is on the Green circuit. Seriously everything else: gauges, wipers, turn/hazard signals, even the reverse lights and radiator fans. So, when there is a break in the green circuit, lots of weird things start to happen. It was odd behavior in this circuit which drove me to replacing the fusebox last fall. On my recent test drive (See MGB - Test Start, Test Drive), most of the things on the green circuit didn't work: wipers, turn signals, tachometer.

Following my own advice, I started with the fuse, and demonstrated to myself that electricity was making it past the fuseblock to the next step in the wiring. Fiddling with the fuse and testing wires must have shaken something free, though, because after simply testing voltage and wiggling wires, the wiper motor started working. So did the cooling fans. So, I started to focus on the turn signals and hazard flashers, recognizing that wire shaking could just as easily cause those things to suddenly stop working too.

taken from MG experience posting

Hazard
The hazard switch is one of the more complicated bits in the MGB electrical system. I covered the early diagnosis of this circuit earlier (see MGB - Ug-lectrical). It has 6 pins coming out of the back, with 4 clustered at one end and 2 at the other. The 2 pins allow voltage through when the switch is in the hazards-are-not-on position. This permits the turn signals to work. The 4 pins clustered together map a brown circuit wire from the hazard relay (into pin #3) to light up the "hazard" light in the center of the dash (pin 4) as well as fire both left and right turn signals (pins 1 and 2).

The Interweb says these switches get crusty and need to be flipped up and down a bunch of times to get them to work again. I tried that, but it didn't work. So, I moved back to testing wires and electrical bits. I thought I'd proven that it was the brown wire which led to the relay, by routing a new wire around it, but I actually proved that the original brown wire wasn't seating. Once I pulled the wire off and onto the relay a few times, the hazards started working.... on just the driver side. I concluded that the switch was faulty. Be forewarned: the new made-in-China switches fail often, and can be short-lived. In fact, the replacement that I got from Moss appeared to have failed right out of the box. Neat.

I shot DeOxit into the plug in which the hazard switch is attached. I took a short stretch of wire and tested the plug, to prove that it was indeed the switch and not the plug. One at a time, I jumpered from pin-hole #3 to holes 1, 2 and 4. The jumpers worked, and I was able to get the dash light and one side and then the other to act properly. This confirmed my suspicions: bad hazard switches, even out of the box. I plugged the originally-on-the-car hazard back in, and tried the turn signals. Now, they worked. So, the DeOxit cleared whatever was wrong on that side, but the hazard part of the switch didn't work. To remedy, I bought a New Old Stock (NOS) part off of eBarf. While expensive at $45, having a hazard switch is a safety item I should not be driving without. Even with the new, proven functional hazard switch, the lights wouldn't flash. The hazard relay, though, made the tick-tick-ticking sound. Now, I've read that the original Lucas hazard flasher relays are a bit touchy. So, I dug around in my electrical stuff and found a bunch of relays from the TDI swap on the bus. One of them matched the pin pattern from my MGB - Ug-lectrical post. With a basic black (black for grounds in British cars) wire grounding pin #31, I connected the wire from the ignition relay and the wire heading for the hazard switch. Viola! We have hazards and directionals!

Stopped
Once I solved the hazards, I moved on to the brake lights. I'd solved these before (See MGB fuse box), but with the work on the pedals and master cylinders, they stopped working again. Knowing what worked last time, I went straight to the brake light switch on the pedal box. With it in-hand, and the ignition turned to run, I pressed the little nub on the end (that lightly rests on the brake pedal arm), and the brake lights lit up. Perfect. This is a basic adjustment issue. I turned the adjustment nut further up the switch. Then, I fit the switch through the side hole and threaded it into the pedal box. I think I got some paint into the threaded hole as the switch started to bind. I cleared the holes with a bolt and re-threaded the switch. I had to employ needle-nose pliers to fully set the switch, but now, when the pedal is depressed about a 1/2 inch, the brake lights fire. I set the adjustment nut down against the pedal box, and one more little nagging issue is solved.

That's it for today. I'm still working through other 80-20 issues and hope to have the MGB ready to put to bed until the rains stop (and my money replenishes) so I can drive it to get a top put on. While I wait for that, I have a radiator replacement I need to do on the bus and C will need a second pair of hands getting the 280ZX road-worthy. And then, there's that broken daily-driver I mentioned last time... its a true target-rich environment.

Thanks, as always, for following along.