MGB Charging System fun

Brief post today about an electrical breakdown deja vu but this time it's Oliver ('78 MGB), not Hapy ('72 VW).

Charging System Basics
Most cars have a relatively simple "primary" electrical system. There's the battery which holds lots of stored power for starting the engine and regulating the secondary power usage. That part is obvious. Wired into that battery are the starter and the alternator (or generator in older cars). These are motors for opposite functions. One, the starter, eats the most power while the other generates power. Connecting these together are wires and the ignition switch. Seriously, that's pretty much it for the charging system. There could be a voltage regulator (VR) between the alternator and the battery, but oftentimes these are integrated into the alternator so you don't need to worry about them independently. The VR is separate on the original '72 bus, mounted on the right side of the firewall in the engine compartment. There are lots of other things connected into the electrical system, of course, but for today I'm focusing on getting the the battery to charge.

And We're Talking about this Why?

from 123ignitionusa

Back when I had the MGB looked at by the local British car experts, they indicated that the charging system was sickly. I swapped out the battery. I still found myself charging the battery up an awful lot so I bought and installed an alternator upgrade (now at 105 amps). Neat. You can find them here. But the little car still died on the side of the road a couple of weeks ago, with an electrical failure. After our adventure coming home from Furthur in 2013 (See Tale of Two Trips) I knew what an electrical failure looked like. Lights dim, wipers slow down, fans slow down and then the engine starts to miss. Soon, it won't keep running and you're gliding to a stop on the shoulder. If there is one. The last tip: the hazard lights don't work. Which is especially great when you break down in an exit lane where there is no shoulder / break down lane. Like here: (45°28'37.3"N 122°47'25.8"W).

Fortunately, it was mid-afternoon on a Friday, so the traffic wasn't too bad. I got AAA on the phone and stood behind the little orange car in the sprinkling rain waving cars around me. While waiting for the truck, an ambulance pulled up behind me. One of the guys helped me push the car some of the way up the ramp. At the intersection, a couple of guys hopped out of a car and helped push the rest of the way up the ramp and around the corner to where I could turn onto Allen Blvd and coast downhill. I came to a stop around the corner on King and Lee where Oliver was loaded onto a flatbed.

Root Cause

LM 11009 (BA7s)
bayonet bulb

In my list of items included in the charging system, I failed to mention one last thing: the charging system failure light (labelled "Ignition"). This seemingly innocuous light bulb does more than just tell you when your charging system is fried. Of course, it does that and it is able to do that because of this other purpose. It provides a light 12v current to the alternator through a means other than the thick battery-direct cable. When that 12v meets another 12v coming from the alternator, no light. When the 12v meets ground, the light illuminates. Cool. This small 12v signal also initializes the magnetic field in the alternator so it can generate electricity. Without this little signal, there is no field, so no power generation, eventual system failure while you're trying to exit OR217. Neat. So, at some point after learning that my charging system was sickly, that little bulb died. Did I need a new battery? Maybe, the old one wasn't too old, but it was a discount brand. Did I need a new alternator? Probably, it was original and had the rust on it to prove it. Besides, 105 amps is far better than the 43 amps you get from the original or direct replacements.

top: blown bulb
bottom: new bulb

So, how do we prove this? Well, first, when you turn the ignition switch to run, you should see that little light. No light? Check the bulb. Assuming you missed that, or just want more proof, start your engine anyway and check the voltage at the battery while running the engine above 2000 rpm. If you're not seeing around 14v, you're not charging. Replacing the bulb is inexpensive ($2.50US per bulb at NAPA), though hard to get to. I was able to access it from under the left side of the dash by running my left hand up along the steering shaft, behind the tachometer. The bulb and harness are pressed into the plastic holder, and held in simply by friction. A slight tug on the harness will free it. The wires are kind of short, so getting the bulb down low enough under the dash so you can see and replace may be the hardest part. Well, that and getting the bulb and harness pressed back into place. Since I was up in there, I checked all of the bulbs, one at a time, to make sure I didn't have to get back up under there again any time soon. Fortunately, there was only the one.

Even if the "Ignition" bulb weren't critical to getting a charge, it is wise to run with all of your warning lights working. Gauges are great, but when you're enjoying the scenery or enduring bumper-to-bumper rubber-banding traffic, you may not note your gauges as often. A bright red light on your dash attracts the eye. And, in my case, I don't have a battery / charging gauge to tell me how well my electrical system is. I like the stock look, so I doubt I'll change that, but if I wasn't thinking about it before, I kind of am now.

Anyway, that's it for today. Thanks, as always, for following along.