My MGB is unique in the entire world

I say this because everybody knows that the alternator on an MGB will not charge when the ignition indicator light on the dashboard is burned out. I could have proven to you that this is true on my car for the past 4 years that I have driven it. Since I have not written a book, journal, article, or produced a video stating therein that said alternator will indeed charge when that light is burned out I'm saying that my car is the only MGB in the entire world that the alternator will work while that light was inoperative. By the same token I could take that alternator from non working to working is less than 5 seconds by doing nothing other than revving up the engine. I discovered this by not accepting as gospel that the alternator would not charge when the ignition light was burned out. My light went out shortly after I had done extensive work behind the dash and I didn't want to go back there again so soon and change that bulb. When I ran a normal voltage check I found only battery voltage like I expected to but decided to try something that used to work on old cars that were old back in the 1960s when I was a professional mechanic. Those old cars had generators with what was called a cut-out relay that could get balky but if you revved the engine up the relay would cut in and the generator would start charging normally. I blipped my throttle and nothing changed, so I revved the engine higher and the alternator kicked in with 13.4 volts, and it held that voltage back down at idle. I went and bought a plug in voltmeter like the one pictured and kept it in the car plugging it in at start up and after revving the engine to between 3500 and 4000 it showed 13.4 volts, I'd remove it and enjoy my drive.

As a test today I removed the burned out bulb, started the engine, and had to rev it up as usual to get charging voltage then I shut it off. Next I installed a new bulb, it came on with the ignition switch, went off when the engine started, and I immediately got charge voltage at idle.

Only you John, only you. :)

I am not worthy....

You really should think about writing that book, JW. We would all buy one!!!!!!!!!

will it charge if the light circuit is shorted?

If the brown with yellow stripe wire shorts hard telling what would happen it's the wire that grounds the circuit and turns the light on before the alternator starts charging. I don't know how it would effect the alternator should it short but it would cause the light to stay on no matter what. If the white wire shorts it will raise hell all over the car because it is not fused and and will heat up or burn until you turn off the ignition switch. That white wire circuit starts at the ignition switch and powers up the ignition relay, alternator light (and evident-ally the alternator exciter too), fuel pump via the inertia switch, overdrive, the electrically controlled fuel shut-off valve and vacuum advance shut-off valve on late model Bs. The white wire is not directly connected to the fuel pump, overdrive, the electrically controlled fuel shut-off valve and vacuum advance shut-off valve but it does supply power to their directly connected wires. The point where this transition takes place is at the right rear corner of the engine compartment where a white wire in the bundle from the inside of the far plugs into the downward harness at that point. I'm thinking about fusing that white wire on my car and if I do I'll install a 10 amp in-line fuse in the white wire exiting the plug that the ignition switch plugs into. That would protect that entire white wire circuit.

Please explain to me why it is necessary for the alternator light has to work before the alternator will produce enough current to charge/run the car. My alternator light does not work and yet the battery seems to have plenty of juice to start the car. My car started many times this year and I always ran with the lights on. The battery produced enough current to start it. The wiring diagram shows the alternator to be grounded. It also shows a + connection from the battery to the alternator through the starter. I admit that I find electrical diagrams to be mysterious but exactly what does the alternator light circuit do except give information about if the alternator is producing current?

Please keep it simple since I am not electrical engineer.

JD,
I think this was discussed a few days ago, a road-safety vs electrical harness protection. Think of sitting on a causeway bridge on a black night with a blown fuse in the white circuit; a road-safety issue.

The white circuit is a 16ga stranded wire? What is its continous current capability? Ten amps continuous might be a stretch for many of our old white circuits.

Many moons ago there was a discussion on "self-exciting" alternators. Seems like many newer alternators are of this type, ie, they don't require an input voltage for the field, its generated internally. This might explain the bulb issue?

I can't explain all this stuff. I just tinkered around and messed with this problem until I figured out what worked. I didn't care why it worked, it doesn't make any difference to me as long as I know it's working and can remember what made it work. It appears yours is doing the same thing as mine was except you have never observed a volt meter and saw when the alternator begins charging. When you take off down the street or road you naturally rev the engine high enough that the alternator begins charging. If not for working on those old cars with generator cut-out relays I would never have thought to see if mine would charge with that light burned out. If I had accepted that my alternator wouldn't charge when that light didn't work I wouldn't have ever known that it will.

As an Aside: Years ago, If I had known you couldn't change u-joints in a 1967 Cadillac I wouldn't have successfully done it in two of them before I found out you had to buy an entire drive shaft. :spinning:

Self exciters does explain the bulb issue. Our alternators are self exciting with fast enough spin put on them.

You may be right about a 10 amp fuse in that wire but I'd want to check the total draw of that circuit before putting anything less in it. A 10 amp limit beats the livin' crap out of the limitless draw potential of an unfused circuit. You have to have something under the car short out one time and smoke out that white wire through the dashboard harness from end to end one time to see what I mean. I lucked out and didn't damage anything but that one wire before getting the switch turned off.

...Kinda like the brake failure switch/light...what's up with that? How many other cars have this?

Well Russ, now that you mention it, most modern cars do have brake failure lights. BUT it cost $1500 to get it turned off!

Hmmm, this is interesting to me because I would have never thought that was possible, let alone done on purpose.

I recently installed a weber conversion which will not allow the anti run on valve to be re-hooked up. After researching why the darn car would keep running even after the key was removed I saw the threads about American wiring and British versions and fixes for those peculiarities. That made me think about just removing the bulb to offset the whole problem. Seemed simple and clean...and it worked. The B shuts down as soon as the key is turned off--every time.

Now I have to wonder, most of the 50 or so miles driven since doing that were at night so the alt got a workout. It started several times without any problems.

Do I need an ignition light or is this a good, simple fix for the run on issues?

This is kinda interesting to me, as my 67 with a generator has been doing the same thing. Ign. light burned out, still making voltage at the generator.
The only difference for me is that nobody told me I couldn't do that.

Have fun

I'll take the one with the inverted Ranger engine

For the first year of my ownership of the 74 1/2. The light never worked for me but the alternator charged fiine. When I changed the alternator out because the bearings were worn I was surprised to have a working light on the dash.

DW

We had a problem on the fishing boat with a Delco alternator on the Perkins diesel engine. It didn't keep the battery up. Try getting a push start in a fishing boat 80 miles off of the coast. Dad figured out that the alternator didn't begin to charge until the engine hit a certain rpm and we were trolling below that threshold. A different Delco took care of the problem. And you ask "How does that relate to the current discussion?" I have no clue other than maybe it isn't so much a higher output as one that starts at a l;ower threshold.

Terry: thanks for the explanation.

It seems to make sense to me. I will have to put fixing the alternator warning light on my fix list for next year. I hope that it is the bulb being burned out.

I may just live with it I'm tired out trying to make everything perfect. As soon as you have it all set up something else breaks. Never ending cycle.:drinking:

If the alternator did charge at idle with a burnt out bulb, as soon as you turned the car off battery power would attempt to run the alternator in the opposite direction like an electric motor, and it would drain your battery very quickly. It starts to charge at high rpm because the power output overcomes the diode bridge. It will only charge at idle with an ignition source from the ign light, which polarizes the field windings like a relay but without any contacts. I wouldn't trust the battery to retain enough juice to start the car without the ignition in from the light....especially if your running two tar tops. Expect to eventually be stranded. Usually at the worst possible time. If the light is too much of a hassle to get to, you could just run the ign light wire from the alt to the fuse box to resume normal operation.

I just have to chime in.

My alternator went out; I was new to the MG scene (bought the car in 2000 from the original owner), so it took a couple of alternators to get one that fit. In the meantime, there were also problems with a relay (maybe the voltage regulator?) Anyhow, I had too much voltage coming from the alternator and it burned out the alternator warning light. In 2001. I have since driven over 12k miles, with no problems.

I also had the "starter motor won't disengage" problem. Installed a new starter, didn't help. Poring over the wiring schematics showed a energized wire with a diode or something on it. I cut that wire and the car has worked perfectly since then.

I have a friend's B over here for an extended stay and also noticed that the ignition light was out and the alternator wasn't charging. I happened to mention this to my friend Melanie at Specialty Auto Electric and SHE told me that it won't work properly without a warning light that also works.
Now, what is the easiest way to change it? Maybe pull the tachometer out and reach in through the hole? It's a 74 with 80mm tach.

If your hands are thin enough and fingers long enough you can reach the bulb from underneath through all the wires, cables, etc. I found that my bulb was OK, but it wasn't making good contract with the socket.

This is all good info, my 74 has been my daily driver for a year or so now and I've had it running for a little over two years or so and the red warning light on the dash has never come on at all, ever...

I knew him well back in the 80's when I was in the parts business, his name isn't actually Lea Francis. He took the name from the car, when I first met him I thought that was his name until I was over there one day and seen one of the Lea Francis cars and I about flippped when I saw one. All my dealings with him were on a professional level, either selling getting him parts or having work done. I never had time to work on my own car back then, and from my dealings with him is where I learned he was a top notch mechanic. Dang he must be getting old, since I knew him back in the 80's.

I went out to the garage last night and thought about this post. Reached up behind the dash and I was able to remove the bulb without taking anything else loose, it was pretty easy to get to. I checked the bulb with a ohmmeter (easier than trying to "see" if the bulb was blown) and it was in fact blown. I dug around and found my spares, replaced the bulb and now it works!! I've never had a problem with charging issues but I do feel better now when I turn the ignition on and the light shines....