I have an American Bosch MJB mag on my 1923 American LaFrance Brockway Torpedo fire engine. The problem is, the engine won't turn off by the switch. I disconnected the mag wire from the back of the switch and started the engine. While it was running, I grounded the wire to see if that would turn the engine off. Nope. So, then I reconnected the wire to the back of the switch and disconnected the same wire from the back of the mag. I knew it would start and it did. I'm concluding that the problem is not in the wire or the switch but is in the mag itself. What would make it not turn off? Any suggestions from you who are far more learned that I?
Ok, so now, I took a brand new, nice wire, connected it to the back of the mag and grounded the other end. The engine still won't turn off.
I think the wire needs to be connected to the points.
You ground the points to kill the engine.
I am NOT an expert on these. However, the few Bosch magnetos I have worked on have a wire connection built into the small (about two inch round) cover (in front of?) over the points. This cover has a "wiper" that connects inside to the center of the points. If that wiper gets bent or broken, it will not "ground out" the points to kill the engine. There may be other things also inside (lint?) that could interfere with the grounding circuit. That is where I would look.
Drive carefully, and enjoy, W2
Hey Wayne, if you look at an MJB, the whole backside of the mag is one big smoothly curved cover. There is no 2 inch round cover like on a DU4. There's an arm that stick out to advance and retard the spark and a screw below that to hook up the grounding wire.
Marty: call Steve at Mainely Magnetos -> (247) 834-2709 (www.mainelymagnetos.com) and he should be able to tell you what's up with yours and what to look for. Honestly, if you really want it to run right you should send it to Steve and have him check it over to see if it needs rebuilding. He does great work and recently finished a 2nd DU4 for me that needed everything plus an impulse coupler added.
Here are a couple of pics of an MJB.
Underneath that spark lever arm is a screw (can see it in second pic) where I would think you would connect the wire to ground the mag and kill the engine. But if it's not working, then we've just crossed out of my depth. If someone wants to talk about a DU-4, then I've had one of those completely all apart and know a tiny bit more. I would think that you could pull the spark lever arm, then remove the grounding screw and the 2 screws holding the cover on and see what's going on behind there. That should give you a big clue as to why it isn't working. Most likely just not making contact with whatever it is supposed to behind the cover.
Thank you Seth! As I said, I am not an expert on magnetos, although I have worked on several different ones. I am not familiar with the MJB at all (sounds like coffee, and I don't drink coffee). I wonder what years those were primarily used?
Drive carefully, and enjoy, W2
Gentleman, thank you. All of you. The problem has been corrected. And, Seth, I contacted Steve at Mainly Mags and he had the fix for me. I emailed him and he was nice enough to call and walk me through the problem. It turns out there is a little spring loaded contact that wasn't making contact because it was stuck. I removed the contact, replaced the spring and that was that. Easier said than done, though, because the mag is a bear to get off and on again.
I know you guys would come through.