Bosch mag

[Tim Williams]

Hi everyone,
My uncle's speedster has a Bosch high tension magneto on it, back in june we were on the SCVMTFC endurance run and broke down cause the car started missing, which we thought fouled plug. After checking plus on side of road, the car would not start. Quick check revealed weak spark from the mag. It was 100 degrees out side that day. Needless to say we crossed finish line on a trailer.
Fast forward to yesterday. He went to try and start car and it fired right up, he drove it for close to an hour with out and trouble, no miss or anything.

My question to any mag expert is what could have gotten hot inside to make mag temporarily not work, next what could be a way to cool it? It is a newer style mag fully encased, not a DU 4 style (sorry can't remember the exact model number). Just looking for thoughts so it doesnt happen again..

Thanks,
Tim

[Mark Gregush]

Could be the condenser or coil that is in the shaft. Good book is; "How to restore tractor magnetos" by Neil Yerigan. There is a guy that comes to the Steam Up in Brooks that could check it, but because it might be temperature related might be shop job. There are a couple of people in the Northwest with the tools to do this work. I will check with Tom at the Model A Ford Works next week or you can give him a call at; 503-282-2212
Mark

[Tim Williams]

Mark,
He is in California and has a bench mag tester that he got from Vic Salla. We were thinking condenser but dont those just go bad and quit working all togther?

[Wayne Sheldon]

Hey there Tim! I am not familiar with the magneto he has. However, in general, condensers do have a tendency to fail in ways that make them temperature sensitive. With all of Califunny's silly notions, I don't know if such things are still available or not. However, we used to use a "freeze" in a spray can. Basically Freon with a wand spray head to test electronic equipment. It also worked on things like automobile ignition systems. What will often happen, is the condenser gets a bit too warm, so you shut the engine off to check it out. While the engine is running, the fan and road speed moves air around the engine cooling everything that is somewhat exposed. Once you shut the engine off, the air stops moving, and heat built up in the block and crankcase migrates out to the surfaces and outer accessories (including your magneto!). Temperature in outer parts may rise as much as twenty to forty degrees (depending on ambient conditions, proximity or connection to hotter areas, etc) causing a failing part to quit completely until it cools back down again. Once that point is reached, it may take minutes, to sometimes hours before the part will cool enough to function again. As the part degrades, the temperature at which it fails (or again begins to function) becomes lower and lower.
The thing about freeze in a can is that one can run a car under warm conditions until it begins to fail. Then stop, spray the freeze until the area cools substantially. If it begins to run right gain, you found the problem. I have done that hundreds of times with electronic equipment, and more than a few times on automobiles.
Electronic equipment doesn't take much "freeze" for diagnosis. A magneto may take a full can or two!

An odd thought. You could try wrapping the magneto in cotton cloth, or burlap, soaked in water to keep it cool. Might be able to diagnose that way? This likely would not work well for a DU4 as too much of it is exposed and the wet material would short it out. Regardless, it would NOT be a permanent fix, but could maybe get you to the finish line with some of the newer magnetos. I suspect a mag repair is in order. If it is the condenser, you are well into borrowed time already.

[Tim Williams]

Wayne,
Thank you so much for the insight. Might have to put a can of freeze into the tool box for next year. I don't trust it not to do that again in hot weather, and don't want to finish run on a trailer again.

Tim

[Dan McEachern]

Hi Tim- have Rich order a new condenser- I recall his mag is an MJB Bosch.
Dan

http://www.mainelymagnetos.com/BoschParts.html

[D Stroud]

I don't know anything about high tension magneto condensers, but if they are anything like the condensers for distributor ignitions they can drive you nuts. I have had a couple of them go bad over the years and know of a few more that went bad for friends. One was a new one made in mexico for a '70's era Ford product that I that I bought at a local auto parts in the late '90's and it worked in one vehicle, but not in the other one. NONE of them acted the same way, almost always mimicking other systems. Just a heads up. Dave

[Tim Williams]

Dan,
I talked to him last night. He has a bunch of mag parts but hasn't check for a condenser. I will pass that website on to him.

Tim