DU4 Bosch Magneto not grounding

[Hubert_25-25]

I have a Bosch DU4 Magneto that is working fine, but the ground connection will not shut off the primary side of the windings when grounded.  Yes there is a pin on the center of the rotating points.  There is a carbon button on the points cap and it is making contact with the center of the points.  When I touch the ground connection on the point cap to the engine, there is a spark, but the magneto continues to output voltage.   To shut it down, I am connecting a ground wire to the secondary side - hoping that this is not causing any problems.  Any help on why the ground is not working would be appreciated.   Thank you.    Hugh

 

 

 

[dibarlaw]

Hugh:

 If no answers here we can post on the Brass Buick site as many have switched over from the Remey and are using the Bosch DU-4 magnetos.

[edinmass]

Give this guy a call....every helpful. He did my mag that had multiple issues, priced fair, and is willing to help over the phone.

 

 

 

 

http://www.rebuiltmags.com

[Mark Shaw]

Hugh,

    Under the points cap there is a flat piece of spring steel mounted to the grounding contact.  The button at the end should be touching the end of the pencil (Q in the drawing).  See if you have continuity between the other end of the pencil and the ground contact.

   

[Hubert_25-25]

Mark, 

    I am going to get over to the car next week and will report back.  The first thing I will try is to see if the button is making a strong contact as that seems to be the most logical source of a poor ground contact. 

 

Edinmass,  I also appreciate the contact information and will give him a shout after I do a little more checking. 

 

Larry,   Thanks for the note about using the brass Buick site.  I have a couple other DU4 Magnetos on some of the other cars that I am managing and they seem to be a pretty good Magneto.  

 

Probably 15 things that keep them from working and only 1 that keeps them from not stopping so I consider myself lucky.     Hugh

[Hubert_25-25]

I believe the carbon brush is worn too much and no longer making contact with the rotating pin at the center of the points.  I ground 2 wide grooves into the insulator to allow the carbon brush to extend further as the insulator was holding it back.  Does anyone have replacement carbon brushes for these?  I did not get a chance to see if this fixed the problem, but will report back after my next visit with the car.     Hugh 

 

[Mark Shaw]

Hugh,

     Evidently, someone modified this component. Your modified part doesn't have enough flex to make contact.

The flat spring steel part acts as a spring to hold the contact tight against the end of the pencil.

I suggest you remove the copper supports and replace them with a single piece of spring steel as shown below.

Also note that the base of the spring part is insulated from the cover as is the threaded ground connection.

 

 

Edited April 14, 2022 by Mark Shaw (see edit history)

[Mark Shaw]

Hugh,

    Did this solve the problem?

[Hubert_25-25]

I made it over to the car today and I meant to get a picture of the back of this magneto for reference.  Making the carbon button make better contact did not fix this problem.  Same thing.  When I touch the wire coming from that center post that the points rotate around, the wire will spark when it is first grounded, but the magneto keeps firing. 

 

I still need to call the number that Edinmass suggested.  I was waiting until I had checked out the first fix.