|Magneto Issues can be daunting. I will talk about how to narrow down the problems and then get to a fix with the help for your A&P. I am not a mechanic and these guidelines for your reference and understanding only.
So you have a mag problem? Or may be you just want to be careful and get them updatedoverhauled any ways.
When to overhaul mags?
What types of magnetos are there?
Mostly Bendix and Slick. I am sure there are more types. I just have not done the research. Bendix is the older, heavier kind.
You can find the advocates for both of them. Slick used to sell a kit to upgrade from Bendix at a very reasonable price. That is not true anymore. So Price wise, there is very less differential from going one route or the other. More then likely if you have slick, you would want to stick with it or the other way around.
Bendix has some hard core fans and is a very reliable system. People who use slick swear by them.
How to diagnose?
The process is generally very simple.
You can also get exchange units from aircraft spruce and chief aircraft (some time even at a good price). In my case; the exchange unit was cheaper then the cost of the overhaul.
Hope that this helps! Let me know if you have questions and any feedback.