I've been frustrated for the past 5 months with mine, but now all is perfect. This is what I did:
New magnet - didn't need it after all.
New hand switch - works perfectly without it anyway, but gets rid of the credit dot if its there.
Try moving it manually at the back where the "tags" pass through the optos and see if they are registering correctly - if not, optos need replacing (unlikely, maybe just cleaning").
check wear of x and y nuts - easily replaced and sourced.
Check wires leading from the hand all the way back to the board - they can get worn/exposed due to movement.

My main problem was the magnet would activate, but drop the ball after a second and couldn't hold it. You could feel it buzzing, but not enough power. Ended up swapping my main board from BSD with JM, and both work fine now. Probably a capacitor or something not working properly, but get your board checked as it's probably at fault.

So much fun when it works, such a pain to play with it disabled.

Hi Dendoc,

Which board did you change? What is the main board, the big one? Would like to try this as well.

thx

icecream - I posted a link to your thread in the JM club just after you posted. Hope someone can help! Dendoc's advice is a good place to start.

Do you have the game manual by chance? Also a great starting place.

I would start with the least expensive options first like replacing X and Y nuts as they wear over time and is a common problem with this pin. I take it you have been over to the JM club and scanned hand problems and solutions there also. I have yet to have a problem with mine,but would be interested to hear what your final fix was.

Yep, replaced the big board, the power driver assembly board.

I owned a Johnny and had a few hand problems. The x nut often strips but that should be easy to spot when the glove doesn't make the lateral trip properly. I've had some of the thin wires come loose off the mechs behind the backboard. The one time I struggled to find the problem it involved a standard switch that should close when the glove travelled towards the player. It registered when depressed by hand but not when the plate travelled over it. Became only a simple adjustment. Maybe the OP's problem.?

The big board won't solve the problem, reseating the connectors on the big board might help. The only thing on the big board is the 20v power. Like others have said, x and y nut first and clean the mechanisms, that will solve most problems. You might also check the connectors on the relay board under the playfield, if you get cold solder joints or cracks the hand might not move enough to catch the ball and then shut itself down for the rest of that game. Biggest thing on this mechanism is that you don't want it binding at all. Those nuts wear out, replace them first.

^This. I just had an odd JM hand issue show up, and the problem seemed to stem from the relay board under the playfield. My issue sounds similar to yours, but not exactly the same. Flowing some solder and adjusting the connectors on the relay board seems to have solved it for now. Here is a link to my post. Hope it helps.

http://pinside.com/pinball/forum/topic/tech-jm-hand-odd-behavior#post-1503209

Just after making the "no problems here " comment above,I turn my machine on to find a F5 switch error at the Hand.

I had this problem as well, mine ended up being the center drop target not working correctly. Check the coil, switch, and return spring. The glove is tied to all of that working correctly to keep the magnet working as engineered. If you knock the drop down to access the glove and the MPU doesn't register the switch closure the glove will grab the ball and allow you to start moving it to the grid, but once the MPU realizes the drop switch never fully closed it drops the ball. If this is your problem replacing the X Y nuts wont solve it. Replacing them obviously won't hurt your pin, but it is a PITA to replace them so keep them in your tool box for later if you don't need any more stress in your life.
Good luck

The glove works fine without the switch. Just ends up with a credit dot.

One of the continuing problems that plaque JM is the continual strain on the wire harness to the hand. As you go through the process of troubleshooting the issues. I suggest that you consider replacing that harness.

Or this. What a mess.

20140315_072610.jpg

This tread may help
https://pinside.com/pinball/forum/topic/tech-jm-glove-problem
I think this is the thread that helped me resolve my issue with JM.
good luck

Hi Guys my JM just started acting up. the magnet went out so i replaced it. now it comes on soon as you start a game and sits there buzzing. does anyone know what turns it off and on? I checked the locator switch and the hand switch that tells it there's a ball in the the hand and both test good in the test mode?

Sounds like the transistor is blown so the coil/magnet stays on. I’d replace the diode (if there is one) and transistor for it at the same time