Make sure you have the correct amount of balls in the game.
With all balls in the trough go into switch test and make sure all trough switches are functioning.
Play the game until you get 2 in the demon and go into switch test again and make sure all switches are functioning in trough and demon.

First thing I would check also - ball count

Can you hear the VUK in the head while your waiting for the game to kick another ball in play. The demon head is a pain in the ass the dial in. Most of the time its the strength on the VUK is set to high and the ball is basically bouncing back down into the VUK.

Had to adjust my power setting. I had this issue as well, works fine now

Correct. After a couple thousand games you will have to dial it in again. Its a PITA.

When you say adjust power setting - what does that mean?

go into your KISS sub menu and adjust the power setting using the red volume buttons.

You can adjust the power of the VUK coil that kicks the ball into the head lock. I wish you can adjust the kickout coil. It would help with the SDTM issue.

You can. I'll have to look later but I adjusted mine and the adjustment is opposite what it is labeled in the Stern menu. Something like Demon lock power (instead of what you would think "Demon eject power"). I dropped mine and the ball just barely pops out of Genes mouth. Cut the SDTMs by 90%.

EDIT: just checked and the setting I changed is called "Demon Lockup Power" I set it to 22. Factory setting is 32. When set at factory, I got STDM almost every single time. The other setting is called "Demon Eject Power". The factory setting is 32 and that is what I have mine set to.

EDIT2:
Just FYI, these setting are in the adjustments menu, then go into "Kiss" sub-menu

I'm running into this exact issue, but with the ball lock one switch. Adjusting the power on the VUK doesn't seem to make it 100% reliable. Any other ideas before I take the head mech off of the game?

Resurrecting because this VUK mech is a pain in the ass to dial in. Sent Stern an email with no response.

I guess they know it sucks, just like the hyper loop in Star Wars.

This is why I'm done with new Sterns. They can't make mechs that work. People should be fired over this crap.

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Did you ever get a response back? I can't get mine 100 %, if I have 2 balls in Demon and start love gun The ball keeps falling into the VUK, I have adjusted the strength in the menu and still can't get it to work right. After 3 attempts it stops trying and it takes 3 cycles to ball search before it will kick all balls from demon head.

Well, yes and no. Mostly yes though.

I read something about the VUK "learning" behavior over time. Mine seems to decide, after 30 plays or so, to stop locking balls in the VUK, but does count them toward Demon MB. It "locks, counts, and ejects" each ball immediately after the lock. So balls don't stay locked, they are ejected. But locks properly count. They device doesn't malfunction so to speak, but it also doesn't lock 3 balls.

This could be due to a bad opto 2 in the demon head. I have a new one to put in, but since the game plays 100%, I didn't install it yet.

Thanks for the response Guinness!

I’m also having heaps of trouble with my LE demons head lock, it’s grabbing and locking balls when it shouldn’t and then I have to wait till the machine does a ball search and activates all the coils and spits the ball back into play.
It’s frustrating me to the point of contemplating selling the machine

Sounds like the first opto is not regersting. Check for loose connection and make sure a wire didn't work loose. Then I would replace that opto, I also went to the virtual locks with the new code. This probably won't help you now as your machine doesn't realize there's a ball in he Demon that's why it goes into ball search.

https://littleshopofgames.com/shop/electronics/stern-opto-receiver-board-for-spike-systems-515-0215-01/ and you need the transmitter pending which one is flaky. 515-0215-02

Hey Hawks, the fix for this in my case was reseating CN5 on Node 11.
Worth a try!