Strange problem with MP5...
 

Hi there. Working on a buddy's TM MP5. It's a few years old, but still in very good shape including the internals.
The problem is, the gun isn't firing. Well it is but...

It wasn't firing so I opened it up and saw the trigger contacts were greasy and not close enough together so I washed it and bent the contacts back. Tested outside the gun and it turned the motor when I pulled the trigger.

Put it back together, put it on semi, took one shot, then nothing. Just dead. No motor noise.

Pulled it apart again, opened up the mechbox, and the motor turned when the trigger was pulled.
Put the mechbox back together, motor still turned when trigger was pulled. Put the gun back together, fired a 3 or 4 shot burst on fullauto, switched to semi, shot once and it died the same way, no noise whatsoever.

I've already looked at all the contacts and they're (obviously) making a connection, but something happens on the first shot, or first few shots.

Could something be pinching or knocked out of place or grounding/shorting out?

This is baffling me. Help?

Try a different motor.

Motor brush gone perhaps?

Nah the motor is fine, like I said it fires sometimes and runs outside the gun perfectly. The motor is in really good condition too.

Selector prongs may be sitting low and not connecting on contact with selector plate. If it still doesnt work then just replace the whole switch assembly.

I do have a spare switch...maybe I'll try swapping.

Problem update. Put the mechbox back together with all original parts and put it back in the grip.

Full auto works fine. If I switch to semi and fire ONCE, the trigger dies. No setting works after that. It will "reset" if I open the mechbox and put it back together, and I can fire full auto again.

Buh? damage you got something for me?

Changed switch assemblies. Same problem persists.

Now I'm gonna try changing the actual trigger part of the switch out...

do you have a spare selector plate? and is the piston drawn back when it dies and you take it apart?

Styrak, it sounds like you have a broken switch stop block, you have the classic symptons. Stock mechboxes will suffer from this due to excessive semi-auto fire.

Nope, it's fine.

I think I figured out the problem: On auto the cutoff lever stops the switch making any vertical movement, but on semi it can move slightly upwards, and the trigger slips off and can't get back "on".

Either needs a new switch assembly or a new trigger.

Problem solved:

Before, when shooting in semi the trigger part of the switch assembly was slipping off the trigger and went back to rest position, and when the trigger went back to rest position, it couldn't pick the electrical trigger up again

So i dremelled off some of the material on a back part of the trigger so it can go EVER so slightly more rearward and pick up the switch again.

FIXED!

Interesting, any idea what caused this to happen in the first place?

It's an old gun, parts wear down. Most likely both the metal trigger and the electrical trigger part of the switch wore down and just didn't work like they used to :p