Weird electrical problem in CA m4

[Flatlander]

I purchased a crane stock and several upgrades so I figured I would do a complete overhaul of the mechbox. Background of latest work:

- systema silent head set and cylinder
- gears cleaned, reshimmed and greased
- modify air nozzle
- stock CA spring for CQB use 'til summer (prometheus 110 to be installed in spring)
- G&P crane stock w/ 9.6v 2200 mah w/ deans
- Fuse REMOVED - didn't think it will fit w/ a crane stock...will it somehow???

Rewired my gun to the rear w/ 16g wire from crappy tire - stock wires not quite long enough so I soldered on extention pieces w/ deans. HOWEVER: before I installed the crane stock I tested the above upgrades with the stock mini connector AFTER wiring to the rear (but before extention wires and deans) - result at first was SA only with FA not working (ie. no juice to motor at all in FA). Switched back and forth from SA to FA and FA would spiratically work after a few times. Finally it started working all the time (or seemed to) so I didn't bother looking into it more.

Fast forward to after crane stock installation complete (next day):

Test fired the gun right away and performed absolutely beautifully! Drastic sound reduction, very high ROF, smooth feeding...everything seemed gravy after a couple low caps. Couple days later (today) I go to test fire it a bit more and right away FA does not work (SA still works)...switch back and forth and then I smell something burning! I remove the buffer tube and inspect my solder joints. The POSITIVE wire/joint heated up drastically and melted some of the solder (hence the burning smell). I then hooked the battery back up...behold SA and FA work flawlessly again while going through 3 mags - nothing suspicious happening at the +ve wire junction.

Anyone know what could cause this? It would seem as the trigger assembly IS functioning (wires heating up/melting solder) but the mechbox *shouldn't* be locking up which would create the huge current draw - FA works great when working + no audible indication of motor trying to turn over. So would this mean there is a short after the trigger assembly and before the motor somewhere which is drawing this current? Why semi and no FA sometimes? This is why I'm taking Mechanical Engineering, NOT Electrical

I was planning to redo the wiring to the crane stock - no soldered joints - in a few months when I upgrade to my outdoor spring. I'm hoping not to have to crack open the mechbox 'til then.

Weird note but may/may not be related: I downgraded back to my stock CA spring for CQB this winter (pre-above mentioned upgrades) and when it went back together the selector switch would sometimes be very stiff from moving from safe/SA/FA...getting harder as you progressed to FA. No firing troubles whatsoever until most recent work and the stiff selector problem is still present and speratic.

Thanks guys.

[ThunderCactus]

Originally Posted by Flatlander

\Fuse REMOVED - didn't think it will fit w/ a crane stock...will it somehow???

It's really unfortunate but no, no G&P guns have fuses with crane stocks, nor does my 249
Sounds like there was a short in your connection somewhere, now would be a good time to open the mechbox and check for arcing and burn marks.
Check your selector plate too, G&P guns don't operate with the same type of selector plate so this may be a problem that's not seen in G&P weapons.

[Flatlander]

It's a Classic Army rifle with a G&P crane stock (had to modify my CA buffer tube as the G&P was not compatible as I found out after...oops)

[Renegade)]

Check all your connections and solder joints, encase them in heat shrink to prevent shorting. Check that your selector plate and all those mechbox electrical parts of the trigger are in correctly.

[mcguyver]

Stiff selector plate can mean that the plate has started to melt. When you have a high current draw in an AEG with a standard switch, all the current must flow from one of the little "fingers" through the selector plate contact to the other. Often, the lower "finger" is only partially in contact with the selector, and this results in increased resistance at that point, and this translates to heat. I've seen this be so severe that it's actually melted the selector plate, and deforming it. I've also seen it melt the plastic over the contact plate and push the lower "finger" out of the way, breaking contact and making full-auto fire intermittant or impossible.

If you pull the mechbox out and look at that point, you'll see if this is the case.

As for a "burning" junction, it's possible that the excess current draw responsible for the burning did not originate there, unless you have a poor solder joint with a high resistance. The overcurrent situation (ie short circuit) causing the burning could be coming from another spot along any wire in the system, and is only manifesting itself at this point, like if it's got a slightly higher resistance across your splice vs the wire itself. Current follows the path of least resistance, but the flip side of that is the spot with the highest resistance will also be the place where thermal problems manifest themselves. It's the same scenario we look for in contact maintenance for large motor contactors or excessive conductor loading. Heat at a spot does not necessarily mean the problem is right there, but often times it does.

Look over all your wiring. Your problem may actually be at the entrance to the stock tube from the body.

[HaZarD SFD]

Wires could of melted from prolonged Full Auto Firing.. I know my CA M15A4 the wires warmed up well when it was on full auto.. its best to burst to be safe.. some wild full autos are ok but not too much.

[Flatlander]

Originally Posted by mcguyver

Stiff selector plate can mean that the plate has started to melt. When you have a high current draw in an AEG with a standard switch, all the current must flow from one of the little "fingers" through the selector plate contact to the other. Often, the lower "finger" is only partially in contact with the selector, and this results in increased resistance at that point, and this translates to heat. I've seen this be so severe that it's actually melted the selector plate, and deforming it. I've also seen it melt the plastic over the contact plate and push the lower "finger" out of the way, breaking contact and making full-auto fire intermittant or impossible.

If you pull the mechbox out and look at that point, you'll see if this is the case.

Hmm. That would make sense. I noticed while checking my shimming I noticed the selector plate seemed 'bubbled out' and thought that somehow the sector gear bushing was pushing it out...double checked and it was shimmed fine so I figured nothing was wrong. I have yet to tinker much w/ the trigger assembly and selector plate so I'll have to take a gander and see now.

Mac, is there any specific cause of this selector plate melting or anyway to prevent it?

[Flatlander]

Originally Posted by HaZarD SFD

Wires could of melted from prolonged Full Auto Firing.. I know my CA M15A4 the wires warmed up well when it was on full auto.. its best to burst to be safe.. some wild full autos are ok but not too much.

I inspected my wires and the trigger contacts while upgrading and re-routing my wires to the back...all in good shape. I keep my trigger pulls short (few seconds max) to prolong the life.

Mac, would it be possible for the sector gear bushing spinning cause enough heat to melt/warp the selector plate? I noticed when testing my gun outside of the body that most of my bushings spin - at varying speeds - with the gears (new bushings are on the 'to buy list' now).

[mcguyver]

Originally Posted by Flatlander

Hmm. That would make sense. I noticed while checking my shimming I noticed the selector plate seemed 'bubbled out' and thought that somehow the sector gear bushing was pushing it out...double checked and it was shimmed fine so I figured nothing was wrong. I have yet to tinker much w/ the trigger assembly and selector plate so I'll have to take a gander and see now.

Mac, is there any specific cause of this selector plate melting or anyway to prevent it?

Some companies make switches that don't have these finger contacts. The fingers are there to provide an electrical safety, to compliment the mechanical safety provided by the safety latch that contacts the trigger.

Fewer contact points are better for the flow of current.

Spinning bushings are more likely to heat the mechbox shell than the selector, as they cintact much more of the shell. But all that metal will only act as a heatsink anyways, so I don't see that being a problem. But look at the selector and if a bushing is heating it, thermal damage will be evident.

More current + more contact points (like small finger contacts) results in more heating of contacts. Heat is what melts things like the trigger contacts and selector plate.

[ThunderCactus]

G&P has a 'fingerless' trigger. The selector plate has no metal on it what so ever, haven't looked at HOW it actually works, but it works really well and doesn't overheat lol

[Flatlander]

Mac:
So if I understand this right, the selector plate also doubles as an electrical safety by cutting the electrical circuit while in the safe position? What companies make these 'fingerless' contacts? Are these the same thing thunder is talking about on the G&P's?

Thunder:
I was troubleshooting a buddies G&P mechbox which had a misterious electrical problems after a rebuild by someone else. It was a new 7mm G&P reinforced mechbox and I believe he had a systema low resistance trigger system. The mechbox came with all v2 selector plates and I'm assuming the G&P m4 version was used. Now this selector plate has the metal contacts on it (it was actually the 'fingers' not making contact w/ the selector plate which was the problem). Is there only certain G&P versions (newer, older, 6mm?) that do the 'no contacts' on the selector plate? The doc may have used a different selector plate that what was supplied with the box, I don't know for sure.

[mcguyver]

Originally Posted by Flatlander

Mac:
So if I understand this right, the selector plate also doubles as an electrical safety by cutting the electrical circuit while in the safe position? What companies make these 'fingerless' contacts? Are these the same thing thunder is talking about on the G&P's?

Yes. G&P uses a switch without the finger contacts. I'm sure a few other companies make them as well. I'm too lazy right now to search. Yes, Thundercactus is talking about the same thing.

When you upgrade a gun, the extra current draw form a heavier spring turns these contacts into a weak point. Sliding contacts are one of the worst for contact pressure, hence the higher resistance and potential for thermal damage. Add a little contamination from dust or grease and matters only get worse.

A selector without a contact plate when used with a "fingerless" switch should not heat up at all. Nor is there the extra contacts that rob your motor of current because their poor contact wastes energy as heat, energy that could be used to turn the motor.

[Flatlander]

Just finished inspecting things. The selector plate and all contacts appear fine. There is, however, a slight 'bow' in the selector plate (bending outwards away from the mechbox) but no signs of heat damage. There is a slight sign of wear just to the front side of the 'horse shoe' shape cutout (for the selector switch thing) which indicates some sort of rubbing. Could the bow in the selector plate cause it to rub against the medal body cause the stiff selector?

I also found a slight tear exposing a bit of bare wire as the pinion gear on the motor must have nicked it - could this be causing my all my electrical problems? I don't see why semi seemed to always work but not FA. Note that it was (seemingly) impossible for the exposed wire to ground itself to the mechbox but possibly the motor.

Side note: would it be recommended/beneficial to remove the contact on the selector plate and just connect the two 'fingers' directly (via a small piece of wire or copper plate soldered onto the two 'fingers')?

[mcguyver]

The bow in the selector may not be allowing the finger contacts to make via the selector in full-auto. If you install it and move it between full and safe while carefully observing the contacts, this should be evident. The selector may also be making contact with the metal body, and if the tear is in the black (-) wire, then a short from - to + would occur via the mechbox and metal body. This would explain your previous over-current problem.

A fuse in your system would have revealed this fault, as it would have blown, or shown thermal damage at the very least. Mini-ATO fuses are very small, smaller than even a mini connector. Install one to protect the system, and to be a diagnostic aid as well.

[Flatlander]

Everything is back together and seems to be functioning fine after around 6-7 lowcaps. Semi and FA are both functioning fine and the resistance in the selector lever appears to have disappeared (hopefully for good).

What I did:

- Put the pins holding the mechbox back in in a different order (trying to pull the mechbox away from the side of the selector plate). This seemed to fix the stiff selector.

- Put a strip of electrical tape over the contacts on the back of the selector plate (between the plate and the body). I believe it was shorting out on the metal body partially caused by the slight bow in the selector plate.

New Concern/Question:

In the process of of moving things around trying to solve the electrical problem, I found a few shims laying near the bevel gear and went "I'm pretty sure these go here!". I figured it best to recheck my shimming - checked gear clearances first; appeared to be as I did them originally - however, I found that when I tried to spin the gears that it was now too tight and wouldn't spin with the 2 halves together. I wound up removing one small shim from the bevel and sector gear (top stacks) to get them spinning smoothly. Well, when everything was back together it performed flawless again with no noticable ROF difference or anything really. The only thing I noticed was that it was just as lound as it was before the silent head install. I'm pretty sure this increase in noise is from the gears/shimming.

So I guess my question is - was the gearbox shimmed better before when it performed quieter or is this too tight? Also, any idea why it would 'tighten up' all of a sudden...this has me baffled!