M4 S-System V2 gearbox "upgrade" = no joy

[n0bby]

Hi y'all,

As my previous visits to the site (a year ago now!) with my trusty old stock TM M4 S-System found it seriously down on power and out-ranged, I thought I'd need to embark on a gearbox upgrade before my next visit. Having decided to get seriously back into my 'softing from 1st August onwards, I naturally left it until the last minute to do the work. In the past I've done a fair bit of modding of my guns - but I've never opened up a gearbox before. However, with the aid of mechbox.com and various online resources I successfully and undramatically disassembled the 'box, replaced the feeble stock spring with a Guarder SP100, replaced the nylon bushings with a metal set, got the gears shimmed up so they meshed sweetly (it took some time...) and then reassembled the whole shebang.

I'd previously powered the AEG with a DIY customised AA cell battery pack (designed for model aircraft) in an empty mag held in a ReadyMag. I did the mod so long ago, I can't even remember what the ratings of the cells are. However, when I charged the batt and tried firing the gun, all I got was the briefest tiny hint of spin (probably only from the motor) and then... nothing. The fuse holder/battery gets hot, but NOTHING is moving. There's not a sound from motor or mechbox.

I wondered if it was the battery, so I attached a 9.6v 2200mah pack (designed for a Crane stock - and I had to bodge a set of leads to change from Deans to Tamiya small which ended up being a mini-adventure in itself..!) to try that as well - but since the initial brief, tiny sound of motion, there's no sign of life whatsoever.

So, given that I'm a complete n00b at gearbox rebuilding and subsequent failure diagnosis, I wondered if a) any of you more experienced modders could immediately suggest a most likely problem based on the symptoms, and b) if I can't get anywhere with re-stripping the gun, if anyone might be able to help out, if I bring it along with me on Sunday, and c) whether anyone has a gun for sale at the moment that I could buy as a back-up for when this sort of thing happens (as it invariably does, I've found, when one only currently has one AEG in one's collection...)

If I can't sort it, I'll be spending Sunday armed only with a TM Deagle Hard-kick and 3 mags...

[Loose Hand Luke]

If the fuse and wire gets hot its shorting out!
You've either got a lead on the wrong way round so that the motor is spinning backwards, OR, there's a screw or component inside the gun shorting the wires out.

PS- Bring the Deagle anyway, my chrome TM version is amazing

[private parts]

I by no means experienced but I'd try running battery wires Straight to motor as this would test motor and bypass trigger contacts. Maybe remove motor and try it just to check motor ok?
Process of elimination

[sniper]

If you mechbox turned freely when you screwed together to test for final free movement in all gears, before dissasembling again to fit spring.
Then you have not adjusted the motor?? since changing the gear set.
This must be done for any new gearset.
what you find is the crown and pinion on the motor can mesh diffrent it can be to lose, which can strip gears ( you will know this noise straight away if its this problem)
or it can be too tite in which case the gun fires very slowly and things heat up and damage the motor as well as wires or in the worst case nothing moves at all.
there is a grub screw or a slot in the bottom of the handle, turn this anti clockwise until you get movment.

or as luke has posted above have you got the motor wires the right way round

[n0bby]

Excellent starting points, gents - thanks. I was very diligent checking the wiring as I went, but maybe there's something shorting/swapped...

I'll try wires straight to the motor and see whether that gets me anywhere...

(And I will be bringing the Deagle Luke - I finally got round to getting a holster it actually fits in, when I ordered the spring!)

[n0bby]

If you mechbox turned freely when you screwed together to test for final free movement in all gears, before dissasembling again to fit spring.
Then you have not adjusted the motor?? since changing the gear set.
This must be done for any new gearset.
what you find is the crown and pinion on the motor can mesh diffrent it can be to lose, which can strip gears ( you will know this noise straight away if its this problem)
or it can be too tite in which case the gun fires very slowly and things heat up and damage the motor as well as wires or in the worst case nothing moves at all.
there is a grub screw or a slot in the bottom of the handle, turn this anti clockwise until you get movment.

or as luke has posted above have you got the motor wires the right way round

Thanks sniper - but as I described, there's NO movement/sound from the internals whatsoever - so I think I can discount mis-meshed gears this time (I previously had this happen when I changed the pistol grip for an ergonomic job - so I know what a wrongly meshed crown/pinion sounds like..!)

And yes, I carefully shimmed the gears and tested that everything turned freely before I did the full reassembly. So, I'm hoping this turns out to be electrical - as the mechbox rebuild seemed to go so well...

[Bio]

With V2's (especially front wired where the motor wires run inside the gearbox) its particularly easy to crush the wires in the casing when you close it, leading to a short.
Inspect all seams around where the wires run to check that you've not got any bits of wire poking out/crushed in there.

Has the fuse blown? Even if it's not, you said it got hot so may be worth replacing. If its part damaged due to the generated heat, the resistance will be high and power wont flow.
Fuse/wire getting hot is a sign of either a short or too much draw for power, often due to an over-stressed (too stiff a spring, bad shim job etc)/jammed gearbox.

Are the motor wires round the right way?
Could be that the motor has burnt out, as parts suggests try connecting directly to the battery and ensure that it runs.

Did the piston slip off its rails?
I've found this happens particularly in the FAMAS gearbox but sometimes in others, during case closure if the spring is too long. Closing with the original spring should prove this point.

Did you use a bearing spring guide? Sometimes if the spring is non-linear and its in the gearbox the wrong way, the spring expands over the bearing (seriously, this happened to me) and gets stuck and jams the gearbox. The piston then pulls back and jams over the top of the now expanded coils and wont release. Only way to resolve is to strip the box again and check. The end with the tighter coils should be at the back of the gearbox.
I would suggest being VERY careful when re-opening the gearbox just in case this has happened as the spring will be fully compressed. Spring guides and pistons like to fly everywhere when in this situation, usually resulting in gears flying everywhere as well as tappet plates and the tappet/anti-reversal latch spring.

Check your trigger contacts. Due to arcing caused every time the trigger is pulled, these blacken up and stop the power flowing from battery > Trigger > motor. Cleaning them up should work if this is the case.



Sadly the only way to know for sure is to strip it down, and piece it back together one bit at a time.

[kingpin]

when re assembled i always put a small screw driver gently down the air nozzle just to see if the piston has behaved during re assemble ,just my tepenth don't forget the chronograph on site if you get it going for Sunday

[n0bby]

Thanks for the comprehensive troubleshooting checklist, Bio - it's much appreciated, and I'll double-check against your advice tomorrow evening when I look into the problem 'box again.

Kingpin - do you mean that you push against the piston to check it can move?

I thought the electrics were a red herring, and the heat was just caused by the motor not being able to turn. All the wiring seemed fine - but the fuse had blown.

I completely stripped the gun tonight, and opened the mechbox. Before doing so, I discovered that the air nozzle was stuck in position slightly back from its default position. When I opened the mechbox, the tappet plate appeared to be out of position, pushed back and bent upwards. It looked like the piston may have come off its rails.

I re-seated everything and got it all in place. Then, I double-checked the alignment of everything was as it should be as I held the compressed spring/piston in place... and then I did 4 times in a row what I managed not to do at all last night... fire the spring and spring guide across the room while trying to close the case. I finally got it together, and reassembled the gun. Then I got to the part where I had to screw the stock tube into the rear of the mechbox... Ah, this is interesting, I have nothing for the bolt to screw into. Damn. Fortunately I hunted around, and found the nut on the window sill where the spring guide had hit the window on its earlier travels! So, I had to disassemble the gun again, open the mechbox, fit the nut in the spring guide, and put it all back together again!

After all that, it was time to test the gun. And exactly the same thing happened as last night. There's a brief whirr from the motor, and then everything locks up and I get no hint of anything moving or even trying to move.

[kingpin]

Yes mate to check the piston is smoooth ahh the joys of the mech box lol

[Bio]

Well that's a royal PITA. This is why I love the A&K M249 gearbox, so easy to work on as you can release the spring in advance. I wish more companies had followed the lead of Star/Ares for this feature in regular gearboxes, and I wish Star/Ares had a little more support from third parties for after market parts to make them a viable purchase.

Anyway, At this point I'd suggest seeing if re-fitting the original spring allows the gearbox to function correctly. That will prove that the original is soft enough that 1) the motor is powerful enough to pull it and 2) that the new spring may be slightly too long causing a piston de-railment during closure.

If it doesn't work when closed with the old stuff either the piston is de-railing anyway and you may need a second person to assist in the closure of the box, sometimes that extra hand pressing the cylinder/piston into place really helps.
Or if its not the piston (the piston could well be another red herring), then it may be the motor itself is damaged or running backwards.

Only things I can really think of. I'm sure someone with more experience will be able to help more than I can. Only done 4-5 gearboxes so would never proclaim to know it all, just ever enough to get me by!

[n0bby]

Kingpin, yes - it's turned out to be quite a trial by fire...

Bio, thanks again for the continuing advice. I quite agree that the best thing I can do next is re-fit the old spring, to at least confirm that it's the larger spring that's at the root of my problems. It does seem to be a heck of a lot of longer and stiffer (oo-er missus) than the stock spring... but I guess that's the idea!

But it does beg the question - should I have gone for an M90 spring instead? Kingpin, could I ask - what sort of internals do you routinely rebuild the site's guns with? I'm assuming wear and tear demands that they get internally overhauled from time to time?

The one good thing to come out of this is that I am at least finding out how the most internal internals of my guns work... As I said, I've never cracked open a mechbox before - my previous gun modding being limited to "cosmetic" alterations (back in the days when such jobs involved actually modifying and scratch-building bits rather than buying and bolting!) - so I at least feel I'm learning something from this somewhat frustrating experience...

[SILVER FOX]

The last time the site guns were looked at was over a year ago and it was by neil and me and they were all and still are running stock springs the only things that needed changing were some bushes/hop rubbers and some busted body parts.

[kingpin]

We have redone most of the original guns now even since the fox worked on them I have I think sp 100 there all 1joule got them from zeroone I have some here at least with these you know u will be site safe

[sniper]

Just be carefull what brand spring you get as they all might say M100 but the FPS will be diffrent, see spring rating guide i had posted on err gunomics

[n0bby]

Just be carefull what brand spring you get as they all might say M100 but the FPS will be diffrent, see spring rating guide i had posted on err gunomics

And a useful starting point your Spring Guide post was too Sniper - cheers!

As I mentioned at the start, I'm fitting (sorry, trying to fit!) an SP100 by Guarder. I customised the M4 so it's got a shorty barrel (the flash hider fits pretty much at the end of the RIS foregrip) and the inner barrel is stock TM diameter, so I was expecting to be getting a final FPS slightly lower than the average rating for the spring.

EDIT: Oh, and thanks for the internals info Mark - IF i can get this spring fitted and working, hopefully it'll result in a gun with similar performance to the hire guns'...

[n0bby]

So, less than 24 hours to skirmish time, and it's the first chance I've had to look into the mechbox again... I've just separated the reciever halves, and the first thing that greets me is the sight of the piston stuck right back - the spring is completely compressed in there! Getting the halves separated and controlling the spring is going to be fun!

So, do I have a timing issue with the gears? I know they were all set as per the instructions - i.e. with the sector gear marker at 1 O'clock. Wish me luck... I'm going in...

[Loose Hand Luke]

Don't worry about timing mate. The piston and tappet plate are both run off the same gear, the others just spin round and round

[Bio]

Sounds like your spring is expanding too much when compressed, possibly caused by having it the wrong way round (tighter coils should be at the back) + bearing spring guide at the same time.
Spring expands and pushes over thrust bearing, piston comes back and jams over spring.

Once you get it apart, double check just to be sure. It's only happened to me once in the past but its not beyond possibility!

[n0bby]

Cheers again for the advice guys.

I got the gearbox apart fine, using the 'screwdriver in the back of the spring guide' technique to control it. I've been fitting the spring the right way round, and once opened up, I really couldn't see what had caused the problem.

I then grabbed the other spring on my desk, and spent the most frustrating 20 minutes of gearbox halves reuniting that I've had so far - it seemed even more difficult compressing and installing the old spring than installing the upgrade one!

I rebuilt the gun again, tested it... same damn result.

I then realised that the spring I'd fitted was the M120 that I bought first as a replacement, until I decided it would probably make the gun too hot. I discovered the original stock TM spring lying in the waste paper basket next to my desk. So, I guess I'll be taking it apart again, to check if the actual original spring works okay...