Pretty good video on transmission gears and forks. Leaves out taking the cases apart,but that's the easy part. Just remember where everything goes when you take one apart. Hard part is getting it out of the frame.Remove the clutches,backing plate,remove the tranny from the drivers side. Plus doesn't mention the chain. Always replace it as it's the cheapest of the parts involved.Plus replace any circlip on the gear shaft,plus use Threebond or Yamabond on the cases. The parts link I showed usually has the cheapest prices. Dirt Cheap Yamaha & Polaris OEM Parts & Accessories – Cycle Parts Warehouse

 

Thanks For the info. I have tried every thing but when you put it in FWD you can feel the detent click in then there is just a hint of grinding if you push on the shifter it does make a louder grinding sound.

 

All it can take too is to have a circlip on the transmission shaft to be worn or even knocked out of it's groove to cause engagement problems. I've found several like this,BUT this kinda sounds like the main problem a lot of them have, the silent chain is probably so worn that it's jumping teeth on the sprockets. I've had them go in to gear ok,but had some grind to where they wouldn't even move. Split the case and see if the chain has slack,should be pretty tight. If it's loose you may only have to replace the chain and two seals if the dog gear,forward gear and shift fork look okay.Plus one other thing a lot of people over look is the small outer sprocket. It's softer than the shaft and the splines in it can strip and cause grinding. Hope it's simple as this so you wont have to split the gear case. Stretched chains,stripped outer sprockets were common on Scramblers, Trail Blazers and the Trail Boss with high/reverse only.

 

OK Here is what I got when I tore it down. The chain has some play but is not jumping that I can see. But there is a grove on the fork and the teeth that have to mess to lock up look warn on the FWD side. I hope the photos show up. https://drive.google.com/file/d/0B7p...ew?usp=sharing
Also there was an orange plastic ring in the bottom of the transmission is is like a washer and has a cut in it like it has came off a shaft or was put on tring to fix somthing.

 

Just like the guy in the video I'd replace the engagement dog gear,forward gear,fork and always replace the chain no matter if you think it's ok.Circlips and two seals also. Better than having to split the gear case again. As far as an orange ring in the bottom,only thing I can think of off hand is someone may have had a piece of a small funnel break off while filling the gear case,or like you say someone may have been in the gear case before. Use Three bond or Yamabond type sealant on the cases.

 

Thats what I was looking at doing man those parts are not cheap but that is the name of this game sometimes.

 

Considering a new gear case is pushing close to a 1000 bucks,a couple hundred on parts doesn't sound too bad..
At least you're still able to get parts on this 15 year old atv. A lot of parts are starting to show up nla on some machines..