how do you know when your diffs are bad?
 

this is for a 3905 emaxx. does it make some sorta wierd noise while driving? or will i just not be able to run it at all?

Diffs
 

I can only guess that if I suspected some kind of problems with one of the diffs, first off I would disconect the center dogbone and check for any resistance in the diff movement. If that is good, then you may have a problem with somthing else. If it does not feel right, then remove the diff open it up and inspect it.

hmm..sounds like a lil too much work to check. is there another way to find out signs of a diff becoming bad?

Depends on whats going wrong - typical prob is ring and pinion are allowed to pull out of mesh enough that the pinion can skip over teeth on the ring. Eventually this will strip a tooth , a few or several teeth off the ring or I have seen teeth break off on the pinion. As long as its only a few and they get pushed out of the way, it'll drive and may/may not make noise where it goes over that bad spot. When it gets to be several teeth, pinion just spins and you get no power to that end.

Spyder gears are diff -when they go, tends to lock up everything. In a typical trx diff (the 16.8 may be better, idk), spyders are the weak link. If its not shimmed, and still in plastic cases, its usually ring gear. When you put diff in alum case with alum diff cup, shim right, then diff probs are usually spyders but usually only after a pretty good service life.

I just put a set of 16.8 diffs under my old Emaxx.

you could open the diff and have a look. But if removing a dogbone is too much work I can't imagine you'd want to do that. Personally I think most of the fun of the hobby is in playing mechanic with the cars. If you don't want to do that maybe your hobby shop can be bothered sussing it out for you.