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  1. #1
    RC poster
    Join Date
    Jul 2019
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    3

    Do I have a center diff problem?

    Hi All,

    I'm hoping for some insight, and hopefully to avoid tearing things apart for no reason.

    At the end of my latest bashing session with my E-Revo 2.0 I got the impression the vehicle was losing traction / labouring. It wasn't a heavy bash session, some small jumps on a bmx track. It didn't have any major crashes, just a couple of rollovers. As the batteries were probably just about done I packed up and came home expecting that I'd find something broken. But nothing appears broken, I took off the cover and checked the spur and pinion and the mesh and all appears good.

    So my main question is, how do I tell if the center diff is working correctly? I have not checked before but I thought the center diff basically locked the 4 wheels except in extreme circumstances. If I spin one tyre by hand the opposite tyre rotates the opposite direction with no grinding etc so seems normal. If I hold any tyre and apply power the other three wheels spins and I can easily hold the tyre still with no real force. Should this happen? Does the center diff require attention or am I just being paranoid?

    I'm hoping somebody knows or has tried this and can tell me what is normal.

    Thanks in advance and Cheers!

  2. #2
    RC Champion olds97_lss's Avatar
    Join Date
    Dec 2003
    Location
    Dekalb, IL
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    2,381
    The center diff runs 20M silicone "oil" in it, it's more like a putty. I have found that if I run the truck pretty hard for a pack or two that it does seem to loosen up quite a bit, but I notice my front tires spinning more, not really any extra "labor". I still get wheelies at that point if I have grip, but the diff does allow the front tires to spin more easily until it sits for an hour and cools down.

    The first time I noticed it, or thought I was having issues, was when I put on a set of MT4 tires. I think it was actually spinning all 4 tires and the tires weren't gripping.

    Regardless, I tore the trans apart, and the center diff seemed fine. I cleaned it and refilled it with fresh 20M stuff and it was very difficult to turn the output shafts while holding the diff in my hand. I'm not 100% sure why they designed it this way and not like a normal center diff that you could actually tune, but I think the gears/space in the diff is too large to run much less than the 20M... I could be wrong. It's a pain to get at it or I'd try some 1M or 500K in it to see if it acted more like a typical center diff.

    Holding one tire and seeing the other 3 spin isn't showing you much. To actuate the center diff you have to hold both the front tires or rear tires to see if it's doing it's job of transferring a lot of force to the stopped tires while allowing the other end to spin, or if it's just allowing the other end to spin very easily.

    Might be worth pulling all 4 tires and hubs and checking to see if you have string or grass wrapped up around behind the hub causing binding. Also inspect the bearings to see if one has seized or something.
    Last edited by olds97_lss; 10-15-2019 at 11:24 AM.
    Expert rigging at it's best!

  3. #3
    RC Racer
    Join Date
    Jan 2015
    Location
    Salado Tx
    Posts
    165
    If there is know obvious grinding or noise from the front or rear diffs check the cush drive in the center diff. Recently my front tires barely engaged and had no drive to them. Turns out it was part #6465 of the transmission assembly. Cheap and easy fix.

  4. #4
    RC poster
    Join Date
    Jul 2019
    Posts
    3
    Thanks for the replies.

    I decided to just rip it apart and have a look. The front and rear diff's themselves were fine and all of the gears still looked like new, but a couple of the bearings were a bit iffy and there was basically no grease. So I've cleaned them up, refilled the diffs with 100k fluid which will hopefully tighten it up a bit.

    In the center diff, the torque biasing unit still feels solid and smooth so I probably won't open it (although the temptation is there!). In the transmission / transfer case again the gears look like new but no grease. It looks like there was basically no grease from the factory. So again I'll clean it up, replace the bearings and grease was should be greased.

    The rear center drive shaft was pretty badly torn up for some reason, like something had been wearing on it all the way along. One of my local spots is a bmx track covered in fairly fine gravel so maybe some of that found its way in there at some stage. Also a couple of the universal yoke pins were loose. Maybe it was the drive shafts vibrating causing the symptoms I observed. So I'm upgrading the front / rear center drive shafts with GPM units. I've had success with their axle drive shafts.

    The cush drive element was indeed moderately torn up, so it's being replaced.

    So basically I didn't find anything obvious I'm just hoping a series of little fixes and some general TLC will have the car back to normal.
    Last edited by morgo1968; 10-16-2019 at 05:43 PM.

  5. #5
    RC Champion olds97_lss's Avatar
    Join Date
    Dec 2003
    Location
    Dekalb, IL
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    2,381
    That's kind of what I found with mine. Since I bought it used, it was worth tearing into to see what was in there, also since it was my first 2.0, good to know what's going on in the truck. I found mine had a traxxas rear steel cvd for the center, so that was nice. I pulled the boots and cleaned/regreased those as well as replaced all the diff bearings and cleaned/greased the ring/pinions before putting it all back together.
    Expert rigging at it's best!

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