Figure 8's vs AMC torque bias for NP249 - JeepForum.com
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Unread 11-10-2013, 07:22 PM   #1
Bullwinkle_98ZJ
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Figure 8's vs AMC torque bias for NP249

It seems according to the posts here that the way most of us are checking the condition of the Viscous Coupler in the 249 is to figure 8 on pavement and check for binding.

What I have noticed is that some still have issues with the figure 8 with a good coupler.

I owned a couple AMC Eagles several years ago with the NP129. This was a full time 4WD transfer case and used a VC not unsimilar to the 249 if I recall.
The VC diagnostic on the 129 was a torque bias check to ensure the amount of slippage. It involved a torque wrench and an elevated tire. Turn the tire with the wrench and if a specific torque was reached, the VC was good.

Is there a similar diagnostic for the 249 using a torque wrench?

PS: I like the 249 and do not intend on swapping it out.

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Unread 11-11-2013, 12:03 AM   #2
dnuccio
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huh. sounds interesting
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Unread 11-11-2013, 01:48 AM   #3
Timo_90xj
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Quote:
Originally Posted by Bullwinkle_98ZJ View Post
The VC diagnostic on the 129 was a torque bias check to ensure the amount of slippage. It involved a torque wrench and an elevated tire. Turn the tire with the wrench and if a specific torque was reached, the VC was good.

Is there a similar diagnostic for the 249 using a torque wrench?
With the 249, when the VC goes bad it usually makes the toque bias closer to constant 50/50 and it will also lock up more than it's supposed to, making it feel a lot like a part-time 4wd. This is especially noticable on the newer '96-'98 cases which usually have around 10/90 torque bias when there is no wheelslip. In other words, if you went out and tested the VC with the said method of raising one front tire in the air, the more stuck (or broken) the VC is the more torque you'd read on the indicator of the torque wrench. On the older 249s, it's always around 50/50 torque bias, so again it would not really work.

I don't really know the reason why the VC clutch (the silicone- based fluid) on the 249s acts like this, because usually failed viscous clutches work just the opposite - they "flex" too much, thus not doing the job trey're supposed to. Think of the older VW Syncro sytem, when the VC fails they really aren't 4wds anymore. Same goes for viscous fan cluthces, they don't cool anymore. In these the torque test works.


Viscous couplings are very sensitive on the amount and type of fluid used inside the clutch unit. People do DIY service on the VW Syncro VC units, and if the amount of new fluid is off by just a few %, the system will not function properly. I'm guessing that's the deal with the remanufactured 249 VC clutches as well, the cheaper ones especially don't seem to be exactly perfect units. I've heard even the OEM factory produced ones when new did not all perform equally.
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Unread 11-11-2013, 07:06 AM   #4
ejschultz
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It should work fine if you want to use this method to test it. For a 93-95, test one front wheel, then test one rear wheel. They should take roughly the same amount of torque to spin. I'd imagine the differentials could possibly alter the number slightly, but I don't think it's anything to worry about. I'd guess if the VC is bad, you probably won't be able to turn it at all because the other end of the vehicle is still on the ground. For a 96-98 249, you'd have to do a little math. The front should take approximately 11% of the torque the rear takes to make it turn. Or, to make it easier, the rear should take 9 times the torque of the front to make it turn. If the rear of the vehicle has an LSD, you'll probably have to lift the entire rear to spin it. Again, if the VC is bad, I doubt you'd be able to turn either the front or the rear because the VC would be locked up.
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