Viscous Couplet; possible issue; NP 249J (97' Grn Cher LTD - JeepForum.com
 
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post #1 of 10 Old 05-14-2018, 11:02 AM Thread Starter
prosman
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Viscous Couplet; possible issue; NP 249J (97' Grn Cher LTD

I am trying to sort out some questionable issues that are probably related to the Viscous Coupler in the NP 249J transfer case in my 97' Grn Cher LTD; which I am newly freshening up. The rear end plate is numbered 52098980; 3 73 605962 3. The rear end has a hexagonal cover which I think means it is a Dana 44. It has the 5.2l v-8. I have read elsewhere that these 97' models that came with a 'towing package'; ie hitch and wiring for a trailer hook up; most likely had a Limited Slip rear differential. I have replaced all the fluids after flushing the front and rear differentials and filled them with Mobil 1 Synthetic. The fluid in the transfer case has also been replaced with synthetic fluid. Here are my questions: When starting up and beginning to drive there seems to be no issues with anything (noise, binding etc). After running for awhile and making anything tighter than about a 1/2 lock left or right turn; in forward or reverse, and at low speeds as in a parking lot; there seems to be a definite 'binding' which can be felt in the steering wheel as well as a feeling that things are straining and 'locked up' way tighter than what should be 'normal'. The tires, (235 70R 16's which are slightly bigger than origin 225's) want to 'chirp' a bit, and everything feels under way too much stress for 'normal'. I also had a 96' Gran Cher of which you could feel a little (very little) 'binding' when making very tight turns at low speeds as in parking lots etc. Any suggestions for sorting this out before going to the effort of re-doing the transfer case with a new Viscous Coupler? Are there any special fluids for either the differentials or the transfer case that would 'loosen up' this sense of 'binding which seems to come on when the vehicle is warmed up and driven a bit, and is a little mysterious and on the intermittent side of the coin? Does increased trans/transfer case temps cause the Viscous Coupler to engage more? Thanks for any thoughts or direction. KTM

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post #2 of 10 Old 05-14-2018, 12:20 PM
wingless
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Welcome to the forum.

Yes, it sounds like the viscous coupler has failed. If remaining w/ the NP249, then a rebuilt exchange is the best option.

Verify the transfer fluid level is correct. Verify the tire pressures are correct. On mine, I suspend the vehicles w/ the tires hanging in the air to measure the outside circumference to ensure they are all identical, like shown in this reply.

Try a short term test if the problem abates w/ the front propeller shaft removed, to identify a failed viscous coupler.

This reply shows my viscous coupler replacement.

The VC function is affected by temperature.

The rear limited slip differential will get maximum lubricant flow by performing 10 to 12 low speed figure eight turns, per the FSM.


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post #3 of 10 Old 05-14-2018, 12:27 PM
kg6mov
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That’s classic early VC failure. The VC operates on heat, tire slip causes friction in the VC to heat the fluid and lock the VC. As the fluid degrades it becomes more viscous at lower and lower temps, so the VC doesn’t act locked when the jeep is cold, but will after things are warmed up.

The fix is a new VC.


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post #4 of 10 Old 05-14-2018, 12:50 PM
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Quote:
Originally Posted by kg6mov View Post
Thatís classic early VC failure. The VC operates on heat, tire slip causes friction in the VC to heat the fluid and lock the VC. As the fluid degrades it becomes more viscous at lower and lower temps, so the VC doesnít act locked when the jeep is cold, but will after things are warmed up.

The fix is a new VC.
kg, you mentioned previously you got 200k out of your VC with proper maintenance.

What was your maintenance interval/procedure for that?

Mine had TC fluid change at 30K and 65K. At 65K the old fluid looked like the color of cola.

I'm at nearly 80K now and wondering if I should change it again.

It's still functioning well by the way, no issues.

Thanks!

Mike
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post #5 of 10 Old 05-14-2018, 01:19 PM
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kg, you mentioned previously you got 200k out of your VC with proper maintenance.

What was your maintenance interval/procedure for that?
Tire rotation, making sure tire PSIs are correct, measure & adjust for as minimal true rolling radius/ circumference difference as possible. That's about all you can do - viscous coupler is a sealed unit. Having the TC fluid in good condition and at correct fill level will aid keeping transfer case as cool as possible.

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post #6 of 10 Old 05-14-2018, 02:54 PM
mkm5
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Thanks Timo!

It would be nice if the fluid capacity for the TC was more than 2.5 pints, it doesn't seem like enough to do the job properly for a 30K maintenance interval. I'm thinking 15K should be the max.

The FSM says ATF+ is the recommended oil; I expect it's okay to use ATF+4 now?
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post #7 of 10 Old 05-14-2018, 03:05 PM
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The VC is sealed, so you could be running soy sauce as fluid for all it cares, cooling being the important part.

On a second gen proper maintenance is pretty simple, proper tire rotation, only buy full sets of tires, don’t run a mismatched spare. It’s also good to take full advantage of the lock in 4lo, I run in 4lo if I’m offroad even when the VC would be capable of keeping up.


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post #8 of 10 Old 05-14-2018, 03:15 PM
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Quote:
Originally Posted by kg6mov View Post
The VC is sealed, so you could be running soy sauce as fluid for all it cares, cooling being the important part.

On a second gen proper maintenance is pretty simple, proper tire rotation, only buy full sets of tires, donít run a mismatched spare. Itís also good to take full advantage of the lock in 4lo, I run in 4lo if Iím offroad even when the VC would be capable of keeping up.
I like the advice of going to lock in 4 Low off-road, this makes good sense. I've wondered whether this would be better or worse for the transmission. I still need to add an aux cooler to the 5.9.

My cooling system never gets above 195 no matter what I do though!
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post #9 of 10 Old 05-14-2018, 07:03 PM
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If you’re keeping it out of OD it will be better for the transmission. It will save a lot of load on the torque converter, which keeps temps down.


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post #10 of 10 Old 05-15-2018, 07:41 AM
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ATF+4 is the correct fluid

93 red ZJ 196k 4.0. Spent more money than I should have ...
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