czettlera

Hi guys my 96 4.0 awd ZJ has an interesting problem. After riding aprox 50km with higher speed, in sharp, slow turns it shakes and I can hear the front tires slipping, just as the front and the rear axle would be closed together. My transfer case has a visco coupling, I would think that is bad, but after a few hours waiting everything works fine. Anybody has any good ideas, or experience?

dave1123

If your front and rear tires aren't matched in wear rate, the viscous clutch may be overheating during a high speed run. After cooling down it functions properly. That's just my opinion, however. With AWD, you have to replace all 4 at the same time and they have to be the same size.

czettlera

They are the same type, size, all four new, and I really pay attention to the pressure. I also think that the problem is somehow connected to the heat, just I don't know where it comes from. Or maybe after 180k miles, the life of the visco is over?

ZJMAX

From what I remember those take a special fluid and if the wrong fluid is used it can have issues once it's warmed up.

Max

dnuccio

if the viscous coupler is original with 180k on it, be happy its lasted this long, and replace it.

czettlera

Yes it's original, and I'm going to replace it. Does anybody has good or bad experience with any brand?

DSMchad

Viscous is shot, your better off to replace the T case and swap it to one thats selectable (NP242 or NP231)

If you get a used NP249 your gonna run into the same problem again quickly, and your eyes will pop out of your head when you see the price of a new VC.

If your going the swap route do your research, there are 3 different input shaft lengths, and 2 different gear cuts on the input/low range planetary which changed in mid 1994.
in my case i have a 93 ZJ with a 95 XJ 4.0 transfer case, and used the front case half, input shaft and 4 low planetary from the original NP249 that had a locked up viscous coupler.

dave1123

^^ what he said^^

When I went looking for my WJ, I looked specifically for one WITHOUT the 249 because of their reputation. I bought one with the 242 and never looked back. I also bought the 4.0 for the same reason.

czettlera

Maybe you are right, the only reason i bought the jeep with 249, is that my wife is also driving it a lot, and in snow if she forgets to switch on the awd, it can be a bad surprise. (we live on a hill and in winter it can be tricky). As i checked the price of the viscous, maybe swap will be the right way, but who knows, maybe after the change it will be good for an other 180k

DSMchad

As long as you dont mind the high cost of replacing the viscous coupler, theres nothing wrong with doing that either. No matter what you do, unless you run across a different T case that has the same input length, you almost always end up having to tear them apart anyway to swap input shafts from your original. But as i said if you really like the AWD that much and dont mind paying for the expensive VC im sure buying a new one and replacing just that will last a good long time again, of course always making sure to replace all 4 tires at the same time and matching brand. Tire rollout is very critical for AWD, especially at highway speeds.

czettlera

I made an interesting experiment today. I have set all 4 tires to factory pressure, than measured the diameter. Because the weight of the car, the front tires were flatter, the difference was 5mm in diameter. So I let out some pressure from the rear wheels (aprox. 0,4bar), till the diameter was the same. After that I had a 300km trip, mostly on highway, and the visco hasn't been blocked. Maybe I can delay the change a little.

DSMchad

well, no matter how flattened the tire is from weight, they still have the same rollout distance, but some tires do expand more than other with pressure.
the true way to measure a tires rollout is to remove them, put a spot of grease in the center of the tread and roll them on the ground one full turn, then measure the distance between the 2 grease marks left behind on the ground, do that to all 4 tires and they should all be within 1/4 to 1/2 inch of each other.
thats a lot of messing around and if yours are the same brand, size, and two arent worn drastically more than the others i wouldnt worry too much about that

if you are to the point of the VC locking up after a long drive its just gonna need to be replaced eventually. mine was doing the same and two of my tires had a slightly farther rollout distance than the other two, so i put those on opposite corners and it took a lot longer before it would tighten up.

czettlera

I think you are wrong on this one. As a very extreme example, check the picture. The rotation speed of the axle can be calculated back from the radius of the wheel, what is in this situation, the distance between the axes of the wheel and the ground. That is the same situation like you have a puncture on one of the front wheels, and the car starts turning. Anyway I think the picture is really cool, good solution for snow

dave1123

Yes they are cool, but have you checked the price of them? You could easily buy another vehicle for what they cost. And they get unstable at high speeds.

The discussion about different wheel diameters is interesting and brings back an experience I had. I had a '67 Camaro SS with a 4.10 Positraction rear axle. I got a flat and mounted the compact spare tire on it and drove home. I toasted the clutches in the rear axle and had to replace all of them. What I should have done was put the spare on the front and put the front tire on the rear for the drive home. That was an expensive learning process. It's hard to think clearly with cars whizzing by you at 70 mph!

DSMchad

a tire has the same travel distance in one rotation weather it is smashed with 5,000 Lbs on it or rolling with just its own weight.
Back to the topic, your VC is going bad like all of them do, just not completely locked up yet.