Vibration after rebuilds

[asymptonic]

Just got the Jeep back after a full AX-15 rebuild and a rear axle replacement (rebuilt 8.25 chrysler 3.07). So all of that stuff is "new". That said I'm having two problems, which probably aren't related. Mostly I'm asking about what might be causing a slight but noticeable vibration above 35mph most noticeable in 3rd gear when accelerating gently. Goes away if I shift into neutral and is better if I let off the gas.

Second problem is 4WD won't engage at all. Feels like maybe just mis-adjusted shifter. I may try the trunion adjustment myself but may just make the shop check it. The vibration worries me more with a big road trip coming up.

 

[RCP Phx]

First problem I think would engine/trans alingment.

 

[asymptonic]

Gonna have the shop take a look at it, but if they strike out, apart from it being annoying should I worry about driving it 900 miles at highway speeds?

 

[RCP Phx]

That would depend on you to determine how bad the vibes are? You didn't say what year but a lot of the tranny mounts have a small offset that's easily reversed.

 

[asymptonic]

Pretty minor vibes, but noticeable. (Edit: if the road is just a bit bumpy you can convince yourself it's that but it isn't). Jeep is a 1998, with a tranny mount that's only about 5 years old. It's a stock style mount.

 

[RCP Phx]

The '98 does have a 1/2" offset!

 

[asymptonic]

1/2" is a lot, I'd expect a ton of vibration if it were that. We'll see what they find, but I think I'd drive it. But now that I think about it I could totally see them botching that.

 

[lawsoncl]

The xfer case shifter may be assembled incorrectly or popped out. A lot of people replace the wonky oem setup with the Azzy or Boostwerks aftermarket version.

 

[asymptonic]

Just to keep this thread updated for searching, 4wd shifting was simple misadjustment of the shift lever trunnion. The FSM says to "lift the vehicle" after loosening the adjustment bolt, and we figured out why today. With the wheels planted it was impossible to get into 4L. We wound up putting the case positively in 4H, then putting the shifter in 4H and tightening there. Then drove it around the block and everything shifts perfectly. I'm guessing if it were lifted 4L would have been easy.

 

[asymptonic]

Well this is damned frustrating. My shop sent the driveshaft off to be balanced, thinking that would fix it, it didn't, but the vibes were mild at 35-45mph. Tire shop that put on the matching tire balanced all four. But after a day of driving and getting it really warmed up at highway speeds, the vibrations occur at most speeds, but peak at 55-60, then at 70, and are tolerable at 75-80. Until it gets really hot, then it's worrying under 60mph, not great at 65, and tolerable but still noticeable above 70.

We didn't feel it safe to drive and put it into a transmission shop half way, who put it on the lift and immediately said it was the transfer case. We let them keep the Jeep and got a rental to keep going. But they pulled the transfer case, said it was fine, and said they thought they noticed the drive shaft was bent. I shipped them a new one and they put it on and said it was solved. Nope! On the return trip today (from the halfway point), all the same symptoms which get worrying after several hours of driving. We're pretty sure it improves again after a full cooldown.

So, to keep score, not the driveshaft, not the transfer case, probably not the differential, not the tires. That leaves the newly rebuilt transmission and/or how it's mounted imho. Any other thoughts?

 

[RCP Phx]

Bad or in-correct trans mount, bad driveshaft angles? Was the driveshaft high speed balanced? Did you also do the front driveshaft? Do you have a SYE? Axle yaw?

 

[asymptonic]

Leaning towards the first one. Several folks have looked at the driveshaft so I would figure the angles would have stood out, I did ask the trans shop about it. Don't know if the drive shaft was high speed balanced but wouldn't expect the same behavior from the old one, balanced, and the new one that came pre-balanced. Didn't do the front drive shaft but it wasn't involved in any new work. No SYE. Drivetrain is stock, no lift. Axle yaw is a new one for me.

Edit: what of any of these options would be milder until things got really hot. I wouldn't think axle angles or front drive shaft (really any drive shaft issue) would be highway-speed heatup dependent.

 

[RCP Phx]

Could very well be a pinion bearing.

 

[asymptonic]

On a newly built diff?

 

[RCP Phx]

Seen it before !!!!

 

[RCP Phx]

Pull the front driveshaft, does it still vibe? Drive it and then put it in neutral at speed, do you still hear/feel anything? Pull the rear shaft and lube the splines, vibes?

 

[asymptonic]

Gonna be a while before I can try any of that. But I will add some details. Vibes get better coming off load at times, and when hot sometimes adding load. Would expect front drive shaft issues to be felt more through the steering wheel but really only the low speed vibes do that, at 35-45. Shifting to neutral (in the transmission) does generally help at low speeds but not when hot at high speeds. Whole lot of confusion in the symptoms.

 

[asymptonic]

The problem, in the end, was a bad end yoke. I wound up with a Jasper rebuilt diff that had an incorrectly speced yoke. My shop finally noticed that the shaft was wobbling under load, and that the yoke didn't seem right. They ordered a series of them that all were wrong. Turns out the tab in the yoke had been ground down on the Jasper one sent, and of all the ones ordered, none of them were correctly sized to spec (no details on how). The shop in the end welded up the bad tab and ground it down to the right size and so far it feels fine, but I still want to get it out on a 50+ mile drive at speed to heat it up and see if it holds.

I'm now 0/2 on being happy with anything coming out of Jasper.