Vibration in drive train.

[tosten]

I have a 97 awd Astro with a high frequency vibration. It sounds like it is in the front end and begins at about 60mph. It has done this since I got it and have put on 16 thousand miles with no change. It seems to be better sometimes on wet and snowy roads. I can run it up to 70mph, put it in neutral, and coast down to 60 and it vibrates the same as if it was in gear. I have rebuilt the front diff., replaced the half shafts, changed the front wheel bearings, tried alternative wheels and tires, tried a new front drive shaft, all with no improvement. If I drive it without the front drive shaft it is noticeably better but I can still feel it somewhat.
I'm stuck. I can't believe this hasn't happened and been solved in other Astros and Safaris. Anyone have any ideas?

[LiftedAWDAstro]

Welcome. I don't recall anyone having a vibration like you describe especially after doing so much work to their van. Let's see...

Are the tires all the same brand, model and size? This is extremely important on the 1990 to 1998 AWD's.

Do you get a pulsation in the steering wheel when braking? Thinking warped rotors throwing a high speed vibration.

How are the rear drive shaft u-joints?

How is the bushing and seal in the tail shaft of the transfer case?

How are the rear wheel bearings?

[dunedog]

Alignment and spin balancing have been done?

Are you running passenger tires or a heavier truck tire? Truck type tire can be a little finicky sometimes, I've found.

[tosten]

Thanks for the quick ideas. The tires are the same and I've tried a set off of an older 2wd that I have. The tires from the 4wd don't shake on the other van either so I assume they don't have balance issues. What throws me off a bit is that it is much better with the front shaft off, and on the hoist it is completely smooth. On the road I can feel a faint vibration. Could the awd link between the front and rear amplify a wheel bearing problem? This thing has me stumped. Is there a chance that worn idler arms could shake like a bad ujoint? I've been told that it isn't likely and it seems that I'd see this noted somewhere if it was a symptom. I've been under it with a stethoscope and can't isolate the noise to any single component but can hear it resonating best in the floor and frame metal.

[LiftedAWDAstro]

Since it is smooth on a hoist, I would be looking towards either tires or more likely a wheel bearing issue.

Another possibility is the rear diff. Have you changed the rear diff oil and inspected the gears and bearings? When the van's weight is on the wheels, the rear axle shafts will transmit some weight to the carrier bearings in the rear diff. If you have something going on inside there you will feel and hear it under load but not unloaded.

[tosten]

I could be wrong but I ruled out a rear diff. problem because the noise changes so much when I remove the front drive shaft. This makes it nearly quiet on the road and the hoist. With the front shaft in on the hoist, it makes the vibration. I do think there might be a bit of noise coming from the rear end and I did change the grease, though. For the last years before me the van was used mainly to slow commute about 4 miles to town. I don't know if that is a significant factor. The front diff was low on grease before I replaced the bearings. I was told by a GM tech that they did have some issues with front drive shaft balance on these vans, though I've never seen another van with this kind of problem myself. I am curious why the sound hasn't changed in so many miles if it is caused by a failing part. I have found myself wishing to break down as a way of discovering the cause of this. I've got an old 92 2wd that runs down the road at 75 as quiet as can be. If need be would the front end parts fit from that as a last resort to eliminate the awd and get a decent driver? The 97 is a very clean van and I hate to give it away because of this phantom vibration.

[LiftedAWDAstro]

Where are you located? Maybe there is a member with another AWD that can listen and feel for the noise and vibration.

Since you rebuilt the front end, I would suspect the transfer case may be worn to the point it is locked up. Does the van turn real tight circles on dry pavement without binding and hopping?

[tosten]

I am near Missoula, Montana. I have wondered about the transfer case too. How do I tell if it is the case? I can turn the front drive shaft with the rear wheels on the ground with a great amount of effort. The transfer case being locked does make sense. It seem to act a lot like a 4x4 with the transfer case shifted into 4wd going down the highway. Thanks for the ideas and help! Are locked up transfer cases common with this type?

[tosten]

I'll try the pavement test the next time I get to town. I don't remember a problem with that but haven't done more than an occasional uturn.

[LiftedAWDAstro]

These Borg-Warner 4472's are very reliable. There have only been a few that have failed. You could see if you can find one locally or on Ebay. They are reasonably cheap and even if it isn't the issue, you can re-sell and break even.