2000 Astro AWD rear end slippage

[gspd750]

Hello everyone, 1st post here. I purchased a 2000 AWD Astro last fall with 100,000 kms. All was fine for the first few months when a whirring noise developed from the rear end. Yup, rear end failure. Bearing kit, crown and pinion, axle bearings were all replaced by the shop.

Only 3 months later and 3000 kms I have noticed what appears to be a slippage sound coming from the rear end when making tight left & right turns on dry pavement.
Yes, I will be taking it to the shop for warrenty work on Monday but I just wanted to put some feelers out here as to what may be the problem. The back of the vehicle will vibrate and shutter during these turns and this can't be normal. Possibly not shimmed correctly when the last shop installed the crown and pinion?
Just a note that when I bought this Astro I removed the OEM P215-75R15 (suggested tire size as on the door panel) and installed relatively new P235-75R15 that were on my old Safari. Tires are all evenly worn and good shape.
Could the larger tires create any havoc on this rear end? I assume not and should only affect the speedo.
Ratio of new rear end the Shop installed is GM7.5-342
Thanks everyone.

[Blue Bowtie]

Welcome aboard!

Hopefully, those tires were put on all four corners and not mixed with 215s on the front.

The '00 Astro could have an open differential on the rear or a locker. (Yes,I know they're rarer on an AWD, but I ordered mine with a locker.) If it is a locker, there could be noise from the centrifugal governor trying to engage, weak or broken springs, or improperly assembled weights and springs. Since it didn't occur right away, I'd tend to think it was not an assembly error.

These are NOT a friction type limited slip unit, so no limited slip lubricant is required. Don't let the shop try to tell you that unless they retrofit in a carrier with a cone/clutch arrangement.

If the carrier looks like this, it's a locker:



Another possibility is a worn side gear, worn shim/Belleville washer, or worn spider gear pinion shaft, allowing excessive axial movement of the axle shafts. Checking play is easy enough with an indicator:



There should be at least 0.018" and less than 0.060" of play. This can be adjusted by pinion gear shaft size, side gear 'C' clip thickness, or replacement axle shafts if they are heavily worn.

[gspd750]

Thanks for the response BB! Great pics too. Yes, all 4 tires are P235's.
I will enquire about the 'open differential' vs's the 'locker' type when I take it to the shop and find out which one I have. You make an interesting point about possible excessive axial play as the rear end seems to almost wag it's tail the way shutters back there. I have a 1 year 20 k kms warrenty on the crown and pinion that was repalced only 3 months ago so I can only hope it won't cost me much more as my last bill was $1,600 parts and labour.
I will post again later and let you know what the shop findings were.

[gspd750]

Just to update my rear end slippage on my 2000 Astro AWD. Took it the shop where the crown and pinion was replaced 3 months prior (warrenty) in which the service manager and I went for a test drive. I was able to duplicate the slippage and he concurred that in fact their was a problem.
He suggested for starters that I drain and replenish the transfer case which seemed to resolve the same problem he had on his own personal '99 Safari a few years back. Can't hurt as it probably has never been done on this vehicle.
He also assured me the shop 'should have' installed a bottle of anti-slip lube additive in the rear diff when they did the job. Did they or didn't they???
Oh well, better pick up a bottle and add it to the rear diff...can't hurt it.

After draining the transfer case ( good call, as it was very dark and not red at all) and adding 2 fresh liters of Amsoil Syth. Dextron III to the transfer case and a bottle of anti-slip additive to the rear diff it was test drive time.....and it was slippage-be-gone.

I like it when a plan works. So..was it the additive I added to the diff or the fresh fluid change in the transfer case? Ya know...I don't care cuz it's fixed.

[Blue Bowtie]

The Amsoil you used for the transfer case is probably fine, but the recommended fill is AutoTrak II (P/N 12378508). It is a synthetic lubricant, like the Amsoil ATF. Several years ago, there was a TSB regarding the transfer case noise applicable to the NV136 case in Astros/Safaris. However, the symptoms were not normally present in normal cornering, but in tight turns (since the front wheels roll farther than the rear) the noise could be produced.

The EP additive for the ear axle was not necessary if the axle is a factory limited slip, but at least it won't cause any problems. If they installed a aftermarket cone/clutch limited slip carrier, that could have created a corning noise problem.

Incidentally, I prefer to change the rear and front axle oil and transmission oil every 30,000 miles, even with synthetics. The front axle is easy, since it has a drain plug. The next time you remove the rear axle cover, make sure the gasket has the oil holes/slots for the outer axle tubes. Most factory gaskets don't have the holes.

[gspd750]

Thanks again BB for your input. Done some research on the Autotrak II and it is in fact blue in colour and has better friction modifiers. Hmmm...that Amsoil I just put in was like $13 a liter. You have me a bit concerned and I just may go down to GM and get the right stuff. That explained why it was not red when it came out. Owners manual and Haynes has absolutely no specs on the transfer case req'd fluid or capacities. Useless. Like it does not even exist. Found on the web that the NV136 (assuming that is the transfer case in my 2000 AWD) does in fact take 2 liters which was what it took pouring in the Amsoil.
Thanks again....this is a great site btw.