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Since the 4wd topic has been coming up lately I was wondering about my system. usually when I drive in snow or ice covered conditions in the frozen tundra of western PA I use the Autotrac because the conditions vary. My problem is the system doesn't engage everytime the wheels spin. I can lock the rear and spin the tires but it takes a quite few seconds for the front to lock and have 4wd; my understanding of the sytem was the hubs were locked and once the computer detects wheel spin, power is provided to other wheels, something that is useful on an icy patch of road, but if the system doesn't kick in until your out of control what good is it? So is it my ignorance of the system or ain't it working right?

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:seeya: From what I understand is on the OEM system, once the transfer case is engaged, the left front axle begins to turn, and the thermal linear actuator is engaged. Thermal Linear actuators create an electrical current that heats the plunger. The left axle and intermediate shaft are connected by spider gears in the differential, and the thermal linear actuator, once heated enough, extends approximately one inch to slide the shift collar and fork over the intermediate shaft to connect it to the right front axle. Both front axles are now turning and 4-wheel drive is fully engaged. When the OEM actuator fails, the shift collar and fork can't connect the right front axle to the intermediate shaft, so the 4-wheel drive never engages.

 

So i does take a few seconds to kick in. I say if you are worried run it 4hi.

 

:sigh:

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Autotrac is virtual seemless when working correctly. My AutoTrac in my '00 Silverado was very smooth and worked great!!!

 

I'm sure I asked before... but where are you in Western PA? I'm in Grove City....

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Are you talking about your 2500HD? According to Chevy's site Autotrac is not availible on 2500HDs only 1500 and 2500 CC

 

EDIT now I see you have an LD sorry. :seeya:

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Are you talking about your 2500HD? According to Chevy's site Autotrac is not availible on 2500HDs only 1500 and 2500 CC

 

EDIT now I see you have an LD sorry. :seeya:

I'm assuming you are reffering to me.... I traded a '00 1500 4x4 on my '01 HD.

 

The HD does not have AutoTrac as it was not an option on the 2500HD with the heavier T-case....

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Are you talking about your 2500HD?  According to Chevy's site Autotrac is not availible on 2500HDs only  1500 and 2500 CC

 

EDIT now I see you have an LD sorry.  :seeya:

I'm assuming you are reffering to me.... I traded a '00 1500 4x4 on my '01 HD.

 

The HD does not have AutoTrac as it was not an option on the 2500HD with the heavier T-case....

It was a brain fart, I though I saw HD in gave20's sig.

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:seeya: From what I understand is on the OEM system, once the transfer case is engaged, the left front axle begins to turn, and the thermal linear actuator is engaged. Thermal Linear actuators create an electrical current that heats the plunger. The left axle and intermediate shaft are connected by spider gears in the differential, and the thermal linear actuator, once heated enough, extends approximately one inch to slide the shift collar and fork over the intermediate shaft to connect it to the right front axle. Both front axles are now turning and 4-wheel drive is fully engaged. When the OEM actuator fails, the shift collar and fork can't connect the right front axle to the intermediate shaft, so the 4-wheel drive never engages.

 

So i does take a few seconds to kick in. I say if you are worried run it 4hi.

 

:sigh:

No Autotrac truck has a thermal actuator. That was only earlier OBS trucks. Also the front axles are never locked together.

It is possible that the front diff actuator is not working correctly, but then the front wouldn't be powered in 4HI or 4LO either.

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The front diff is alway "semi engaged" in auto trac. most of the off on engagement comes from the t-case. your clutches in the t-case could be getting worn, may need replacement of theses clutches.

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I think I covered how AutoTrac works in a previous thread. Still, we can go over it again. There's a raging blizzard outside my home office so I'm not going anywhere, and the cat is sleeping in my lap, so why wake him up??

 

:seeya:

 

The +1999 trucks use an electric actuator for the front axle: a DC electric motor is hooked up to a tiny worm gear drive that moves a pawl back-n-forth, with a fork at the end of the pawl. In the front axle there is a splined collar that slides on a splined shaft, the fork fits into that collar.

 

When the collar is commanded by the electric actuator to "disengage" the worm gear pulls the pawl one way, the fork moves the collar, and the R&P are disengaged from the halfshafts. The transfer case also disengages to 2WD so the front driveshaft stops rotating.

 

Note: there is no "free" position like the older trucks with manual hubs. The front wheels are still turning the CV joints and halfshafts, and the case bearings in the front axle are still turning as well.

 

This is why it's so important to regularly service your front axle: even if you NEVER push AUTO 4WD or any 4WD mode and leave it in 2WD, the front axle case bearings are still getting a good workout. If you let the gear oil run low or never change it, the oil quickly degrades and at the very least the case bearings are shot. It only holds around 3.5 pints of fluid, so there isn't much there for any margin of error.

 

If the electric actuator is dead or if it jams, no 4WD mode will work and you should get a "SERVICE 4WD" in the scrolling Message Center. Depending on the failure, there could also be a grinding/ratcheting noise from the front axle. If you EVER get a grinding/ratcheting noise, immediately stop driving the truck and have it towed for inspection. You could ruin the front axle.

 

Folks in colder climates have also had problems with delayed engagement. To save $0.07 per truck (I'm being sarcastic here) GM fills the front axle with a mineral-based 80W-90 GL-5 gear oil. According to API specs, that 80W-90 is solid at -12 F, and it's flow properties are marginal at around 2 F.

 

There is a TSB regarding this that I also covered in a previous post: GM recommends their $$$ GoodWrench Synthetic 75W-90. Since I have pails of commercial synthetic 75W-90 in my shop, it was a no-brainer for me to use the commercial synthetic gear oil in my front axle from new. I changed my front axle at 600 miles.

 

Although the TSB warns of vent leaks when changing to a synthetic gear oil, I've never had a problem. Also have never had a leaky pinion seal or leaky case seal. I did have a leaky rear axle pinion seal, but it came from the factory that way and the GMC truck dealer promptly fixed it. No leaks since.

 

The TSB warns that even if engagement is ok, the thick 80W-90 will cause increased case bearing wear and premature failure in cold climates. Since this can start at around 2 F, figure out if it applies to your ambient conditions.

 

Inside the transfer case, it has a regular chain and planetary gears for the regular 4HI and 4LO modes. This is quite similar to any chain-driven transfer case. In AUTO 4WD mode, something different happens.

 

When you poke AUTO 4WD, the front axle electric actuator engages and the collar is pushed to engage the R&P. The driveshaft is then allowed to rotate, but little torque is applied . It's "along for the ride."

 

At the same time, a clutch pack in the transfer case is starting to bias. The PCM keeps track of the rear VSS (Vehicle Speed Sensor) and the front driveshaft VSS. I made a boo-boo in the previous post that the front ABS rings are used to determine an average speed to calculate bias: that's how the Stabilitrac system works on the GMC Yukon Denali. Sorry bout that, my bad.

 

As long as the front/rear VSS are close to agreement, the clutch pack only receives light bias. Once the front/rear VSS differ, the clutch pack is biased stronger. Depending on the nature of front/rear VSS difference, the clutch pack can actually bias so most, though not all, of the torque is sent to the front axle.

 

The best example of this is when I'm driving in town and am at a stop light or stop sign. The sander only appears to sprinkle sand/salt at the very front of the intersection, so the front wheels are sitting on sanded/salted ice, and the rear wheels are sitting on glare ice. In AUTO 4WD mode, when I press the gas at a green light, the front wheels smoothly pull the truck away. I've NEVER felt a grab OR a delay.

 

This bias is very quick and there should NOT be any apparent delay. If you have a problem with the clutch pack, say old fluid in the transfer case or WRONG FLUID, it won't work properly and you will get a delay. It may even stop working altogether.

 

If you accidently fill the Automatic Transfer Case with regular Dexron III ATF, expect the clutch pack to become sluggish. The correct fill is that AC Delco AutoTrak II stuff and it's blue, like a Smurf. It contains special additives for the clutch pack. You can use this stuff in a regular 2 speed transfer case but there is no reason to and you'd be wasting money.

 

I serviced my transfer case at 600 miles initial, then every fall since. I figure it's WAY cheaper than a new clutch pack and/or transfer case. My AutoTrac appears to work flawlessly, and it sure gets a workout in our long winters here.

 

Sometimes you have to drain/refill several times with the correct fluid to see if the clutch pack comes back to life. Also try a drain/refill of the front axle using the correct GL-5 gear oil for your ambient conditions (Approaching 0 F use a synthetic 75W-90 GL-5 gear oil) If it doesn't come back to life, you have a problem.

 

The correct servicing tips have been posted by moderator MountineerTom: at the left pane of the LD Forum, look under "Features" and "Guides." Should be in there, also at the root LD Forum at the very top of the page.

 

Let us know how it turns out.

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Simlar example: Yesterday evening, the exposed upward smooth concrete ramp out of my parking garage contained a wet, icy coating. It is fairly steep. My rear tires (even though locking) would spin as I went up in 2WD. I stopped (though maybe I didn't need to), hit the Autotrac button, and after a very slight rear wheel slip (about as along as it takes to snap your fingers) I went up with no more slipping.

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Shaners -I'm in butler

 

Jayman- whew thanks for the info it's been so cold here the last couple of days and when I was using it that the heavy weight oil was probably causing the sluggishness as it warmed up to 25 and it worked better.

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Shaners -I'm in butler

 

Jayman- whew thanks for the info it's been so cold here the last couple of days and when I was using it that the heavy weight oil was probably causing the sluggishness as it warmed up to 25 and it worked better.

That's probably it then. When you get a chance, drain/refill the front axle with the correct synthetic 75W-90 GL-5 rated gear oil. Since there are no clutch packs in the front axle to worry about, feel free to use any synthetic GL-5 rated 75W-90. It's good to use year round.

 

Don't be surprised when you take out the front axle drain plug that it's all full of goop and metal shavings. That's normal, though actual metal chunks are NOT normal. The best way to change the gear oil in cold weather is to drive in 4HI on a snow covered gravel road for a few miles.

 

This warms up the front axle but you won't get torque-bind or excessive wear like you would driving in 4HI on pavement. I live at the end of a 3 mile gravel sideroad, so I have a pretty easy way to warm up my front axle before changing the fluid.

 

If the gear oil looks real nasty, I mean gross and black, you may want to change it again a week or two later, just to flush things out. I had to do that with only 600 miles on the thing. But after I changed the front axle to synthetic, instant engagement even at -40 F.

 

Use your best judgement on whether you want to drain/refill the AutoTrac transfer case as well. Remember to only use the correct AC Delco AutoTrak II blue fluid.

 

The AutoTrak II fluid appears to be at least a partial synthetic, maybe a full synthetic. I have a couple of quarts in the shed where I park my snowblower. Even at -35 F it appears to be quite fluid and frisky.

 

Good luck! :seeya:

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It is amazing how much one can miss on here if you don't keep up on a daily basis. I was in Orlando for a week and didn't have access and I have a ton of catching up to do. :jester:

 

I just wanted to add to this post that GM AutoTrac II is the only fluid for the Auto Trac transfer case. I have searched for another major brand replacement and there is non out there to my knowledge. AMSOIL does not even make a replacement synthetic. So make sure that you use only the GM brand Auto Trac II fluid. :rolleyes:

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Welcome back wondered where you you disappered to.

 

Expect more manufacturers to come out with unique fluid requirements. Can you say "profit margin?"

 

At least that AutoTrak II stuff isn't what I consider to be terribly outrageous, not like their GoodWrench synthetic rear axle fluid. Local dealer wants $7/qt for the AutoTrak II and $25/qt for the synthetic rear fluid.

 

I wonder what they blend into that rear axle fluid? Gold and diamonds??

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