# B&B? how does silverado 4x4 system work?



## ontario026 (Dec 22, 2008)

Just curious if I understand the operation of a silverado 4x4 system... My truck is a 2000 2500 silverado, if I understand correctly, when driving down the road normally in 2wd, my front drive shaft should not be spinning, while the driver's cv shaft will be rotating, thus spinning the spider gears in the front diff, and the left cv axle will be spinning 'free-wheeling' free of the front diff due to the electronic disconnect actuator??


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## ontario026 (Dec 22, 2008)

and is it possible for the front axle actuator to fail, resulting in the front end being locked in 4wd all the time, so that the front drive shaft is alway rotating? Without any DTC's being set in the pcm?


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## B&B (Nov 4, 2006)

ontario026;1048076 said:


> Just curious if I understand the operation of a silverado 4x4 system... My truck is a 2000 2500 silverado, if I understand correctly, when driving down the road normally in 2wd, my front drive shaft should not be spinning, while the driver's cv shaft will be rotating, thus spinning the spider gears in the front diff, and the left cv axle will be spinning 'free-wheeling' free of the front diff due to the electronic disconnect actuator??


Close but not quite with the Autotrac 4WD system in your truck.

When in the 2Hi position the internal clutch pack in the T-case is applied fully thus there's rotation applied to the front diff via the driveshaft. So in 2WD the D-shaft DOES rotate as does the ring & pinion and differential. However since the axles are disconnected from one another by the central axle disconnect (the actuator) there is no power applied to the wheels and no binding issues, everything is just being freely rotated by the T-case.

When you press the "Auto" position the actuator can immediately engage the pass side axle since the T-case has already been turning it and the other front axle components at road speed so there's no gear clash when shifting on the fly. After the axle is engaged by the actuator the clutch pack in the T-case is immediately released so there's still no power being applied to the tires (hence you don't get any 4WD "binding" on a a hard surface). So now the axle, D-shaft and all other front axle components are still being turned, but now they're being turned by the tires, not the T-case. In this configuration if wheel slip is encountered the clutches can then apply power smoothly as needed and as much as necessary to reduce rear wheel slip. It has the capability to apply zero up to a full 50/50 split between the front and rear until wheel speeds are matched or very close.

It's never a wise idea to plow in the auto position as it adds premature wear on the T-case clutches. I've repaired more smoked clutches in these systems from guys insisting on plowing in the auto position than I care to discuss. Some require a $1500 repair bill before following my original recommendation. 4Hi is the wise choice.

I'm sure you understand 4Hi and 4Lo so no real need for an explanation with those, it simply (and physically) locks both front and rear axles together so neither can rotate faster or slower than the other. Simple there.


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## B&B (Nov 4, 2006)

ontario026;1048082 said:


> and is it possible for the front axle actuator to fail, resulting in the front end being locked in 4wd all the time, so that the front drive shaft is alway rotating? Without any DTC's being set in the pcm?


4WD system codes are not tagged by the PCM, they're tagged and stored by the TCCM (transfer case control module) so you won't find any with a generic scanner as most can't read the TCCM. To access that you need a good professional scanner and if you suspect that the axle actuator is stuck you will find at least one DTC because the TCCM monitors the actuators position at all times with the ignition on.

So, if you suspect a 4WD system issue I'd recommend scanning it for possible codes with a scanner capable of accessing these codes before concluding that there are no codes stored. Because if there is I suspect you'll find at least one B series (body/4WD systems) code.


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