auto trac 4wd question

hoorders · August 8, 2005

Hi,

I have a 2000 Z71 4x4 1500 silverado. I had it for some service recently and noticed that even with the truck in 2wd, the front diff was still engaged. However, if I take the truck off the hoist, put it on some loose dirt, it will spin only the back tires. The 4wd seems to work fine but I was curious about the diff being engaged on the hoist.

Any thoughts on this?

Thanks very much.

redvett · August 8, 2005

If you have autotrac when you put it in auto the front axle is engaged so when there is a wheel spin differental between the front & rear the transfer case engages the front axle. In 2 hi only the axle shafts turn not the front drive shaft.

hoorders · August 8, 2005

If you have autotrac when you put it in auto the front axle is engaged so when there is a wheel spin differental between the front & rear the transfer case engages the front axle. In 2 hi only the axle shafts turn not the front drive shaft.

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Thanks for all the replies.......................I understand how autotrac works. My question was if the front diff should be engaged all the time when its in 2wd? I know the transfer case sends a small percnetage of torque to the front drive shaft when its in 2wd so the front drive shaft is spinning all the time. I was just curious about the diff. When the truck is on the ground, in 2wd, it will spin only the back wheels. Whens its up on a hoist in 2wd all four wheels seem to spin..............im wondering if this is normal and if not what engages and disengages the front differential?

redvett · August 8, 2005

If you have autotrac when you put it in auto the front axle is engaged so when there is a wheel spin differental between the front & rear the transfer case engages the front axle. In 2 hi only the axle shafts turn not the front drive shaft.

<{POST_SNAPBACK}>

Thanks for all the replies.......................I understand how autotrac works. My question was if the front diff should be engaged all the time when its in 2wd? I know the transfer case sends a small percnetage of torque to the front drive shaft when its in 2wd so the front drive shaft is spinning all the time. I was just curious about the diff. When the truck is on the ground, in 2wd, it will spin only the back wheels. Whens its up on a hoist in 2wd all four wheels seem to spin..............im wondering if this is normal and if not what engages and disengages the front differential?

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There tends to be a small amount of drive line drag that makes the driveshaft turn when on a hoist but in reality its disconnected at the differental when in 2 wd.

firstruck · August 9, 2005

Holy hyper there redvette..lay off the caffine!

To answer the original question, yes the front "differential" is always engaged. As in when the front wheels spin the front drive shaft rotates. The tranfer case is not actively driving the front shaft in 2HI. So everytime you accelerate you have to over come the "dead" weight of the idle front drive line to get it moving. Hence why a 4x4 in 2HI doesn't get the same gas mileage as a std. 2wd p/u. I do believe redvette is correct that there is a small amount of "drag" in transfer case that would let an unloaded 4x4 rotate all four corners, say up on a lift. But then again I'm usually talking out my blow hole so take it with a grain of salt....

Oh, and redvette I'm sure you meant it's disconnected at the TRANSFER CASE, right?

redvett · August 10, 2005

Holy hyper there redvette..lay off the caffine!

To answer the original question, yes the front "differential" is always engaged. As in when the front wheels spin the front drive shaft rotates. The tranfer case is not actively driving the front shaft in 2HI. So everytime you accelerate you have to over come the "dead" weight of the idle front drive line to get it moving. Hence why a 4x4 in 2HI doesn't get the same gas mileage as a std. 2wd p/u. I do believe redvette is correct that there is a small amount of "drag" in transfer case that would let an unloaded 4x4 rotate all four corners, say up on a lift. But then again I'm usually talking out my blow hole so take it with a grain of salt....

Oh, and redvette I'm sure you meant it's disconnected at the TRANSFER CASE, right?

<{POST_SNAPBACK}>

It is disconnected at the front axle . There is an actuator that engages when you push the "auto" button then if the transfercase controller sees a speed difference between the front & rear axles it engages the transfercase to fourwheel drive. To answer your question in 2wd the front axle is NOT engaged.

hoorders · August 10, 2005

Holy hyper there redvette..lay off the caffine!

To answer the original question, yes the front "differential" is always engaged. As in when the front wheels spin the front drive shaft rotates. The tranfer case is not actively driving the front shaft in 2HI. So everytime you accelerate you have to over come the "dead" weight of the idle front drive line to get it moving. Hence why a 4x4 in 2HI doesn't get the same gas mileage as a std. 2wd p/u. I do believe redvette is correct that there is a small amount of "drag" in transfer case that would let an unloaded 4x4 rotate all four corners, say up on a lift. But then again I'm usually talking out my blow hole so take it with a grain of salt....

Oh, and redvette I'm sure you meant it's disconnected at the TRANSFER CASE, right?

<{POST_SNAPBACK}>

It is disconnected at the front axle . There is an actuator that engages when you push the "auto" button then if the transfercase controller sees a speed difference between the front & rear axles it engages the transfercase to fourwheel drive. To answer your question in 2wd the front axle is NOT engaged.

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Well there seems to be some mixed feelings on whether the front diff is always engaged or not. Im just basing my question on seeing my truck on hoist and all 4 tires rotating in 2wd. But if you put the truck down on the ground, throw the rear end up on jack stands, put it in drive in 2wd, only the rear tires spin.

Maybe we can get some clarification from a gm tech.

GM-Tech · August 10, 2005

"The front axle on Selectable Four Wheel Drive model vehicles uses a central disconnect feature in order to engage and disengage the front axle. When the driver engages the 4WD system, the Transfer Case Control Module sends a signal to the electric motor actuator to energize and extend the plunger inside. The extended plunger moves the clutch fork and clutch fork sleeve across the inner axle shaft and the clutch fork shaft and locks the two shafts together. The locking of the two shafts allows the axle to operate in the same manner as a semi-floating rear axle. A propeller shaft connects the transfer case to the front axle. The differential carrier assembly uses a conventional ring and pinion gear set to transmit the driving force of the engine to the wheels. The open differential allows the wheels to turn at different rates of speed while the axle continues to transmit the driving force. This prevents tire scuffing when going around corners and premature wear on internal axle parts. The ring and pinion set and the differential are contained within the carrier. The axle identification number is located on top of the differential carrier assembly or on a label on the bottom of the right half of differential carrier assembly. The drive axles are completely flexible assemblies consisting of inner and outer constant velocity CV joints protected by thermoplastic boots and connected by a wheel drive shaft. "