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1 9th June 06:20
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Posts: 1
Default Autotrac Transfer Case (NP236)

GM's online service manuals, but they are not available to the public.
I can paste a paragraph or two below.

It's like that for the non-autotrak t/cases and even some models
that use the automatic feature do not spin the front driveshaft.
I suspect that spinning the ring and pinion doesn't really cause
that much drag anyway.

It's not "my" explanation. It's just the way it is.

Ian (see below)

Transfer Case Description and Operation
General Operation
The New Venture Gear model NVG 246 RPO NP8 transfer case is a two speed
automatic, active, transfer case. The NVG 246 EAU provides 5 modes, Auto
4WD, 4HI, 4LO, 2HI and Neutral. The Auto 4WD position allows the capability
of an active transfer case, which provides the benefits of on-demand torque
biasing wet clutch and easy vehicle tuning through software calibrations.
The software calibrations allow more features such as flexible adapt ready
position and clutch preload torque levels. The technology allows for vehicle
speed dependent clutch torque levels to enhance the performance of the
system. For example, the system is calibrated to provide 0-5 ft lb of clutch
torque during low speed, low engine torque operation, and predetermined
higher torque for 40 km/h (25 mph) and greater. This prevents crow-hop and
binding at low speeds and provides higher torque biases at higher vehicle
speeds, in order to enhance stability.

Transfer Case Shift Control Switch

The NVG 246 EAU transfer case features a 4 button shift control switch
located on the instrument panel. When the ignition key is in the RUN
position, the transfer case shift control module monitors the transfer case
shift control switch to determine if the driver desires a new mode/range
position. At a single press of the transfer case shift control switch, the
lamp of the new desired position will begin flashing to inform the driver
that the transfer case shift control module has received the request for a
new mode/range position. The lamp will continue to flash until all shifting
criteria has been met and the new mode/range position has been reached, or
has been engaged. Once the new mode/range position is fully active, the
switch indicator lamp for the new position will remain ON constantly.

During normal driving situations, the transfer case can operate in the Auto
4WD mode. In the Auto 4WD mode, the transfer case shift control module
monitors rear wheel slip speed, based on the inputs from both the front and
rear propshaft speed sensors. When the vehicle experiences a rear wheel slip
condition, the transfer case shift control module sends a pulse width
modulated (PWM) signal to an electronic motor, which is the transfer case
encoder motor. This motor rotates the transfer case control actuator lever
shaft, applying a clutch pack. This clutch pack is designed to deliver a
variable amount of torque, normally delivered to the rear wheels, and
transfers it to the front wheels. Torque is ramped up to the front wheels
until the front propshaft speed sensor matches that of the rear propshaft
speed sensor. Torque is ramped down to the front wheels. The process would
repeat if rear wheel slip is detected again.

The NVG 246 EAU transfer case has the added feature of also providing the
driver with 3 manual mode/range positions:

a.. 4HI - 4 Wheel Drive high range
b.. 2HI - 2 Wheel Drive high range
c.. 4LO - 4 Wheel Drive low range
The driver may choose to select any of these mode/range positions while
driving the vehicle. However, the transfer case will not allow a shift into
or out of 4LO unless the following criteria has been met:

a.. The engine is running.
b.. The automatic transmission is in Neutral.
c.. The vehicle speed is below 5 km/h (3 mph).
This transfer case also has a Neutral position. A shift to the Neutral
position allows the vehicle to be towed without rotating the transmission
output shaft. Neutral position may be obtained only if the following
criteria has been met:

a.. The engine is running.
b.. The automatic transmission is in Neutral.
c.. The vehicle speed is below 5 km/h (3 mph).
d.. The transfer case is in 2HI mode.
Once these conditions have been met, press and hold both the 2HI and 4LO
buttons for 10 seconds. When the system completes the shift to neutral, the
red neutral lamp will illuminate.

The NVG 246 EAU case halves are high-pressure die-cast magnesium. Ball
bearings support the input shaft, the front output shaft, and the rear
output shaft. A thrust bearing is located inside of the input shaft gear to
support the front of the rear output shaft. The transfer case requires Auto
Trac® II Fluid GM P/N 12378508 (Canadian P/N 10953626) which is blue in
color. The fluid is designed for smooth clutch application. An oil pump,
driven by the rear output shaft, pumps the fluid through the rear output
shaft oil gallery to the clutch and bearings.

There are two versions of the NVG 246 EAU, which depend on the transmission
applications and vehicle applications. If the vehicle is equipped with a
transmission RPO M30, the transmission splines in the input gear will have
27 teeth. With this application the planetary carrier assembly will have 3
pinion gears. If the vehicle is equipped with transmission RPO MT1 or MN8,
the transmission splines in the input gear will have 32 teeth. The planetary
carrier assembly on this application will have 6 pinion gears.

Some of the changes to the transfer case, that will take place over this
model year, include the following:

a.. The oil pump suction pipe and screen will change to a one piece
b.. The case half bolts will have a cup style washer and the bolt can be
used again.
c.. The clutch apply plate, clutch hub, and clutch oil restrictor plate
will have a design change.
d.. The 4 pinion gear planetary carrier assembly is changing to a 3 pinion
gear design.
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