NV246: Understanding
the Tricks
It is easy to see that any external
variations in wheel or tire size or tire pressure can fool the
computer into detecting a slipping condition. Before doing
anything else on one of these vehicles you must make sure that
all tires are of the same measured circumference and that the
tire pressures are equal on all four wheels. A difference of 15
rpm between front and rear propshaft speeds can activate the
clutch pack or set DTC codes. This translates into a difference
in tread wear of 1/16 inch. There is no room for error here,
and if you neglect to measure the tires with a stagger gauge or
a tape measure around the center of the tire, this unit will
turn into a diagnostic nightmare.
This is a complex operating system and is
not well understood in the field, which leads this unit to
create exceptionally high amounts of traffic on our technical
help line. This article is meant to give you an overview of the
operating system to help simplify diagnosis and repairs.
Shift modes
Auto – In this mode the ATC module
will vary the torque available to the front axle as needed
without intervention by the driver. The front-axle actuator is
always engaged, locking the front propshaft to the front
differential.
2WD – In this mode the 4WD operation
is disabled and the front propshaft is disconnected from the
front differential. There are no active or automatic functions
in 2WD.
4WD High – The transfer case is fully
engaged to both axles with a 50/50 torque split. This mode
should be used only on snow- or ice-covered roads and never on
dry pavement, or else crow hop and wheel chatter will begin.
4WD Low – Creates the same conditions
as 4WD High, except that power now flows through a planetary
gear set, providing a 2-1 gear reduction. This mode is for
off-road use only and should never be used on dry pavement.
Neutral – There is no power flow
through the transfer case, allowing the use of accessories or
flat tows for short distances. There is no button for Neutral.
To get into neutral, the driver presses the 2 High and 4 High
buttons at the same time and holds them for at least 10
seconds. Vehicle speed must be less than 3 mph, the transfer
case must be in the 2 High range, the transmission must be in
neutral or the clutch depressed, and there must be a speed
signal from the engine.
The NV 246 has the following control
components:
• ATC module
• An encoder with a
pulse-width-modulated controlled shift motor and motor brake.
This multifunction unit has the ability to shift the transfer
case into different modes, apply and release the internal
clutch pack, and lock the motor to a determined position.
• Three vehicle-speed sensors
• A throw-out bearing within the
transfer case
• An internal multiple-disc clutch
pack
• Vehicle-control-module (VCM)
interface
• ATC-control-switch assembly mounted
in the driver compartment.
To diagnose and repair one of these active
transfer cases properly, you must have a scan tool. You also
will need to access the proper diagnostic reference material
online to obtain codes, wiring diagrams and other necessary
information. The scan tool also can be used to activate various
circuits for voltage and resistance readings and to make sure
the ATC module is flashed properly for the vehicle it is in. If
you need to replace an ATC module you must use the scan tool to
program the new ATC module to the vehicle you are working on,
or else it will not be able to communicate with the VCM
interface.
Scan-tool-data definitions
2WD High indicator light – The scan
tool will read on or off and should indicate on when the
transfer-case shift-control module (TCSCM) asks for 2WD high
mode.
4WD High indicator light – The scan
tool will read on or off and should indicate on when the TCSCM
asks for 4WD High mode.
4WD Low active – The scan tool shows
yes or no. It should indicate Yes when the transfer case is in
the 4WD Low lock mode.
4WD Low indicator light – The scan
tool shows on or off. It should read on when the TCSCM commands
4WD Low mode.
ATC application – The scan tool will
display a numeric value indicating the model number assigned to
the transfer case.
ATC software ID – An alphanumeric
value will be displayed, indicating the numbers or characters
assigned to the current version of software used by the TCSCM.
Auto 4WD indicator light – The scan
tool shows on or off. It should display on when the TCSCM
commands the Auto 4WD light on.
Battery voltage – A voltage reading
of 0-25.5 volts will display, indicating the voltage measured
by the TCSCM at the TCSCM battery feed.
Current slip adapts – A numeric value
displayed by the scan tool that indicates the corrective
adjustments being made by the control module for wheel-slip
occurrences. Note: This is useful in helping to determine
whether tire size or pressure is causing codes to set.
Encoder Gear Position – The scan tool
will display the mode in which the transfer case is operating.
Encoder Return Voltage – The scan
tool will display the voltage that the ATC module sees at the
encoder signal return. The voltage will be 0-7.5 volts. Never
put battery voltage to the encoder unless you wish to buy a new
one.
Encoder Supply Voltage – This
indicates voltage seen by the ATC module at the encoder power
supply. The scan tool will display 0-7.5 volts. Again, never
put battery voltage to the encoder unless you wish to buy one.
Front Axle requested – The scan tool
will show yes or no. Yes should be displayed if the ATC module
has commanded any range besides 2WD High or Neutral. If the
scan tool shows yes, the front-differential actuator should
lock the front driveshaft to the front differential.
Front Axle switch – The scan tool
will read locked or unlocked, indicating the state of feedback
of the front propshaft.
Ignition Cycles Since Last Current DTC
– The scan tool will display a numeric value indicating
how many ignition cycles have occurred since the latest current
DTC.
Mode Switch Return voltage – A
reading of 0-5 volts, which indicates return voltage from the
transfer-case control switch.
Mode Switch Selected – The scan tool
will display the current mode selected, and this should match
the shift-control-switch button pressed by the driver.
Motor A Current – Amperage displayed
on the scan tool indicates the flow of current when the motor
moves in one of two directions.
Motor B Current – Amperage displayed
on the scan tool indicates the flow of current when the motor
moves in one of two directions.
Neutral indicator light – The scan
tool will read on or off. On should be indicated when the ATC
module commands the neutral position.
Rear Propshaft Speed – Indicates the
rotational speed of the rear propshaft; the scan tool will
display 0-8,192 rpm.
Service 4WD Lamp – The scan tool will
read on or off. On indicates that the ATC module has commanded
the Service 4WD lamp or message on.
Slip Adapt PWM – Indicates the duty
cycle applied to the transfer-case encoder motor for clutch
application. The scan tool will display 0-100%.
Total Slippage – This indicates the
total slippage of both front and rear propshafts and will be
displayed as 0-8,192 rpm.
Transfer Case Lock – The scan tool
will show enabled or disabled and should indicate enabled if
the ATC module commands the transfer-case-motor brake on. If
the motor brake is on, rotating the encoder motor by hand is
impossible.
Scan-tool output controls
The scan tool can be used to control many
of the transfer-case functions for diagnosis and testing. The
scan-tool output controls may require additional menu
selections to access all the functions.
2WD High indicator light – allows
technician to turn on or off the 2WD High light on the TC
control switch.
4WD High indicator Light – allows
technician to turn on or off the 4WD High light on the TC
control switch.
4WD Low indicator Light – allows
technician to turn on or off the 4WD low light on the TC
control switch
ATC Motor A Control – permits the
technician to shift the transfer case through all five modes.
Motor Control A moves in the opposite direction of Motor
Control B.
ATC Motor B Control – permits the
technician to shift the transfer case through all five modes.
Motor Control B moves in the opposite direction of Motor
Control A.
Auto 4WD indicator light – allows the
technician to turn Auto 4WD light on or off on the
transfer-case control switch.
Engage Front Axle – permits the
technician to engage or disengage the front-axle actuator. When
engaged, the actuator should lock the front propshaft to the
front differential.
Neutral indicator light – allows the
technician to turn the neutral indicator light on or off on the
transfer-case control switch.
Transfer Case Lock – allows the
technician to turn the encoder-motor lock on or off. With the
encoder-motor lock on, shifting the transfer case is
impossible. Note: This is an important function to access when
you’re replacing an encoder motor. With the lock disabled
you will hear a click, and you can move the encoder motor by
hand to line up with the shift lever when replacing an encoder
motor. Be extremely careful when rotating the encoder motor by
hand not to over-rotate the motor or it will need to be
replaced. An alignment tool supposedly is available, but as of
this printing I have not been able to find a part number.
Diagnostic procedures and trouble codes
To diagnose and test these transfer cases
properly you must have a proper scan tool and access to online
diagnostic information. I will give you an overview of the
codes, but there isn’t room here to go through the test
procedures to solve the problems. Begin all diagnostic routines
by accessing the Diagnostic System Check-Transfer Case. This
will review three important areas through the scan tool. It
will identify the control modules that are in command of the
system, it will check the ability of the control modules to
communicate through the serial data circuit, and it will
identify any stored DTCs and their status.
The 246 has at least 19 available trouble
codes. Where many technicians get into trouble is in
identifying which codes work with which system. None of these
codes is a powertrain code. They are chassis, body and
universal codes and are identified by the letter prefix of the
code. “C” is a chassis code; “B”
indicates a TBCM (truck body-control module) circuit failure;
and “U” codes, known as universal DTCs, indicate a
data-transmission-network concern.
C0324 T/case lock circuit high
voltage
C0327 Encoder circuit
C0357 Park-switch circuit high
voltage
C0362 4 Low circuit low voltage
C0367 Front-axle circuit high voltage
C0374 General system malfunction
C0376 Front vs. rear propshaft speed
incorrect
C0550 ATCM processor malfunction
C0611 VIN information incorrect
C0895 Voltage incorrect
B0768 Service light low voltage
B2725 Mode-switch malfunction
U1016 VCM communications
U1041 EBCM communications
U1064 TBCM communications
Encoder motor: important information
The encoder motor is a complex design.
There are two electrical connections, one six-pin on the motor
and a four-pin pigtail, for which I have provided pin
schematics in the illustrations here. Never put 12 volts to
this system. This system is designed to handle a 5-volt
reference and 7.5 volts for operating power. The encoder motor
provides several functions: It shifts the transfer case through
the five modes, applies the clutch pack and informs the ATC
module of the mode in which the transfer case is operating.
When you are going to disassemble the
transfer case, make sure it is in 2WD High mode before removing
the encoder motor. Failure to do so will require releasing the
encoder-motor brake before you replace the encoder motor on the
transfer case to index the motor to the shift rail correctly.
You can do this with a scan tool and the motor plugged into the
harnesses. It also is possible to do this on the bench. You
will need a 9-volt radio battery and two leads. Connect the
positive lead to pin C (battery positive voltage) on the
four-pin pigtail (orange wire) and the negative lead to pin D
(lock control circuit), the tan wire. You should hear a click
and be able to rotate the motor by hand. Use extreme caution
not to over-travel the motor or else it will be expensive junk.
These are sophisticated transfer cases, as
one would expect from the price tags of the vehicles in which
they are used. A complete understanding of their function and
electronic controls should make diagnosis and repairs quicker
and easier to achieve. Welcome to the computer world, where
information is the most-important commodity. 
©2005 Transmission Digest
