Transfer Gearbox Control Unit Diagnostics (41/03/97/NAS)

GENERAL DESCRIPTION

In order to understand and diagnose transfer gearbox control system related problems, a complete understanding of the range shift control process is necessary.

The TCU receives and transmits information to and from:

1. Shift Control Motor Position Switch
2. High/Low Request Switch
3. Transfer Gearbox Speed Sensor
4. Drivers selected range on the H-Gate
5. GEMS ECM
6. Automatic Transmission Control Module (TCM)
7. The BeCM

Using this information the TCU energizes an electric motor to shift ranges in the transfer gearbox. Study the block diagram (Fig 1 ) to understand the relationships between the various components in the transfer gearbox shift control system.

The information received by the TCU from the shift control motor encoder switches regarding the motor's position and from the driver's selected range results in the following:

• Movement of the shift control motor from one range to another.
• Information to the driver through the Message Center and H-Gate illumination.

To change range, reduce speed to below 5 mph (8 kph), move the gear lever across the H-Gate and wait in neutral for the light to stop flashing and the audible warning to stop. When the change has been completed, select a gear and drive away. lf the vehicle is driving too fast or the lever is moved into gear before the range change has taken place, the range change will not occur and a message will be displayed on the Message Center. By following the instructions displayed (e.g., "Slow Down" or "Select Neutral") the change will then take place.

Normally a shift will be completed in approximately 1 1/4 seconds with a current draw of 1 1/2 amps. lf the shift is blocked or the motor is stalled against resistance, the TCU will make three attempts before timing out The first attempt will last for two seconds, followed by a half second pause, followed by a one second attempt and a four second pause, then a final two second attempt. The TCU will not attempt any further range change until the ignition is cycled or the request is repeated by moving the shift lever across the H-Gate and back again.

Because one of the components in the system can affect another, it is important that the technician confirm that all the circuits between components are in good condition without any short or open circuits Refer to the ETM for circuit details. Failed communication between ECUs can cause problems seemingly unrelated to the transfer gearbox control system (e.g., poor shifting and inoperative cruise control).

GEMS

On 1996 MY and later vehicles, three GEMS codes can be set by problems with the Transfer Gearbox Control System:

• 1701 - Transfer gearbox line fault.
• 1703 - Transfer gearbox line open circuit fault.
• 1708 - Transfer gearbox line short circuit fault.

GEMS stores a fault code when the fault line is activated. The MIL (Malfunction Indicator Lamp) is illuminated upon completion of two successive trips where a fault is recorded.

At ignition ON, the link between the TCU and the TCM is driven near ground, and then near battery voltage and stays there if no faults are present. lf a fault is present, the line is driven near ground.

TCU READINESS CHECKS

When the transfer gearbox receives a range change request, the TCU carries out the following three checks:

SPEED SENSOR CHECK

The resistance of the speed sensor is checked. If the speed sensor is found to be open or short circuited, the fault output line is activated. If the speed sensor passes the resistance test, it will be used to determine the speed of the vehicle. The TCU compares this speed with a value stored in memory and decides if a range change is allowed. Presently the range change speed is set at five mph (8 kph) High to Low and Low to High

SHIFT CONTROL MOTOR POSITION CHECK

The TCU reads the position of the shift control motor as a binary code produced by the four shift control motor encoder switches Given this information the TCU can determine the position of the shift control motor and where the motor should move to engage the requested range. If the shift control motor should move outside the normal range (i.e., Left of High) then the TCU can move the motor back to the requested position.

The TCU will attempt three times to move the shift control motor to High Range. If High Range position is not reached, the fault output line is activated.

SHIFT CONTROL MOTOR PLAUSIBILITY CHECK

As the shift control motor moves from one range to the next, the TCU monitors the signals from the four motor encoder switches. If a position is sensed that does not conform with the position expected, the motor stops and the fault output line is activated.

AUTOMATIC TRANSMISSION CONTROL MODULE (TCM)

The automatic transmission control module (TCM) receives a signal from the high range status line. If the transfer gearbox is in high range, the high range status line will read 0V to ground, near battery voltage in low range. The high range status line is also used to light the High (green) side of the H-gate and it will remain lit while the line is at 0V. If the TCM does not receive the range information then an incorrect shift strategy will be selected, e.g., Manual rather than Sport in High Range and vice versa in Low Range. Because of the different shift strategies of High and Low range Economy, a poor shift pattern and quality can result.

BECM

The TCU informs the BeCM, which then informs the Instrument Pack, about the range the vehicle is in using three status lines, high, low and neutral. The BeCM uses the high range status line to determine if cruise control can be activated. A shift between ranges is only possible when the transmission is in Park or Neutral. The TCU senses this from the Park/Neutral Position switch. This signal comes from the BeCM.

When a range change is requested, the desired range will flash on the display (e.g., "HIGH") and continue flashing until the range change is completed. When the range change is complete, the message will change to constant and be displayed for a few seconds.

If a range change is requested and not all of the conditions are satisfied, then the range message will flash and continue flashing until all of parameters are satisfied or the request is removed.

If the transfer gearbox neutral is selected (by inserting a fuse in BeCM fuse position F11), then, after a five second safety delay; the transfer gearbox shift control motor moves to neutral and the display shows "TRANSFER NEUTRAL".

BECM FUSES

Fuses for the TCU and shift control motor are located in the BeCM under the front passenger seat.

• Fuse No. 4 (30A) is the power feed for the TCU output drive circuits to drive the shift control motor.
• Fuse No. 6 (10a) is the ignition feed for the TCU.

Installing a fuse in position No. 11 will place the transfer gearbox in neutral.

REDESIGNED TCU

On vehicles built up to VIN VA372390 there have been a number of reports of TCU failure because of over-current protection problems. A redesigned TCU was introduced at VIN VA372390. It is identifiable by its large finned heat sink and features improved internal over-current protection. See Fig 2 . It is unlikely that this TCU is the cause of shift problems look closely at the wiring.

SYMPTOM/COMPLAINT - NUMBER 1

• No range change
• Burning odor
• H-gate lights on, flashes if shift requested
• "Select Neutral." warning (possible)

DESCRIPTION - NUMBER 1

The output drive circuits in the TCU may have failed. The shift control motor may have failed.

REPAIR - NUMBER 1

1. Check TCU output to shift control motor. CHECK FOR PROPER OUTPUT TO SHIFT CONTROL MOTOR.
2. Check Fuse No. 4 in BeCM.
3. Inspect Shift control motor (Check 4).

SYMPTOM/COMPLAINT - NUMBER 2

• No range change
• Both sides of H-gate dim/out
• Poor shifting of automatic transmission in high range (possible).
• No response to range change requests

DESCRIPTION - NUMBER 2

TCU may be inoperative or lack ignition feed.

REPAIR - NUMBER 2

1. Check for blown fuse Position No. 6.
2. Check ignition feed to TCU.
3. If 1 and 2 check out, replace TCU.

SYMPTOM/COMPLAINT - NUMBER 3

MIL illumination.

DESCRIPTION - NUMBER 3

Code P1701 - Transfer Gearbox line fault.

REPAIR - NUMBER 3

1. Fill out DTCR and FAX to LRNA.
2. Check speed sensor resistance. Resistance should be approximately 1,400 ohms.
3. Read Shift Control Motor encoder switches. CHECK SHIFT CONTROL MOTOR ENCODER SWITCHES.
4. If motor encoder switch information does not match motor position, replace shift control motor.

SYMPTOM/COMPLAINT - NUMBER 4

MIL illumination.

DESCRIPTION - NUMBER 4

Code P1703 - Transfer Gearbox line open circuit line fault.

REPAIR - NUMBER 4

1. Fill out DTCR and FAX to LRNA.
2. Check if power supply has been interrupted to the TCU (e.g., battery replacement, blown fuse or a loose battery terminal causing an intermittent open circuit). Even if the power is only briefly interrupted the GEMS ECM will "wake-up" before the TCU and set a 1703 code.
3. Check circuit between TCU and GEMS ECM for open circuits

SYMPTOM/COMPLAINT - NUMBER 5

MIL illumination.

DESCRIPTION - NUMBER 5

Code P1708 - Transfer Gearbox line short circuit fault.

REPAIR - NUMBER 5

1. Fill out DTCR and FAX to LRNA.
2. Check circuit between TCU and GEMS ECM for short circuits.

SYMPTOM/COMPLAINT - NUMBER 6

Vehicle shifts poorly in high range.

DESCRIPTION - NUMBER 6

The High Range Status line should be near GND (0V) when the vehicle is in high range, informing the TCM that the vehicle is in high range. If the high range status line is near battery power, when the vehicle is in high range, the TCM will enable the incorrect shift strategy.

REPAIR - NUMBER 6

1. Ensure that the vehicle is physically in high range.
2. Check if the High side of the H-gate is lit (it should be brighter than the low side) when in high range.
3. If it is lit properly, refer to Automatic Transmission Diagnostics.
4. If it is not lit, check the connection at C-603, pin 35.
5. Check the motor encoder switches for appropriate motor position. CHECK SHIFT CONTROL MOTOR ENCODER SWITCHES. If the switches DO NOT indicate high range, the TCU will not pull down the High Range status line to near 0V and the TCM will select the shift strategy for low range.
6. If the encoder switches do not indicate high range, replace the shift control motor and check the vehicle for proper operation. CHECK FOR NORMAL OPERATION.
7. If the high range status line and connections are good, replace the TCU.

SYMPTOM/COMPLAINT - NUMBER 7

Vehicle shifts poorly in Low Range.

DESCRIPTION - NUMBER 7

The High Range Status line should be near battery voltage when the vehicle is in Low Range, informing the TCM that it is in low range If the high range status line is near GND (0V) the TCM will enable the incorrect shift strategy.

REPAIR - NUMBER 7

1. If the High side of the H-gate is dim, check the operation of the H-gate switch with Test Book. CHECK HIGH STATUS LINE & SPORT/MANUAL SWITCHES.
2. If Test Book Test indicates a malfunction, check the circuit between H-gate connector C617 pin 8 and TCM C601 pin 46.
3. If the H-gate is bright in both ranges, check for a short to ground in the High Range Status line.

SYMPTOM/COMPLAINT - NUMBER 8

• "Select Neutral" warning displayed in Message Center.

DESCRIPTION - NUMBER 8

Warning displayed if gear was selected before shift was completed.

REPAIR - NUMBER 8

1. Inspect operation of TCU. CHECK FOR NORMAL OPERATION.
2. Pause in neutral to allow range change.

SYMPTOM/COMPLAINT - NUMBER 9

Cruise control inoperative.

DESCRIPTION - NUMBER 9

The high range status line should be low (near 0V) while the vehicle is in high range. lf it is high (near battery voltage) the BeCM thinks that the vehicle is in low range and cruise control operation is not enabled.

REPAIR - NUMBER 9

1. If the High side of the H-gate is lit, the problem is not TCU related, check elsewhere for the problem.
2. Check the connection at C603 pin 35. Repair as necessary.
3. Check the shift control motor position encoder switches. CHECK SHIFT CONTROL MOTOR ENCODER SWITCHES. If they do not indicate high range, the TCU will not pull the high range status line down near 0V and the BeCM will not enable the cruise control system. Repair as necessary.
4. If H-gate is not illuminated properly, check the motor position switch pins at the TCU. If no fault is found, replace the TCU.

SYMPTOM/COMPLAINT - NUMBER 10

"Transfer Neutral" displayed in Message Center.

DESCRIPTION - NUMBER 10

Displayed if Neutral range status line is low (near 0V). Transfer neutral is activated by installing fuse in position No 11.

REPAIR - NUMBER 10

1. Check that a fuse has not been mistakenly inserted in position No. 11.
2. Check Neutral status line for short to ground.
3. Check for open circuit to BeCM.

SYMPTOM/COMPLAINT - NUMBER 11

"Slow Down" warning displayed in Message Center.

DESCRIPTION - NUMBER 11

REPAIR - NUMBER 11

1. Check ABS system.

CHECK FOR NORMAL OPERATION

During normal operation, the range display on the H-Gate should be lit brightly in the range the vehicle is in, and the other side should be dim. When a range change is requested, the requested range display will flash until the change is complete. When the range change is complete, the flashing will stop, the selected range display will be bright and the other range display will go dim. If the displays on both sides are dim, the TCU may have lost power or may have failed. If the lights continue to flash, the drivers in the TCU may be damaged and the TCU is unable to request a shift or there may be a shift control motor problem.

CHECK FOR PROPER OUTPUT TO SHIFT CONTROL MOTOR

1. Disconnect the TCU and install the breakout box inline between the TCU and the harness connector.
2. Set up a digital volt-ohm meter (DVOM) to measure voltage between Pins 1 and 25.
3. Turn ignition on and request a range change and observe voltage reading.
4. Voltage should read approximate battery voltage with either positive or negative polarity while the shift control motor is running
   NOTE: The TCU controls shift motor direction by reversing polarity.
5. If the correct voltage is present, but the shift control motor is inoperative, check the circuit between the TCU and the shift control motor. If the proper voltages are present at the shift control motor, check the following and repair as necessary:
   • Shift control motor.
   • Oil intrusion into shift control motor
6. If the correct voltage is not present, check for battery power at Pins 23 and 24 (from BeCM) and Pin 21 (Ignition Power).
7. Repair as necessary and repeat steps 2 through 5.
   CAUTION: If it is determined that the TCU is to be replaced, check the shift control motor before energizing the system. CHECK SHIFT CONTROL MOTOR.
8. If power supply tests good, replace the TCU.

CHECK SHIFT CONTROL MOTOR ENCODER SWITCHES

The Transfer Gearbox TCU has a seven wire connection with the shift control motor, consisting of four motor position encoder switches, the ground for the switches and drives to move the motor one way or the other. See Fig 3 .

The TCU reads the position of the motor as a binary (on/off) code. The shift control motor encoder switches can be checked several ways:

1. Use Test Book shift control motor encoder check.
2. If Test Book is unavailable, the encoder switches can be tested manually by measuring at the TCU with the breakout box installed.
   • Measure between the motor pins (17, 32, 31 or 7) and encoder ground pin (19) with a DVOM switched to measure voltage.
   • You should observe either 5 volts (open) or 0 volts (closed).
   • Use the chart below to interpret the motor position.
3. Additionally the encoder switches can be checked at the shift control motor harness connector. Disconnect the connector and measure continuity between encoder ground and the four switch connector terminals. See Fig 4 for terminal positions.

CHECK SHIFT CONTROL MOTOR

If the TCU is found to have failed, the shift control motor should be inspected to avoid damaging the replacement TCU Use the following procedure to check the shift control motor.

1. Remove bolt securing the end bracket to the transfer gearbox.
2. Remove the two nuts securing the bracket to the motor and remove bracket.
3. Remove the lower through-bolt from the motor, see Fig 5 , ref. 2, and inspect the bolt for speckles of burned varnish and/or burned odor.
4. If automatic transmission fluid (ATF) is present on the bolt, the motor and the selector shaft seal should be replaced.
5. If the bolt is clean and dry, reinstall the motor through-bolt.
6. Install bracket on motor and loosely install nuts onto motor through-bolts.
7. Install bolt through bracket, see Fig 5 , ref. 1, and tighten to 10 N.m. (88 in-lb).
8. Tighten nuts securing bracket to through-bolts.

CHECK HIGH STATUS LINE & SPORT/MANUAL SWITCHES

Test Book has a quick test to verify the high status line and the Sport/Manual mode switch signals to the Transfer Gearbox ECU.

1. Hook up Test Book.
2. Go to Diagnosis, Gearbox, Real Time Monitor, Hi/Low Range.
3. Follow instructions to test the Hi/Low switch and the Sport/Manual switch from this menu.

SELECTOR SHAFT SEAL REPLACEMENT - SRO: 41.20.59

REMOVAL

1. Place vehicle on lift and raise for access.
2. Unplug both shift control motor electrical connectors and move wiring out of the way.
3. Remove two nuts and one bolt securing end bracket.
   WARNING: DO NOT loosen three Torx screws securing encoder switch to shift control motor. The switch adjustment is set from the factory and MUST NOT be changed.
4. Remove three bolts securing shift control motor and remove motor.
5. Use a suitable tool to remove selector shaft seal from transfer gearbox.

INSTALLATION

1. Lightly oil selector shaft seal and install into transfer gearbox. Use a socket the same size as the seal (a 15 mm Snap-On) deep well can be used).
2. Set seal flush to flange surface, DO NOT pass the shaft flange.
3. Apply Right Stuff RTV to the mating surface of the shift control motor.
4. Install new shift control motor onto transfer gearbox with four bolts. Tighten to 10 N.m (88 in-lb).
5. Reconnect both electrical connectors.
6. Check transfer gearbox fluid level and replenish as necessary.
7. Check transfer gearbox vent line to ensure that it is clear.

TRANSFER GEARBOX ECU REPLACEMENT - SRO: 41.30.01

REMOVAL

1. Adjust the front left hand seat all the way forward and raise to the highest level.
2. Obtain radio code and disconnect the battery negative lead.
3. Remove two screws securing rear floor heater air duct. See Fig 6 , reference 1, and remove duct.
4. Raise carpet and insulation mat for access.
5. Remove two screws securing heater duct mounting. See Fig 6 , Reference 2. Remove mounting.
6. Remove left hand screw and loosen right hand screw securing TCU.
   CAUTION: Ensure that the white anti-backout strip remains in the TCU harness connector.
7. Remove TCU and disconnect multi-plug.

INSTALLATION

1. Connect multi-plug. Position new TCU and secure with screws.
2. Position rear floor heater duct mounting and secure with screws.
3. Position insulation mat and carpet. Ensure seat harness IS NOT trapped.
4. Position rear floor heater air duct. Secure with screws.
5. Connect negative battery lead.
6. Position front seat to original position.
7. Code radio, set windows and sunroof.
8. Synchronize remote.

Fig 1: Block Diagram

Fig 2: TCU - Identified By Large Finned Heat Sink

Fig 3: Transfer Gearbox TCU - Identification

Fig 4: TCU Shift Control Motor, Face & Motor/Encoder Connector Table

Fig 5: Motor & Bracket Bolts

Fig 6: Transfer Gearbox ECU - Screws

Fig 7: TCU Pin Layout - Table