Diagnostic Checks

The TCU will seldom fail by itself, so it is necessary that the technician investigate the entire system to find the root cause of the TCU failure. The following items should be checked for failure:

CAUTION:
Whenever the harness connector is removed from the TCU, ensure that the anti-backout remains with the wire harness connector.

1. Check the operation of the system.

2. Check TCU for proper output to Shift Control Motor.

3. Check Shift Control Motor Encoder Switches

4. Check Transfer Gearbox Shift Control Motor.

5. Check High status line and Sport/Manual Switches for proper function

1 - 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.

2 - CHECK FOR PROPER OUTPUT TO SHIFT CONTROL MOTOR

NOTE:
Each dealer has received a dedicated break out box (Land Rover Part No. DTCO051A, Hewlett-Packard No. 29055-60016) with the first TestBook shipment.

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 baffery 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 # 4).

8. If power supply tests good, replace the TCU.

3 - 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 (Figure 2).

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 TestBook shift control motor encoder check.

2. If TestBook 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 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 Figure 3 for terminal positions.





NOTE:
The switch state information in some older ETMs is not correct. Use the table below for shift control motor encoder switch state analysis.

4 - 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 (Figure 4, 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 (Figure 4, Ref. 1) and tighten to 10 Nm. (88 in.lb.).

8. Tighten nuts securing bracket to through-bolts.

5 - CHECK HIGH STATUS LINE AND SPORT/MANUAL SWITCHES

TestBook 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 TestBook.

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.

NOTE:
This is what the TCM sees when these switches are operated. The TCM reads the high range status line to ensure that the correct shift strategy is selected.