Automatic Transmission Control Module (TCM)

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 Figure 2.

It is unlikely that this TCU is the cause of shift problems, look closely at the wiring.