Keep Those Trannys Rolling - January/February - 2016

Allison Inhibiting Gear Ranges or Shifts

In this issue of Keep Those Trannys Rolling, we’re going to look at one of the most common problems plaguing Allison’s LCT transmission group: inhibited gear ranges or shifts.

These conditions are usually a result of the Transmission Control Module (TCM) receiving incorrect inputs from onboard sensors.

The TCM monitors inputs from several onboard systems to operate the transmission properly. If the TCM receives an input that’s far enough out of range to affect transmission operation, the TCM will enter Transmission Abuse Protection Mode.

In this mode, the TCM can inhibit engagements into drive and reverse and any or all shifts, depending upon what caused the TCM to enter this mode.

What Is the TCM Monitoring?

The TCM monitors inputs from numerous sensors from several onboard systems to control the transmission operation. These sensors are:

  • Crankshaft (CMK) and CamshaftPosition (CMP) sensors (figures 1 and2) for engine RPM.
  • Throttle Position Sensor (TPS)or Accelerator Position Sensor(APPS; figures 3 and 4) forengine load. 
  • Manifold Absolute Pressure(MAP) or Barometric Pressure(BP) sensors (figure 5) also forengine load.
  • Engine Coolant Temperature(ECT) sensor (figure 6) andTransmission Fluid Temperaturesensor (TFT), located in the pressure switch manifold (PSM).
  • Neutral Safety Backup Switch(NSBU) (figure 7) or Park/NeutralPosition (PNP) switch.
  • Input Speed Sensor (ISS/TSS;figure 8).
  • Output Speed Sensor (OSS).
  • Transmission Fluid PressureSwitch (PSM; figure 9).
  • Transfer Case Position sensor or switch (figure 10).

Let’s look at each of these inputs to determine how the TCM uses them to control the transmission operation.

Crankshaft and camshaft position sensors — Provide the TCM with engine RPM signals, which it uses to calculate TCC slip.

Throttle position sensor or accelerator position sensor — Provide the TCM with engine load signals to calculate line pressure rise and shift timing.

Manifold absolute pressure and barometric pressure sensors — Provide the TCM with engine load signals for calculating line pressure rise and shift timing.

Engine coolant and transmission temperature sensor — Provide the TCM with temperature signals to allow it to determine actual operating temperatures of engine and transmission.

Neutral safety backup switch or park/neutral position switch — Provide the TCM with signals to indicate selected gear range.

Input speed sensor — Provides the TCM with a signal to measure the rotating speed of the turbine shaft, from which it can calculate transmission RPM and slip.

Output speed sensor — Provides the TCM with a signal it can use to determine vehicle speed and calculate transmission slip.

Transmission fluid pressure switch manifold — Provides the TCM with signals for monitoring clutch engagement and shift timing.

Transfer case position sensor or switch — Provides the TCM with a signal to identify transfer case range.

Now let’s discuss how each of these inputs can cause the TCM to inhibit transmission operation. When the TCM inhibits the transmission operation, you may notice the PRNDL display flashing on and off or display Gear Inhibited or Shift Inhibited. These conditions can cause the TCM to inhibit transmission operation.

High engine RPM on engagement — Check engine RPM while shifting into gear. If engine RPM exceeds idle specifications, the TCM will keep the transmission in neutral. Check CMK and CMP inputs and idle speed adjustment; repair as needed.

High throttle or high torque on engagement — If the throttle or accelerator position sensor signal exceeds 25% during transmission engagement, the TCM will keep the transmission in neutral. Check sensor input signal to TCM at idle; repair as needed.

Erratic input speed sensor signal — If the TCM receives an erratic input speed sensor signal during engagement, the TCM will keep the transmission in neutral. Check input speed sensor signal; repair as needed.

Erratic output speed sensor signal — If the output speed sensor signal is erratic or over 300 RPM during engagement, the TCM will keep the transmission in neutral. Check the output speed sensor signal;
repair as needed.

Erratic neutral safety backup or park/neutral position switch signals — If the TCM receives an erratic signal from the neutral safety backup or park/neutral position switch, it’ll keep the transmission in neutral. Check the switch signals at TCM, check wiring and connections, and check for water contamination in the neutral safety backup switch; repair as needed.

Erratic pressure switch manifold signals — If the signal from the
pressure switch manifold is erratic, the TCM will keep the transmission in neutral. This may be caused by low fluid level, valve body problems, a faulty pressure switch manifold, or problems with the wiring or connections; repair as needed.

Low transmission fluid temperature — If the transmission fluid temperature sensor signal indicates the transmission temperature is below -49ºF (-45ºC), the TCM will keep the transmission in neutral. Verify the transmission temperature. Check the transmission fluid temperature sensor circuit, wiring, and connections; repair as needed.

Transfer case in neutral when shifting transmission into gear — If the transfer case control system indicates the transfer case is in neutral while shifting the transmission into gear, the TCM will keep the transmission in neutral.

If it indicates the transfer case was shifted to neutral while driving, TCM will wait until the vehicle output speed drops below 300 RPM to shift the transmission into neutral. Check and repair transfer case input to TCM as needed.

Transmission slip — If TCM receives a signal that the transmission is slipping, it’ll prevent the transmission from shifting into that gear range. Check transmission operation; if it seems to be working properly, check the appropriate sensor inputs. If the transmission is slipping, repair as needed.

Well there you have it: the most likely causes and possible cures for the most common problem plaguing the Allison LCT1000 transmission: inhibited shifts or gear ranges. With a better understanding of how the Allison LCT transmission group operates, you should have no problem keeping those trannys rolling.