Shift Fork Control

This guide covers setup and calibration of the hydraulically actuated Shift Forks used to control gear selection in Dual Clutch Transmissions (DCT).

Concepts

Axes

A DCT has two input shafts (“axes”), each carrying its own clutch. While one axis is transmitting drive, the other is free to pre-select the next gear so that the shift itself is just a clutch hand-off, with no torque interruption. Every gear (including Reverse) belongs to one of these two axes:

The active axis is whichever axis currently has a gear engaged and is transmitting drive.
The inactive axis is free to move its fork(s) to pre-select the next gear.

Shift Forks

A shift fork is a physical selector that can sit in one of three positions (“slots”):

Each fork belongs to one axis (or, for a shared Reverse fork, both axes) and is responsible for up to two gears, one on its Low side and one on its High side.
Up to 8 forks and 8 shift solenoids are supported.

While driving:

• The active axis must already have its current gear’s fork engaged (all other forks on that axis in Neutral).
• The inactive axis may have, at most, the pre-selected next gear’s fork engaged (all others in Neutral).

Only one fork is ever moved at a time. A fork on the active axis is never moved unless it’s explicitly required (e.g. a shift out of Neutral/Park), and a fork that needs to return to Neutral is always preferred over one that’s about to engage a new gear, so the way is cleared before a new gear goes in.

Fork Position Sensing

Each fork has a position sensor that is scaled and calibrated to report the fork’s measured position in millimeters. Some forks also have a secondary tracking sensor used purely for cross-checking / diagnostics.

Fork Position 1-8 inputs are configured in Input Setup.

The fork’s calibrated Low / Centre / High positions and tolerance bands are what translate this raw position into a slot (Low / Centre / High / Moving).

Fork Management

At all times the shift fork system is actively performing the following tasks:

• Each fork’s target is set:
  • Active-axis forks target the current gear.
  • Inactive-axis forks target the pre-selected gear.
  • Everything else targets Neutral.
• Each fork’s own position/slot state is updated.
• Axis “binding” is detected if more than one fork on an axis reports being in gear at once.
• An axis fault is raised if any fork on that axis is in a position error state.

Fork Movement

• If a fork is currently moving, its movement is monitored until complete.
• Otherwise, the next fork that needs to move is found (inactive axis first, unless an active-axis move is explicitly allowed) and its movement begins.
• The physical shift solenoids are driven to move the current fork, or to hold the idle/default solenoid pattern when nothing is moving.
• If Axis Pressure Control solenoids are in use, their pressure is modulated to control fork movement speed.
• The moving fork is driven either toward its target position (default), or through a series of engagement phases when Engagement Phase Control is enabled.

Shift Procedure

1. The currently engaged gear and its axis are marked as active.
2. The Preselection system predicts the next shift direction (up or down) and selects the next gear on the inactive axis.
3. On a shift request, the next gear’s fork is determined and pre-selected (if it’s not already) on the inactive axis.
4. Once the pre-selected gear’s axis is ready (fork in target slot, no axis error), the clutch hand-off for the shift can proceed.
5. After the shift completes, the new gear’s axis is marked as active, the offgoing axis is marked inactive.
6. The Preselection system resumes prediction of the next shift on the newly inactive axis.

Configuration & Calibration

Shift Fork Setup (Global)

These apply to whichever fork is currently moving, regardless of which fork it is.

Shift Fork Setup (Per-Fork)

Each fork (numbered 1–8) is independent and must be configured individually. Below, “#” stands for the fork number, eg: “Shift Fork 3 Axis”.

Notes:

• The Low and High slot positions do not need to be numerically low/high — the firmware automatically detects whether the fork’s travel is inverted (i.e. the Low position is a larger number than Centre) and corrects the shift-direction logic accordingly.
• Whichever gear ends up on the Low slot vs. High slot is purely a function of fork travel direction — set the Low/High gear assignments to match the physical gear each end of travel actually engages.
• A fork does not need both slots populated — a single-sided fork (e.g. Reverse-only) should leave the unused Low/High gear assignment at 0.

Shift Solenoid Selection

Which solenoids are energised to move a fork in a direction is controlled by the solenoid select tables.

Per-Fork Position Sensor Calibration

Shift Fork n Position is reported in mm. Each fork has a position sensor that can be calibrated in Input Config > Transmission Inputs > Shift Fork Positions.
It is critical that these position sensor inputs are assigned and calibrated correctly.

Fork Movement Pressure Control

While a fork is moving, its physical movement can optionally be driven by a closed-loop position PID acting on hydraulic pressure (rather than a fixed pressure). The need for pressure control will depend on the transmission design.
When enabled, the Axis Pressure solenoids are controlled to achieve the desired pressure.

With Engagement Phase Control ON, the PID is not used. The pressure is instead set by the force of the current engagement phase, as a percentage of the maximum fork movement pressure (see Engagement Phase Control).

Fork Movement Current Control

Transmissions that set fork force through the shift solenoid current rather than axis pressure (e.g. Getrag GS7) use Variable Force shift solenoids. While a solenoid is energised, its current is taken from its Shift Solenoid # Current table. The table’s axis can be chosen to suit the fork movement mode:

Leave Axis Pressure Override OFF on these transmissions.

Torque Limiting During Fork Movement

Active Axis Fork Movement Torque Limit can be applied applied while a fork on the currently active axis is moving (e.g. Shifts out of Neutral/Park, where the axis being engaged is also the “active” axis).

Engagement Phase Control

Engagement Phase Control is an alternative way of driving a moving fork, built around how a synchroniser engages a gear. Instead of simply pushing the fork toward its target position, the TCM steps the fork through a series of phases (approach, synchronise, engage and settle), each with its own force. If the fork stalls, it’s backed off, rested and tried again with a little more force, rather than being held hard against the synchroniser.

This makes the biggest difference when engaging a gear from Neutral or Park at a standstill, where drag from the open clutch keeps the shaft spinning. A fork that is just pushed harder tends to sit on the synchroniser, using it as a brake against the clutch drag. Engagement Phase Control detects this, backs off and retries, and limits how hard and how long the synchroniser is worked.

It works with both pressure-actuated forks (e.g. Nissan GR6) and forks whose force is set by the shift solenoid current (e.g. Getrag GS7).

Enabling Engagement Phase Control

Set Engagement Phase Control to ON in Shift Fork Setup. The Shift Fork Engagement Phase Control folder will then appear, containing the settings and tables described below.

All fork forces are set as a percentage (0 - 100%) of the fork’s maximum force. How that percentage is turned into an output depends on how the forks are actuated:

Fork Travel and Sync Slip

Two channels are central to how Engagement Phase Control works, and to tuning it:

• Shift Fork Travel is the moving fork’s progress, from 0% at the slot it’s leaving to 100% at its target slot. Where the synchroniser sits within that travel is set by Sync Zone Start and Sync Zone End. The sync zone is shared by all forks in both directions, so it must cover every fork’s synchroniser.
• Shift Fork Sync Slip is the speed difference the synchroniser has to remove before the gear can engage: the speed of the target axis clutch compared to the speed the target gear would turn it at. The shaft is considered synchronised once this falls below Synchro Slip Complete Threshold. Correct clutch speed and output shaft speed readings are required.

Move Phases

Each fork move steps through the following phases, reported by Shift Fork Move Phase:

A fork returning to Neutral has no synchroniser to work against, so it goes straight from Approach to Engage.

Stalls and Retries

When a fork stops making progress toward its target, the TCM records why in Shift Fork Stall Type, backs the fork off, rests, then tries again.

Retries work as follows:

• Each retry raises the Sync force ceiling by Sync Retry Force Step (up to 100%). The first attempt can be gentle, with force only increasing when the synchroniser needs it.
• Each fork is allowed its Shift Fork # Retry Limit retries after the first attempt. Once they have all failed, the fork enters its position error state.
• The fork’s Error Delay still limits the total time of the whole move, including every attempt.
• After a synchroniser stall (Blocker Ring or Equilibrium), the rest ends early as soon as the shaft is synchronised.
• After a Dog Teeth stall, the rest ends early as soon as the shaft has turned relative to the gear, giving the teeth a new position to engage.

Shift Fork Crash Count counts (per fork, since power up) engagements where the dog teeth went in while the shaft was still slipping above Synchro Slip Complete Threshold, i.e. the gear was forced through the synchroniser. A rising count means the Sync force ceiling is too high, or there is too much drag on the target axis.

Synchroniser Heat Protection

Repeated attempts heat the synchroniser. Shift Fork Sync Energy estimates this for each fork by accumulating slip x force x time while the synchroniser is loaded, and decays over roughly 20 seconds.

While Shift Fork Sync Energy is above Sync Energy Limit, the rest between attempts is extended from Retry Rest Time to Sync Energy Limit Cooldown. Set Sync Energy Limit to 0 to disable.

Standstill Engagement Aids

Two optional functions help engage a gear from Neutral or Park while the vehicle is stationary.

Dog Engagement Stall Clutch Pulse
If the fork stalls on the dog teeth with the shaft already synchronised, the teeth are likely sitting tip to tip. When enabled, the target axis clutch is briefly applied during the rest, at its touch point + Dog Engagement Stall Clutch Pulse Pressure for Dog Engagement Stall Clutch Pulse Time. This turns the shaft a few degrees so the teeth can line up on the next attempt.

• Only the clutch on the axis being engaged is pulsed, and only while nothing is engaged on that axis.
• It’s never used after a synchroniser stall.
• Shift Fork Clutch Pulse Status is ON while the pulse is applied, and the clutch status reports On - Shift Fork Clutch Pulse.

Fork Move Idle Target (Shift Fork Setup)
A lower engine idle speed means less clutch drag for the synchroniser to overcome. While a gear is being engaged from Neutral or Park at a standstill, the Fork Move Idle Target is output on Shift Fork Idle Speed Request (0 at all other times). To use it, transmit Shift Fork Idle Speed Request to the ECU in a CAN message and configure the ECU to use it as its idle target. Set to 0 to disable.

Engagement Phase Settings

Found in Shift Fork Engagement Phase Control > Shift Fork Engagement Phase Setup.

Force Tables:

Tuning Engagement Phase Control

1. Complete the standard Calibration Procedure first. Fork travel is measured between the calibrated slot positions, so these must be accurate.

2. Check Shift Fork Sync Slip reads sensibly during a fork movement. It relies on correct clutch speed and output shaft speed readings.

3. Set up the force range for the fork actuation type, see Enabling Engagement Phase Control.

4. Enable Engagement Phase Control and log a number of engagements from Neutral at a standstill, as well as pre-selections while driving. Log Shift Fork Move Phase, Shift Fork Stall Type, Shift Fork Move Attempt, Shift Fork Travel, Shift Fork Sync Slip, Shift Fork Force Demand and Shift Fork Crash Count, along with each fork’s Position.

5. Set the sync zone from Shift Fork Travel at the point where each fork stalls on the synchroniser. Sync Zone Start should be below the lowest stall point, and Sync Zone End above the highest.

6. Set Shift Fork Sync Force Start low enough that the synchroniser has a chance to work before the force ramps up to its ceiling. Only then raise Sync Force Max if forks still fail to engage.

7. Watch Shift Fork Crash Count. A rising count means the force ceiling is overpowering the blocker ring rather than letting the synchroniser work. The fix is less force or less drag (e.g. a lower idle with Fork Move Idle Target), not more force.

8. Review stall types at a standstill. Repeated Equilibrium stalls mean clutch drag is too high for the synchroniser to overcome, so reduce drag rather than adding force. Repeated Dog Teeth stalls with the shaft synchronised can be helped by the Dog Engagement Stall Clutch Pulse.

9. Set each fork’s Retry Limit and Error Delay last, once engagement is reliable.

Errors & Diagnostics

Per-Fork Position Error

If a fork’s error-detection is enabled, a fork that fails to reach its target slot within its Error Delay (or whose position sensor reports a fault) raises a Fork # Movement fault and enters an internal error state.

From the error state, the firmware automatically retries the move, up to the fork’s Retry Limit, 1 second apart. With Engagement Phase Control ON, retries are made within the move itself (see Stalls and Retries), so there are no further automatic retries once the fork has entered the error state. If Error Auto Clear is enabled, the error clears automatically once the fork successfully reaches its target slot again (and the position sensor is healthy) — this is normally only allowed to happen when the fork’s target is Centre (Neutral), so a fork always has to prove it can get back to a safe state before being trusted again.

Using the tuning software’s “Clear All Codes” action also resets every fork’s retry count, in addition to clearing the fault codes themselves — so a fork that had exhausted its retries gets a fresh set of attempts on its next movement, rather than immediately re-latching into a fault.

Axis errors

Each axis is also monitored as a whole:

While an axis is in fault, no fork on that axis will be moved until the fault clears.

Related channels for diagnosis

See Runtime Channels for the full list. Each fork’s Status and Position Error, and the Moving Shift Fork channel, are usually the first things worth logging when diagnosing a shift fork issue.

Runtime Channels

Per-Fork Channels (# = fork number, 1–8):

System-Wide Channels:

Engagement Phase Control Channels:

Calibration Procedure

1. Enable each fork — set the Axis setting for every physical fork fitted (Axis A, B, or Both for a shared Reverse fork). Leave unused fork slots OFF.

2. Wire up and calibrate position sensors first, via the standard analog input mapping/translation tools, so that each fork’s Position channel reads a correctly-scaled value (mm) across the whole range of travel.

3. Set tolerance bands (Low/Centre/High Tolerance) tight enough to confirm the dog teeth/synchro are actually engaged, but loose enough to tolerate normal sensor noise and mechanical play.

4. Assign gears to each slot via Position L Gear / Position H Gear. You may have to put some educated guesses into the positions initially.

5. Assign shift solenoids per fork (Positive/Negative Solenoid Select) and confirm direction — command a move with Test mode and verify the fork moves the correct way and its Status reports moving in the expected direction, then settles into the correct slot.

6. Set idle/default solenoid patterns (Idle Solenoid Select, Default Select / Enable) as required by the actuator hardware.

7. Find and record the three slot positions for each fork with the vehicle safely supported and the driveline free to move by hand, or using the fork’s Test setting to command it to each slot in turn (only works while the requested gear is Neutral or Park):

   • Force to Low, record the settled Position reading into Position L.
   • Force to Centre, record Centre position.
   • Force to High, record Position H.
   • Set Test back to Off when done.

8. Tune move-complete detection — Stable Velocity and Stable Time — so a fork isn’t reported as “done” while still settling, but without adding unnecessary delay to every shift.

9. Tune fork movement pressure (if using closed-loop pressure control): set the Movement Pressure Base and PID gains, and the Pressure Override Min/Max and Integral Min/Max clamps, then set Axis Pressure Override to ON if pressure-based fork actuation is desired instead of fixed axis pressure targets. If using Engagement Phase Control, follow Tuning Engagement Phase Control instead.

10. Set error detection last, once movement is proven reliable. Set each fork’s Error detect / Auto-Clear and Error Delay, generous enough to allow for normal shift timing but tight enough to catch a stuck fork.

11. Verify full shift sequences in all gears, both directions, watching the Active Gear Shift Fork, Preselected Gear Shift Fork and Moving Shift Fork channels. Watch each fork’s Status & Position, for consistent, movement and engagement. Confirm no Axis Bound/Axis Fault faults occur across the full gear range.