Torque Converter Lockup

The torque converter lockup clutch mechanically joins the engine to the transmission input, bypassing the converter’s fluid coupling. With the clutch locked there is no converter slip: less heat in the transmission oil, better fuel economy, and a direct connection between the throttle and the wheels. With it unlocked the converter can slip and multiply torque, which helps launches and low-speed driving.

The TCM decides when the lockup clutch should be locked or unlocked, how much pressure to apply, and how quickly to apply and release it. Each of these can be tuned separately for each phase of operation (takeup, launch, in gear, up shifts and down shifts).

Torque Converter Lockup applies to transmissions with a torque converter, such as the ZF 8HP. Dual-clutch transmissions don’t have a converter and don’t use it.

Enabling

Assign the lockup solenoid to an output in Output Config > Transmission Functions (Torque Converter Lockup Solenoid Output). Once an output is assigned, the Torque Converter Lockup folder appears.

The TC Lockup Setup page holds the general settings:

Solenoid Translation

TC Lockup Solenoid Translation converts the target lockup pressure (bar) into a solenoid current (A). Fill it in from the solenoid and valve body characteristics, the same way as the clutch solenoid translation tables (see Clutch Pressure Control).

The TCM always looks this table up by the lockup pressure, so the X axis values must be in bar. If the X axis is set to any other channel, the TC Lockup Config Invalid DTC is set.

Operating Phases

At any moment the lockup system is in one phase, and each phase has its own set of tables. Phases are listed here from highest to lowest priority; the first one that applies is used.

The Launch folder is only shown when Launch Control is enabled, and the Dyno Mode folder is only shown when Dyno Mode is not Off.

State Tables: When to Lock

Each phase’s State table decides whether the lockup clutch should be locked:

Fill the cells with 0 and 100 and let the interpolation between neighbouring cells form the hysteresis. The value has to reach 100 to lock and 0 to unlock, so the lock and unlock points are exactly where the 100 and 0 cells are. For example, with the X axis on Output Shaft Speed, a cell of 0 at 1200 RPM and a cell of 100 at 1600 RPM gives:

• Lock when rising to 1600 RPM.
• Unlock when falling to 1200 RPM.

Spacing the two cells further apart widens the hysteresis.

The axes can be any channel. Typical choices are Output Shaft Speed or Drive Speed against Pedal Position or Throttle Position. Useful extras:

• Transmission Fluid Temperature: unlock earlier or lock later as the fluid temperature changes. For a simple cold-fluid inhibit or hot-fluid lock, use the built-in options instead (see Fluid Temperature and Brake).
• Pedal position: unlock at high pedal so the converter can multiply torque when accelerating hard.

For standing starts, the TC Lockup Takeup State and TC Lockup Launch State tables are normally left at 0, so the converter can slip and multiply torque. Lock after the launch using a speed axis.

Lockouts

Any of the following lockouts unlocks the clutch, regardless of the State tables:

Lockouts release the clutch at TC Lockup In Gear Release Rate. Setting a minimum to 0 disables that check. If more than one lockout is active at the same time, the status shows only one of them.

The User Enable lockout can be used to add your own conditions, for example a dash switch or a User Function based on other channels.

Fluid Temperature and Brake

Cold Fluid Inhibit

With Cold Fluid Inhibit on, the lockup clutch stays unlocked while Transmission Fluid Temperature is below Cold Fluid Threshold (Lockout - Fluid Temp Low). Cold fluid makes the lockup slow and harsh, and letting the converter slip warms the fluid up faster. The lockout ends once the fluid is Fluid Temp Hysteresis above Cold Fluid Threshold.

Hot Fluid Lock

With Hot Fluid Lock on, the lockup clutch is locked in forward gears while Transmission Fluid Temperature is above Hot Fluid Threshold, whatever the In Gear or Gear # State table asks for. The status shows On - Fluid Temp High. A slipping converter is one of the biggest heat sources in the transmission, so locking it helps the fluid recover. Once the fluid has cooled Fluid Temp Hysteresis below Hot Fluid Threshold, the State table is in control again.

• Only the In Gear phase is forced. The Up Shift, Down Shift, Launch and Takeup State tables still apply. Fill the shift State tables with 50 to keep the clutch locked through shifts while hot.
• All lockouts still apply, including the speed minimums and the brake, so the clutch is never forced locked at a standstill.
• Reverse is never forced.

Neither Cold Fluid Inhibit nor Hot Fluid Lock acts while the transmission fluid temperature sensor has a fault DTC.

Brake Unlock

Pressing the brake can unlock the lockup clutch (Lockout - Brake), using either or both of:

• Brake Switch Unlock: unlocks while Brake Switch 1 is on.
• Brake Pressure Unlock: unlocks while Brake Pressure Front is at or above this pressure. The lockout ends once the pressure falls below half of it, so a light brake pressure hovering around the setting doesn’t make the clutch lock and unlock. 0 turns it off.

The clutch unlocks at any speed. When the brake is released, it locks again if the State table asks for lock.

Lockup Pressure

While lockup is requested, the TCM calculates the lockup clutch pressure from the engine torque:

1. Take the engine torque, in either direction (driving or engine braking), and add TC Lockup Clutch Torque Margin.
2. Convert the result to a pressure using TC Lockup Clutch Torque Capacity.
3. Add TC Lockup Clutch Touch Point.
4. Limit the result to Max Pressure.

So the clutch is applied firmly under load and gently at light load, and the pressure follows the engine torque while the clutch is locked. The engine torque comes from Engine Torque (Supplied), so an accurate torque source matters (see TCM Torque Model and Engine Torque).

With Inertia Corrected Torque on, Engine Torque (Inertia Corrected) is used instead. A locked clutch also has to carry the torque from the engine speeding up or slowing down, which is large through an up shift or a rev-matched down shift. Using the inertia corrected torque sizes the pressure for that, instead of relying on TC Lockup Clutch Torque Margin. Check Torque Converter Lock Up Pressure in a log after turning it on: the inertia term can be noisy, and the pressure follows torque increases immediately while locked.

When line pressure is set to target automatically, the TCM keeps it at or above the lockup pressure, so the lockup clutch always gets the pressure it is commanded.

Fast Fill

When the clutch is applied from fully released, TC Lockup Clutch Fast Fill Pressure is commanded straight away for TC Lockup Clutch Fast Fill Time to fill the lockup piston quickly. When the fill ends, the pressure drops straight to the calculated lockup pressure if the fill pressure was higher, or rises to it at the Apply Rate if the fill pressure was lower. Fast fill is only used again once the clutch has been fully released.

Too much fast fill pressure or time makes the lockup harsh. Too little makes it slow to lock.

Apply and Release Rates

Each phase has an Apply Rate and a Release Rate table (bar/s) that limit how fast the lockup pressure changes:

• Apply Rate: sets how quickly the pressure rises from the end of the fast fill up to the calculated lockup pressure. This is what sets how firmly the clutch locks. Once the pressure has reached the calculated lockup pressure, it follows increases in engine torque immediately, so the clutch doesn’t slip when the throttle is opened while locked.
• Release Rate: sets how quickly the pressure falls when unlocking, and when engine torque drops while locked.

The rates of the phase that is active at the time are used. For example, an unlock requested by the Up Shift State table releases at TC Lockup Up Shift Release Rate. Lockouts always release at TC Lockup In Gear Release Rate.

A rate of 0 means no limit: the pressure changes to its new value immediately.

Slip Monitoring

Slip monitoring checks that the lockup clutch actually locks. Once the lockup pressure has reached its calculated value, if Converter Slip stays above Slip Threshold for longer than Slip Time, the TC Lockup Slip DTC is set and the Slip Protection action is applied:

A slipping lockup clutch at load puts a lot of heat into the lining and the fluid, so it’s worth setting up even if you only use DTC Only. Common causes are a Torque Capacity that is too high (pressure too low for the torque), an engine torque reading that is lower than the real torque, and a worn clutch.

Dyno Mode

Dyno Mode makes the lockup clutch follow the TC Lockup Dyno State table in every phase. The In Gear Apply and Release Rate tables are used, and are also shown in the Dyno Mode folder (they are the same tables).

The lockouts still apply in Dyno Mode. On a dyno without a vehicle speed signal, set Drive Speed Minimum to 0.

Monitoring

The lockup system generates the following runtime channels:

Torque Converter Lock Up Clutch Status reports Open whenever Torque Converter Lock Up Pressure is at or below TC Lockup Clutch Touch Point. Above the touch point it reports Lock when Converter Slip is within Lock Threshold (plus Lock Hysteresis once locked), and otherwise Sync, Slip or Open by comparing the estimated lockup capacity against engine torque. It is for display and logging only and doesn’t change how the clutch is controlled.

Tuning Procedure

1. Output and translation. Assign the output and fill in TC Lockup Solenoid Translation with its X axis on Torque Converter Lock Up Pressure. Set Max Pressure to the highest pressure the lockup clutch should ever see.
2. Start unlocked. Set every State table to 0, and set Engine Speed Minimum comfortably above idle as a stall backstop.
3. Touch point. Use the transmission’s specification if you have one. Otherwise, lock the clutch at a steady light-throttle cruise with a slow In Gear Apply Rate, and log Torque Converter Lock Up Pressure and Converter Slip. The touch point is the pressure at which Converter Slip first starts to fall.
4. Capacity and margin. Lock the clutch at cruise, then add load. If Converter Slip rises while the clutch should be locked, the pressure is too low: lower TC Lockup Clutch Torque Capacity or raise TC Lockup Clutch Torque Margin. If lockup is harsh at light load, the pressure is higher than needed.
5. In Gear State. Lock in the higher gears above a cruising speed, and unlock well before the car slows to the lockout minimums. Optionally unlock at high pedal for torque multiplication. Turn on Cold Fluid Inhibit, Hot Fluid Lock and brake unlock as needed.
6. Apply and release feel. Adjust TC Lockup Clutch Fast Fill Pressure/Time and the Apply Rate tables until lockup is quick without a bump. Adjust the Release Rate tables so unlocking is smooth but finishes before the car needs converter slip.
7. Shifts. Decide per shift direction whether to unlock (0), hold (50) or lock (100), then check shift quality in the logs.
8. Takeup and Launch. Normally leave these at 0 and let the In Gear table lock the clutch once the car is moving.
9. Slip monitoring. Once lockup is working, choose a Slip Protection action, then set Slip Threshold and Slip Time from your logs.

Channels worth logging while tuning:

• Torque Converter Lock Up Status
• Torque Converter Lock Up Request
• Torque Converter Lock Up Pressure
• Torque Converter Lock Up Solenoid Current
• Torque Converter Lock Up Clutch Status
• Converter Slip
• Engine Torque (Supplied)
• Output Shaft Speed
• Gear
• Your State table axes