In short
The TCU (transmission control unit) is the unit that runs an automatic or dual-clutch (DSG) gearbox: it decides when to shift, how hard the clutches clamp and how much torque it accepts from the engine. If the engine is remapped and the gearbox has a programmed torque limit, it may cut power or protect the transmission, so sometimes the TCU needs adapting too. And after replacing clutches or the mechatronic unit, adaptations have to be run so the gearbox learns the new engagement points. The first step is checking the gearbox is healthy: a mechanical fault isn't fixed with software. Price on request, in Alcalá de Henares or with us coming to you anywhere in the Community of Madrid.
What the gearbox control unit controls
A modern automatic gearbox is a hydraulic and mechanical system run by electronics. Its control unit, the TCU (transmission control unit), is sometimes inside the gearbox itself, built into what is called the mechatronic unit: a block containing the electronics, the solenoid valves and the sensors. What the TCU decides:
- When to shift, based on speed, pedal position, load and driving mode.
- How much pressure closes the clutches or the gearbox's internal brakes. More pressure means more torque transmitted and firmer shifts.
- How much torque it accepts from the engine. The TCU and the engine ECU talk to each other constantly over the car's network: during a shift, the TCU asks the engine to drop torque for an instant; and if the engine tries to deliver more than the gearbox accepts, the gearbox limits it or protects itself.
- How the gearbox has adapted to its own wear, with learned values that keep being corrected with use.
The gearbox as the limit of an ECU remap
This is why many remaps on automatic cars end up here. If the engine's torque is raised with a Stage 1 or a Stage 2 and the TCU has a lower maximum torque programmed, three things can happen:
- The gearbox cuts back: the car doesn't perform as it should in the gears where the TCU limits it.
- The clutches slip: if the programmed pressure isn't enough for the new torque, the clutch slips, heats up and wears.
- The gearbox goes into limp mode: it detects slip it isn't expecting and protects itself.
That is why, on an automatic, we read both control units before deciding anything. Sometimes it is enough to limit engine torque in the lower gears; other times it is better to adapt the TCU so it accepts the new torque with matching pressures.
When the TCU needs work
| Situation | What is done in the TCU | What has to be checked first |
|---|---|---|
| Engine remap that exceeds the torque limit | Raise the torque limit and adjust pressures | That the gearbox has genuine mechanical headroom |
| Behaviour you don't like (shifts up too early, hesitates on downshifts) | Adjust shift points and times | That it isn't a symptom of a fault |
| Replacement of clutches, mechatronic unit or gearbox | Adaptations and learning | Correct oil, level and temperature |
| Replacement gearbox or one from another vehicle | Coding and adaptation to the car | That the part number is compatible |
Types of gearbox and what each one allows
Not all automatic gearboxes are the same, and what can be done with each varies a great deal:
- Dry dual-clutch (dry DSG). The clutches run without an oil bath. They are light and efficient, but suffer with heat and repeated launches. They have little headroom for more torque and it isn't wise to make the shifts more aggressive.
- Wet dual-clutch (wet DSG). The clutches are oil-cooled and cope with more torque and more heat. They usually accept TCU adaptations for remaps, provided the oil and filters are up to date.
- Torque converter automatic. The classic planetary gearbox with a hydraulic converter. It usually has mechanical headroom, and the TCU also manages the converter lock-up, which affects response and fuel consumption.
- Continuously variable transmission (CVT). It drives through a belt or chain between variable pulleys; there is little headroom and the work is usually diagnostics and adaptation.
- Automated manual. A manual gearbox with actuators that operate the clutch and change gear by themselves. The usual job is adapting the clutch engagement point after a repair.
The torque each gearbox accepts depends on its exact version, not its trade name, so we don't give generic figures: we check it when we read your TCU. Each manufacturer uses different gearboxes and mechatronic units; in the Volkswagen Group, for example, DSG gearboxes are very common, as we explain under VAG Group.
Adaptations and learning after a repair
An automatic gearbox doesn't work properly just because the parts are fitted. After replacing clutches, the mechatronic unit or the whole gearbox, the TCU has to learn the values of the new parts:
Clear the learned values
those from the old parts are no longer valid and would confuse the gearbox.
Starting conditions
oil at the level and temperature the procedure requires, engine idling, car stationary.
Adaptation procedure
with the diagnostic equipment we run the routine that measures each clutch's engagement points and the pressures.
Road test
some gearboxes finish learning on the road, with shifts under different conditions.
Final read-out
we check there are no faults left and that the learned values are within the expected range.
Skipping any of these steps is the most common cause of gearboxes that "have been jerky ever since the repair".
When not to touch it
Software doesn't fix a mechanical problem; it only hides it for a while. We don't program a TCU if the gearbox has:
- Slipping under acceleration, revs rising without the car moving forward.
- Clunks when engaging or going from neutral into gear.
- Noises from bearings or gears.
- Burnt oil, oil with metal particles or oil not changed at the proper interval.
- Gearbox temperature warnings in normal use.
In those cases the right thing is electronic diagnostics and the repair. Once the gearbox is healthy again, we can talk about software.
Price
It is price on request: it depends on the gearbox, on whether the job is an adaptation or TCU programming, and on whether it goes together with an engine remap (which starts from from €200 + VAT (€242 including VAT)). We give you a firm price with the make, model, engine, year and type of gearbox.
Tell us the vehicle, which gearbox it has and what is happening or what you want to do. We will tell you whether it is a matter of software, adaptation or mechanics.
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