First, make sure the control unit is to blame
Symptoms similar to a faulty control unit can also be caused by too little oil in the gearbox, a weak battery, a poor earth, moisture in the connector or worn gearbox internals. Before you pay for a repair, read the fault codes with a diagnostic tool that can talk to the gearbox control unit, and check the power supply, the connector and the oil. We describe the whole procedure and what the fault codes mean in our guide on how to check the operation of a transmission control module - symptoms and tests.
Control unit, mechatronic unit or gearbox mechanics - what goes wrong
On older cars, the gearbox control unit is a separate module that can be replaced without opening the gearbox. In many newer gearboxes (e.g. DSG, S tronic, 7G-Tronic), the electronics are combined with the valve body into a mechatronic unit, which works in oil, at high temperatures and under vibration. What fails then is either the electronics of the mechatronic unit or its hydraulic part (valves, sensors). If the gearbox mechanics are damaged (clutches, bearings), replacing the control unit alone will not help.
What does gearbox ECU repair and reconditioning involve?
An automotive electronics specialist tests the control unit, opens the housing and looks for the damaged component. During reconditioning, the parts most often repaired or replaced are:
- cracked solder joints - they crack as a result of vibration and changes in temperature, and the fault comes and goes, e.g. once the gearbox has warmed up;
- the components that drive the solenoid valves - transistors, which usually burn out after a short circuit in a solenoid or its wiring;
- power supply components - damaged by a voltage spike;
- sensors in the mechatronic unit, e.g. speed sensors, if they can be replaced separately.
After the repair, the control unit should pass a test, and once it is back in the car, the fault codes need to be cleared, the adaptations carried out and the car taken for a test drive.
Which faults can be repaired and which cannot
A transmission control unit repair has the best chance of success when a single, known component is damaged and the circuit board, processor and memory are intact. The chances are slim when:
- the control unit has been flooded or has burnt out - corrosion carries on spreading even after the unit has dried out, and a short circuit destroys the tracks inside the circuit board (find out more in our guide to a flooded automatic transmission control module or burnt TCM);
- coolant or metal particles from a worn gearbox have got into the mechatronic unit;
- the same fault comes back after an earlier repair;
- the gearbox mechanics are damaged - in that case, repairing the control unit will not cure the symptoms.
How much does gearbox ECU repair or replacement cost?
There is no single price, as it depends on the type of gearbox, the kind of fault and the car model. When comparing quotes, add up the whole cost:
- for a repair - the diagnosis, the repair itself, removing and refitting the control unit or mechatronic unit, shipping to the workshop and the time the car is off the road;
- for a replacement - the price of a control unit or mechatronic unit with the same numbers, fitting, programming and adaptations, and for a mechatronic unit also the oil and often the gearbox filter.
A new control unit or mechatronic unit from a main dealer is usually much more expensive than a used original, and for older cars it may no longer be available. That is why a repair is most often compared with a used, tested part of the same type. You will find current prices in our Transmission control unit (TCU) category.
Repair vs a used gearbox control unit - comparison
- Your car's data. After a repair, you get back the same control unit with your car's data and adaptations. A used control unit has the previous car's data, so it has to be programmed or have the data from your old control unit transferred to it.
- Certainty. After a repair, you cannot always be sure that other components on the circuit board have not been weakened. We check every used part on a test bench before dispatch.
- Time. A repair means shipping to the workshop, diagnosis and the repair itself, and the car is usually off the road in the meantime. We dispatch a used part from stock straight away.
- When to choose a repair: when the electronic fault is minor and known, the part is hard to find or you want to keep the original unit.
- When to choose a replacement: after water damage or a burnout, with a fault that keeps coming back, or when you need the car back quickly.
How to match a used gearbox control unit
A control unit is matched by its part numbers, not just by the car model. The gearbox software depends on the engine version, model year and specification, so control units differ even within the same model. Note down or photograph:
- the car manufacturer's part number (OE) - e.g. on Volkswagen Group DSG gearboxes, in the format 0AM 927 or 0B5 927;
- the electronics manufacturer's number - e.g. Bosch, Continental, Temic or Delphi;
- the hardware and software version numbers (HW, SW), if they are on the label.
On gearboxes with a mechatronic unit, also check whether the listing is for the complete mechatronic unit or the electronic module only, and choose the one that suits your gearbox. The numbers in the listing must match the numbers on your old part. If you are not sure, send us a photo of the label.
Replacing a gearbox control unit step by step
- Make sure the control unit is to blame: check the battery, the power supply, the connector, the oil and the fault codes.
- Note down the numbers from the old control unit or mechatronic unit and choose a part with the same numbers.
- Fix the cause of the failure, e.g. moisture in the connector, a short circuit in the wiring or coolant in the oil.
- Replace a separate control unit with the battery disconnected. Fit a mechatronic unit in clean conditions, as any dirt in the gearbox causes damage, and top up the oil according to the manufacturer's instructions.
- Program the control unit to the car. On many cars, the gearbox control unit is linked to the VIN, and on some makes also to the immobiliser. An automotive electronics specialist can also transfer the data from your old control unit to the one you have bought (cloning), as long as the old unit's memory can be read.
- Carry out the adaptations (gearbox learning), clear the fault codes and take the car for a test drive.
Transmission control units at Auto24Parts
We mainly sell used, original transmission control units, and some models have been reconditioned. Before dispatch, every control unit is checked on a test bench: electrically, electronically, visually and mechanically. Used parts come with a 6-month warranty. You can return a part within 14 days of receiving the parcel (see our warranty and returns policy).
Not sure which control unit fits?
Email us all the numbers from your old control unit or a clear photo of its label to contact@auto24parts.com. We will check compatibility before you place your order. We ship by DHL Express or FedEx Express, and delivery within Europe, including the UK and Ireland, usually takes around 1-2 working days from dispatch. You will see the exact shipping cost at checkout once you enter your address. The UK is outside the EU, so customs duties and import charges may be payable by the recipient (shipping and delivery).
Frequently asked questions
Is gearbox control unit reconditioning worth it?
It is worth it for a minor, known electronic fault, when the gearbox mechanics are working and the part is hard to find. After water damage, a burnout or a fault that keeps coming back, a used control unit or mechatronic unit with the same numbers usually works out cheaper and more reliable.
Does the control unit need programming after a repair?
Usually not, because you get back the same control unit with your car's data. After fitting, however, the fault codes need to be cleared, and the gearbox adaptations often need to be carried out.
Mechatronic unit or control unit only - which should I buy?
It depends on the gearbox and the fault. If only the electronics are damaged and the electronic module can be replaced separately, the module on its own, with matching numbers, is enough. If the hydraulic part is damaged too, or the module cannot be replaced on its own, you need the complete mechatronic unit.
Can I replace the gearbox control unit myself?
You can swap a separate control unit in the cabin yourself, but programming and adaptations need to be done by a garage with the right diagnostic tool. Replacing a mechatronic unit is best left to a garage that knows that gearbox.
What is gearbox control unit cloning?
An electronics specialist reads the data from your old control unit and writes it to a working control unit with the same numbers. The control unit you have bought then takes on your car's data. Cloning is only possible if the memory of the old control unit can be read.
How long will I have to wait for the control unit?
We dispatch control units from stock as quickly as possible. With DHL Express or FedEx Express, delivery within Europe, including the UK and Ireland, usually takes around 1-2 working days from dispatch, and the exact shipping cost is shown at checkout.
