Engine Control Units (ECUs), ABS Pumps and Automotive Electronics

A faulty transmission control module (TCM) most often causes limp mode with the gearbox stuck in one gear, harsh jolts when you select D or R, erratic gear changes, flashing PRND letters and a gearbox or engine warning light. However, the same symptoms are often caused by a low gearbox oil level, a weak battery, a poor earth or a faulty sensor. To test a transmission control module, start by checking the battery and reading the fault codes with a diagnostic tool that can talk to the gearbox control unit, then check the power supply, the connector, the oil and the live data. Below we explain the symptoms, the diagnostic steps and what the most common fault codes mean.

A burned engine control unit (ECU) usually causes a sudden no-start: the engine cranks but will not start, the diagnostic tool cannot connect to the ECU, and the engine management light does not come on when you switch on the ignition. The most common causes are a voltage spike when jump-starting, reversed battery polarity, a faulty voltage regulator and a short circuit in the wiring loom or in an actuator. Below we explain how to tell a burned ECU from a power supply fault, what causes this kind of failure and how to choose a replacement ECU.

A burnt-out or flooded automatic transmission control module (TCM) usually causes sudden symptoms: limp mode with the gearbox stuck in one gear, harsh thumps when you select D or R, no response to the gear selector, flashing or blank PRND letters and no communication between the diagnostic tool and the gearbox control unit. Flooding is most often caused by blocked scuttle panel drains, washing the engine bay or coolant in the gearbox oil, and a burnout by overheating, a short circuit or a voltage spike. Below we explain the symptoms, what to do straight away and why drying out or repairing such a control unit rarely gives a lasting result.

A water-damaged or burnt-out ABS pump control module most often lights up the dashboard like a 'Christmas tree' (ABS, ESP and the red brake warning light), leaves the diagnostic tool unable to communicate with the ABS module, makes the pump motor run non-stop or stops the speedometer working. Water usually gets into the module when the scuttle panel drains are blocked, after the engine bay has been cleaned with a pressure washer or after driving through a deep puddle, while a burnout is most often caused by a short circuit or a voltage spike. Below we describe the symptoms, what to do straight away and why drying or repairing such a module rarely gives lasting results.

A flooded engine control unit (ECU) most often causes a no-start, a 'Christmas tree' of warning lights on the dashboard, no communication with the diagnostic tool, or rough running in limp mode. Water usually gets into the ECU from blocked scuttle drains, after the engine bay has been cleaned with a pressure washer, or after driving through deep water. Below we describe the symptoms of a water-damaged ECU, what to do straight after the ECU gets wet, and why drying or repairing it rarely gives lasting results.

The most common symptom of a faulty ABS pump or ABS module is an ABS warning light that stays on after the engine starts, often together with the ESP light. The brakes still work, but without ABS the wheels can lock under hard braking. To test an ABS module, start by reading the fault codes with a diagnostic tool that can talk to the ABS unit, then check the power supply, the earth and the wheel speed sensors. Only when all of these are fine is the module itself usually to blame.