Melett Newsletter 12

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Common Turbo Failures:
REA/SREA Failure Modes
What is variable geometry and why are these systems used?
When a turbocharger is matched to an engine, it is essential to balance the low speed
response with high speed efficiency. The nozzle ring assembly is designed to change the
exhaust gas inlet area with the engine speed to closely match the desired boost
requirements of the engine. As the engine speed increases, the electronic actuator moves
the nozzle ring assembly vanes to the fully open position to maximize the exhaust gas flow.
REA / SREA electronic actuators are highly complex and intricate. They are not
interchangeable with different gearboxes or black caps. The calibration settings are
programmed in the software within the black cap and are unique to each turbocharger.
Benefits of Electronic Actuators Include:
• Rapid response to changes in speed;
• Accuracy of minimum and maximum air flow;
• Feedback loop to the vehicle’s ECU - indicates where the actuator position is and
learns what boost is required.
Faulty Electronic Actuator Symptoms:
• Complete loss of power, causing the vehicle to go into limp home mode;
• Flashing engine management light;
• Intermittent low pressure, or over boost.
Common Failure Modes:
• Gearbox Failure - REA and SREA will fail as a result of the variable geometry
mechanism sticking due to sludging or carbon build up. When the variable
geometry mechanism sticks, it pulls a higher current through the motor than it is
designed to take, this therefore causes the motor to burnout or the plastic worm
gears to fail. This restriction can reduce boost pressure and can result in the
vehicle being put into ‘limp home mode’. In most failures, the black cap and
electronics are unaffected by the gearbox failure and can be reused.
• Board Connectors - The wire connectors can expand and contract, eventually
breaking and causing failure of the electronic actuator. This fault can often go
undetected during repair and workshop testing, as the fault can only be detected
once the actuator heats up.
• Mishandling of the Turbo - If the turbo connector is knocked or banged, it will break
and the whole unit will need replacing.
• Water Ingress - The location of a turbo in the engine compartment can mean the
electronic actuator is more susceptible to water ingress. The actuator can become
rusty and contaminated, giving the wrong signals and ultimately breaking.
• Engine Vibration - Constant vibration from the vehicle can wear the electronic
actuator out, causing it to fail over a period of time.
• Loss of Power - If the electronic actuator has failed to open the nozzle ring
assembly vanes under acceleration, the turbo will fail to operate efficiently. If the
vanes are set to a closed position, it can cause choking of the engine or
overspeeding of the turbine. Alternatively, if the vanes are open more than is
required, the turbo will have too much ‘lag’ and be slow to respond. If this occurs,
it is essential to check if the electronic actuator has failed as a consequence of
other failure modes.
Notes for Repair:
• When repairing the electronic actuator the worm gear and motor must be the
correct ratio to avoid immediate failure.
• REA actuators must be calibrated to the turbo after repair. Incorrect calibration of
the electronic actuator when assembled on the turbocharger can result in poor
performance.
• Soldering to repair broken connections in the gearbox and black cap is not advised.
Solder is susceptible to cracking in environments with temperature variance and
vibration. For this reason, the motor and motor choke assembly connector must be
spot welded.
For further information on this or other topics, visit the TurboHub
, or contact our team via sales
REA (Rotary Electronic Actuator) or SREA (Simple Rotary Electronic Actuator) are fitted to many variable
geometry turbos and control the variable vane movement. This article explores the benefits of the REA and
possible causes or symptoms of failure.
Worm gear failure due
to restricted VNT mechanism
CIPOS gear, worm gear and motor
failure. A complete gearbox is
required with failures of this nature.