Here’s a detailed breakdown of the ABS pump / actuator unit as used in the 2009 Altis (GD series) — its function, components, location, part-numbers and what to check if you’re diagnosing issues.

What the unit is and what it does

The ABS system (''Anti-Lock Braking System'') prevents wheel lock-up during hard braking so you retain steering control. The main hydraulic/actuator component is sometimes called the ABS pump, ABS modulator or ABS actuator assembly. 

In the Altis, the ABS actuator assembly includes:

• A hydraulic pump/motor (to restore pressure after the valves release it)
• A set of solenoid valves/modulator valves (to direct brake fluid pressure based on wheel speed sensor input)
• An electronic control module (sometimes housed together or mounted close) that takes sensor inputs (wheel speed, brake switch, etc) and drives the valves & pump.  
• A structural block/housing where brake-lines feed in/out.

When a wheel is detected as about to lock (wheel decelerating too fast relative to vehicle speed), the system releases pressure at that wheel (via the modulator valve) and then uses the pump to restore proper pressure once slip is corrected. 

Part location & typical part numbers

• On the GD series Altis (for example 2008-2011 generation), the actuator assembly is mounted in the engine bay, attached to or near the brake booster/master cylinder area (or close to the firewall) depending on market.
• Example OEM part numbers for similar models include: 44050-12D90 for ''Actuator Assembly, Brake / Modulator Valve'' on a 2008 Toyota Corolla.  
• Another source lists for 2008 Corolla: Actuator Assembly, Brake = 44050-01060 (for rear disc brake version) and other variants 44050-02130.  
• For the Altis listings in Malaysia, I found used-original listings with descriptions like ''Toyota Altis / Corolla ABS pump actuator brake pump GD Altis 2008'' which indicates compatibility with the GD body style.  
• Since your vehicle is 2009 Altis GD, you should confirm the VIN or build code and match the exact actuator assembly part number for your market (Malaysia etc.), as right/left, with or without VSC/traction control can affect differences.

Key components & how to recognise them

• Hydraulic block: a metallic block with multiple brake-line ports (input from master cylinder and output to wheels).
• Pump/motor unit: typically attached to the hydraulic block, an electric motor with gear/piston that pumps brake fluid when needed.
• Valves/solenoids: typically incorporated into the hydraulic block, internally switching fluid paths.
• Electronic module / harness: The ABS ECU connector plugs into the actuator unit; wheel-speed sensor wires and brake-switch signal feed into the control side.
• Brake-line ports: You’ll see brake-lines coming from the master cylinder to the unit, and multiple smaller lines out to each wheel or axle.
• Mounting bracket: Usually bolted to firewall or support bracket; the module may have rubber bushings to reduce vibration.

Symptoms of failure / what to look for

If the ABS actuator/pump is malfunctioning, common symptoms include:

• ABS warning light (and/or brake warning light) on the dash.
• Pedal behaviour: e.g., sudden pulsation when normally it doesn’t, or the pedal may feel ''hard'' or ''soft'' in a way different from normal braking.
• The hydraulic pump motor may run continuously or not at all (you might hear unusual whine or buzzing from the brake-booster area).
• Fault codes stored: often speed-sensor circuit faults, pump-motor or valve faults, internal pressure sensor faults. For example, owners of 2009 Corolla/Altis report that:
  ''Mechanics now say this is the ABS module … when it gets confused by the faulty ABS sensor readings''  
• Brake performance may be degraded; on wet or slippery surfaces you may feel the ABS engaging incorrectly.
• Corrosion or brake-fluid leaks at the actuator unit or brake-lines connected to it.

Maintenance / replacement considerations

• When replacing the actuator assembly, you may need to bleed the brake system thoroughly afterwards (ABS units often have built-in pressure reservoirs).
• Ensure the electrical connector and ground are clean—with no corrosion. Poor connection will cause malfunction.
• If you’re buying a used unit, verify that the software adaptation or coding (if applicable) is correct for your vehicle. Some markets allow re-programming or ''clear adaptation'' of ABS unit.
• After installation, clear fault codes and test the ABS system safely (e.g., on a low speed, low-grip surface) to ensure it activates correctly.
• Use correct brake fluid specification as per manufacturer; contamination or incorrect fluid can cause internal valve sticking in the hydraulic block.
• It may be advisable to replace wheel-speed sensors or check their wiring/harness because many ABS failures stem from sensor faults which then cause the actuator to go into fault mode.

If you like, I can check the exact OEM part number for the 2009 Altis GD in Malaysia, plus a wiring diagram for the ABS actuator block so you can cross-verify. Would that be helpful?