REFERENCE · LIVE

A4 Power Steering Diagnostic

Three architectures, three test paths. Hydraulic (pump + lines + rack), electric (motor + module + sensors), or hybrid (motor-driven hydraulic pump). Identify the system first, then test.

HYDRAULIC (HPS)

Belt-driven pump + rack

Pump pressurizes fluid; rack uses fluid to assist. Most cars before ~2010, all trucks/HD.

FIRST TEST: Fluid level + condition + pressure gauge at pump output.

ELECTRIC (EPS)

Motor + module + sensors

Motor on column or rack provides assist. Module reads steering angle + torque sensor.

FIRST TEST: Scan for codes + module live data (assist level, torque PID).

EHPS (HYBRID)

Electric-driven hydraulic pump

Pump driven by an electric motor, not the belt. Common on early hybrids and EVs.

FIRST TEST: Confirm electric pump is running + hydraulic pressure spec.

Hydraulic — Pressure Spec

Pump output at idle (off-lock)
80-120 psi
Pump output at full lock
800-1600 psi (varies)
Hydroboost-shared system
Brake booster runs off same pump
1300-1800 psi
Flow rate at idle
2.5-4 gpm typical
Fluid type
PSF / ATF Dexron III / Pentosin — match OE
Healthy fluid color
Red/amber, clear
Fluid replacement signal
Dark, burnt smell, foam in reservoir

Electric — Diagnostic Flow

  1. Scan for codes — EPS modules store specific torque sensor and motor codes. Don\'t guess.
  2. Verify B+ and ground at the EPS module. Drop test under load. Voltage drop > 0.2V kills assist intermittently.
  3. Read live data — torque sensor input, motor current draw, assist level. Static at no input, dynamic when you turn the wheel.
  4. Test motor circuit — bidirectional command via scan tool. Listen for motor activation.
  5. Confirm steering angle calibration — required after EPS replacement, suspension work, or battery disconnect on many platforms.

BAY-FLOOR RULES

Symptom triage