Chassis, generic

C05A9 ABS Pump Motor Supply Voltage Circuit A/B Correlation

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

With two feeds for the pump motor, the module can compare them: both come from the same vehicle supply and should read almost alike. C05A9 is set when the two drift apart beyond the allowed margin, even though each may still lie in its own window. A Hitachi patent for electric brakes uses this idea: the control unit compares the voltages of several branch supply lines and, if the largest difference exceeds a limit, concludes that a main line is broken and switches in a backup path. Hyundai Mobis gives each winding of a dual brake motor its own power module and supply input, and Ipgate runs its brake-by-wire electronics on two redundant supplies. No maker documents C05A9 publicly. Likely causes are a high-resistance connection in one feed or a measuring error in the module.

Likely causes

Corroded terminal, fuse or connector in only one of the two feeds
Voltage measurement channel in the brake module drifting
Poor ground affecting only one supply path

Symptoms

  • ABS/ESC or brake system warning lamp on
  • Pump operation or brake redundancy restricted

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€30–1,500
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

abs pump motor supply aabs pump motor supply b

Related codes

C05A5C05A8C052FC055C

References: patents.google.com, patents.google.com, patents.google.com.