Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairHard
A customer complains that their vehicle's brake pedal occasionally feels 'spongy' at low speeds, and the ABS warning light illuminates intermittently. A scan tool shows no active fault codes, but historical codes indicate 'C1210 - System Voltage Low'. Further testing reveals normal battery voltage and charging system operation. Which of the following is the MOST likely cause of the intermittent sponginess and voltage code?
- AIntermittent high resistance in the ABS pump motor circuit
- BA defective wheel speed sensor tone ring
- CA faulty brake master cylinder
- DAir trapped in the hydraulic brake lines
Show answer & explanationAnswer & explanation
Correct answer: A. Intermittent high resistance in the ABS pump motor circuit
An intermittent high resistance in the ABS pump motor circuit could cause the pump to draw excessive current or operate inefficiently when activated, leading to a momentary system voltage drop that triggers a 'low voltage' code. This could also affect brake feel if the ABS/ESC system attempts to intervene with insufficient pump power.
Why the other options are wrong
- B. A defective tone ring would cause wheel speed sensor errors (C00xx codes) and ABS activation issues, not a 'system voltage low' code related to the ABS module's main power supply.
- C. A faulty master cylinder would typically cause a consistently spongy pedal, not intermittent, and wouldn't directly cause a 'system voltage low' code.
- D. Air in the lines would cause a consistently spongy pedal, but not a 'system voltage low' code, especially if the charging system is good.
ABS Pump Motor Circuit Resistance
High resistance in the ABS pump motor circuit can reduce power to the pump, causing it to draw excessive current or operate weakly, leading to system voltage drops and potential malfunction.
- Can cause 'system voltage low' fault codes (e.g., C1210).
- May lead to intermittent ABS/ESC activation issues.
- Diagnosed by checking voltage drop and resistance across the circuit.
Memory trick: The 'Low Voltage' is a tricky foe, hiding in high resistance flow.