Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairHard
A technician is diagnosing an ABS warning light and a C0121 fault code: 'Low System Voltage to ABS Control Module'. After verifying the battery voltage is 12.6V and the alternator is charging at 14.2V, the technician checks the ABS module's main power supply circuit. The voltage drop across the positive battery terminal to the ABS module's power input terminal (with the system active) should be no more than:
- A0.5 V
- B2.0 V
- C1.0 V
- D0.1 V
Show answer & explanationAnswer & explanation
Correct answer: D. 0.1 V
For critical electronic modules like the ABS control module, a maximum voltage drop of 0.1 volts (100 millivolts) across any power or ground circuit is generally considered acceptable. Anything higher indicates excessive resistance in the wiring or connections, which can lead to low voltage conditions at the module, especially under load.
Why the other options are wrong
- A. 0.5V is too high for a critical power supply circuit to an electronic control module, indicating significant resistance.
- B. 2.0V indicates a severe voltage drop, which would almost certainly cause the module to malfunction or report low voltage codes.
- C. 1.0V is definitely too high and would cause operational issues.
Max Voltage Drop (ABS Module)
For critical electronic control modules like ABS, the maximum allowable voltage drop across power or ground circuits from the battery/ground point to the module should generally not exceed 0.1 volts (100mV).
- Ensures adequate voltage supply.
- Higher drop indicates excessive resistance.
- Critical for sensitive electronics.
Memory trick: ABS Power: Max Drop is a Tenth!