Circuit Description
When the antilock brake system (ABS) is active, the valve relay provides voltage to actuate the solenoid valves. The valves do not use this voltage unless the ABS control module provides the ground for each solenoid coil.
DTC C0121 will set if the valve relay voltage is low or if the relay supply line is at 12 volts when the ABS control module is not requesting it. This diagnostic trouble code (DTC) will also set if the ABS control module detects 3 or more solenoid valve circuits are open or shorted during the self-test.
This procedure checks whether there is a poor ground connection for the electronic brake control module (EBCM).
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC C0121 ABS Relay Circuit
Conditions for Running the DTC
| • | Vehicle speed is greater than 6 km/h (4 mph) with the brake pedal not applied |
| or |
| • | Vehicle speed is greater than 15 km/h (9 mph) and the brake pedal is actuated. |
Conditions for Setting the DTC
| • | A connector terminal is corroded. |
| • | The wiring harness is damaged. |
| • | The ground terminal is not conducting properly. |
| • | The EBCM is defective. |
Action Taken When the DTC Sets
The ABS/EBD is disabled, and the ABS/EBD warning lamp is turned ON for the remainder of the ignition cycle. If the failure is intermittent, the control module will enable the system at the next ignition cycle and set a history DTC C0121.
Conditions for Clearing the DTC
The condition responsible for setting the DTC no longer exists and the scan tool Clear DTCs function is used.
Diagnostic Aids
It is very important to perform a thorough inspection of the wiring and the connectors. Failure to do so may result in misdiagnosis, causing part replacement with reappearance of the malfunction.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Antilock Brake System Schematics Connector End View Reference: Antilock Brake System Connector End Views | ||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |
2 |
Does DTC C0121 set again? | -- | Go to Step 4 | Go to Step 3 |
3 |
Did you complete the action? | -- | Go to Step 11 | -- |
4 |
Is there damage or corrosion at terminal 19? | -- | Go to Step 5 | Go to Step 6 |
5 | Repair the terminal or the connector, or replace the ABS harness or ABS unit as required. Did you complete the action? | -- | Go to Step 11 | -- |
6 | Measure the resistance from terminal 19 of the harness connector J1 to a good chassis ground. Is the resistance equal to the specified value? | 0 ohms | Go to Step 7 | Go to Step 8 |
7 | Replace the ABS unit. Did you complete the replacement? | -- | Go to Step 11 | -- |
8 | Measure the resistance from terminal 19 of the harness connector J1 to the ground lug at G106. Is the resistance equal to the specified value? | 0 ohms | Go to Step 9 | Go to Step 10 |
9 | Repair the chassis connection at the ground lug. Did you complete the repair? | -- | Go to Step 11 | -- |
10 | Repair the open or the high resistance in the ABS harness, or replace the harness as required. Did you complete the action? | -- | Go to Step 11 | -- |
11 |
Does the DTC reset? | -- | Go to Step 2 | System OK |
