ECM Power Supply & Ground Tests

Check Main Relay

Turn ignition off. Remove main relay, located at left kick panel. Connect battery voltage to main relay terminals No. 1 and 2. Check resistance between main relay terminals No. 3 and 5, and terminals No. 4 and 6. See Fig 1. Resistance should be less than 10 ohms in both tests. If resistance is as specified, go to next step. If resistance is not as specified, replace main relay.

Fig 1: Testing Main Relay
Courtesy of SUBARU OF AMERICA, INC.

Check ECM Ground Circuit

Turn ignition off. Disconnect ECM harness connectors B134 (35-pin connector) and B136 (30-pin connector). Using DVOM, check resistance between specified ECM connector terminals and chassis ground. See ECM HARNESS CONNECTOR & TERMINAL IDENTIFICATION FOR GROUND CIRCUIT TEST . See Fig 3. Resistance on each listed terminal should be less than 5 ohms. If resistance is as specified, go to next step. If resistance is not as specified, repair open circuit between ground and ECM harness connector or check for poor terminal contact in harness connector. See appropriate wiring diagram in WIRING DIAGRAMS article.

ECM HARNESS CONNECTOR & TERMINAL IDENTIFICATION FOR GROUND CIRCUIT TEST

ECM Connector No.	Terminal No.
B134 (35-Pin Connector)	7, 8 & 27
B136 (30-Pin Connector)	21 & 22

Check Input Voltage Of ECM

Turn ignition on. Using a DVOM, measure voltage by backprobing between specified ECM connector terminals and chassis ground. See ECM HARNESS CONNECTOR & TERMINAL IDENTIFICATION FOR INPUT VOLTAGE TEST table. See Fig 3. Voltage reading at each listed terminal should be more than 10 volts. If voltage reading is as specified, go to next step. If voltage reading is not as specified, check and repair power circuit. See appropriate wiring diagram in WIRING DIAGRAMS article.

ECM HARNESS CONNECTOR & TERMINAL IDENTIFICATION FOR INPUT VOLTAGE TEST

ECM Connector No.	Terminal No.
30-Pin Connector (B136)	1, 2 & 9
28-Pin Connector (B135)	7

Check Harness Between ECM & Main Relay Connector

Turn ignition off. Using a DVOM connected to ground, measure resistance by backprobing ECM connector B135 (28-pin connector) terminal No. 19. See Fig 3. If resistance is more than one megohm, go to next step. If resistance is not more than one megohm, repair short to ground in harness between ECM connector and main relay connector, then replace ECM.

Check Output Voltage From ECM

Ensure all connectors are properly attached to ECM. Turn ignition on. Using a DVOM connected to ground, check voltage by backprobing ECM connector B135 (28-pin connector) terminal No. 19. See Fig 3. If voltage reading is more than 10 volts, go to next step. If voltage reading is not more than 10 volts, replace ECM.

Check Input Voltage Of Main Relay

Reconnect main relay. Disconnect main relay harness connector. Turn ignition on. Using DVOM, check voltage between main relay connector terminals Nos. 2, 5 and 6 and chassis ground. See Fig 2. Voltage reading on each listed terminal should be greater than 10 volts. If voltage reading is as specified, go to next step. If voltage reading is not as specified, check and repair power circuit to main relay harness connector. See appropriate wiring diagram in WIRING DIAGRAMS article.

Fig 2: Identifying Main Relay Harness Connector Terminals
Courtesy of SUBARU OF AMERICA, INC.

Check Main Relay Ground Circuit

Reconnect main relay. Disconnect main relay harness connector. Turn ignition off. Using DVOM, check resistance between main relay connector terminal No. 1 and chassis ground. See Fig 2. Resistance should be less than 5 ohms. If resistance is as specified, check ignition control system. If resistance is not as specified, check and repair ground circuit to main relay harness connector. See appropriate wiring diagram in WIRING DIAGRAMS article.

Fig 3: Identifying ECM Connectors & Terminals (Reverse Image To Backprobe Terminals)

Courtesy of SUBARU OF AMERICA, INC.