Purpose
To determine correct power supplied to operate the ECM
The ECM requires 7 V minimum for correct operation.
Voltage Measurement with Breakout Harness at Main Power Relay
Tools
• Relay Breakout Harness
• DMM
Procedure
NOTE: Batteries must be fully charged before doing the following steps.
1. Turn the ignition switch to OFF and ensure all accessories are turned off.
2. Connect Relay Breakout Harness between ECM main power relay and power distribution center or chassis harness depending on application.
NOTE: Depending on application, the relay could be one of two kinds. Check power distribution center or cab cowl.
3. Connect DMM POS to lead 87 and NEG to ground terminal on cowl.
4. Crank engine for 20 seconds and measure voltage.
5. Record the lowest voltage on Diagnostic Form.
• If the voltage is below 7 V, the ECM main power relay may be resetting, due to low voltage and current from the batteries, or problems in the ignition circuit and power feed circuits. See Electronic Control Module Power (ECM PWR) in Section 7 (page 381).
• If the voltage is above 7 V, continue with Hard Start and No Start Diagnostic tests.
NOTE: Results can be above 7 V, but there may be a problem between the main power relay and the ECM. If a Hard Start / No Start problem remains after all Diagnostic Form tests are complete, do Voltage Measurement at ECM
with Breakout Box.
Possible Causes
Low battery voltage
• Failed batteries
• High-resistance at battery cable connections
• Wiring to the ECM
Low or no battery voltage to the ECM
• High-resistance or an open power feed circuit to the ECM or ECM main power relay.
• The ECM power circuit fuse in battery box may be open.
• ECM main power relay may have failed.
• VIGN circuit problem
• Failed ECM
Voltage Measurement at ECM with Breakout Box
NOTE: If the breakout box was used to do Test 6 – EST Data List, the following procedures do not have to be done.
Use the following procedures when any of the following situations exist:
• A Relay Breakout Harness is not available
• Expected voltages were not to spec, when using the Relay Breakout Harness
• Voltages were to spec, using the Relay Breakout Harness and Hard Start No Start Diagnostics is complete – but a concern remains
Tools
• Breakout Box
• Digital Multimeter (DMM)
Procedure
NOTE: Batteries must be fully charged before doing the following steps.
1. Turn the ignition switch to OFF and ensure all accessories are turned off.
2. Remove two white connectors (X3 and X4) from ECM.
3. Connect breakout box connectors (X3 and X4) to connections on ECM.
4. Connect chassis harness connectors to breakout box header (X3 and X4).
5. Connect leads of the DMM to the following test points on the breakout box:
• POS X3–3 to NEG X3–7 (V IGN Pwr)
• POS X4–2 to NEG X3–7 (ECM PWR)
• POS X4–1 to NEG X3–7 (ECM PWR)
6. Crank engine for 20 seconds and measure voltage.
7. Record the lowest voltage on Diagnostic Form.
• If the voltage is below 7 V, the ECM power relay may be resetting, resulting from low voltage and current from the batteries, or problems in the ignition circuit and power feed circuits. See “ECM PWR, Electronic Control Module Power” in Section 7 (page 381).
• If the voltage is above 7 V, continue with Hard Start and No Start Diagnostic tests.
