Circuit Description
The main relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly to the relay coil and the armature contact at all times. The engine control module (ECM) supplies the ground path to the relay coil at all times. The engine control module (ECM) supplies the ground path to the relay coil control circuit, via an internal integrated circuit called an output driver module (ODM). When the ECM commands the relay ON, the relay coil creates an electromagnetic field. This electromagnetic field overcomes the spring tension and pulls the armature contact into the stationary contact of the relay load circuit. The closing of the relay contacts allows current to flow from the battery to the following fuses:
| • | ECM/CAM FUSE |
| • | INJ/MANF FUSE |
| • | EMISSION FUSE |
When the ignition switch is turned to the OFF position, the main relay will continue to be energized for three minutes after an engine run cycle. The main relay will continue to be energized for five seconds after the ignition switch is turned OFF, from an ignition ON, engine OFF condition. Once the power is interrupted to the output driver module in the ECM, the relay electromagnetic field collapses. This allows the spring tension to separate the relay armature contact from the relay load circuit contact, which interrupts current flow to the fuses.
If the main relay fails to close the engine will crank, but will not run. The ECM class 2 communications will not be available with the use of a scan tool. A body control module (BCM) Code U2105-Lost Communication with Engine Control System will set.
The main relay system diagnosis table assumes that the vehicle battery if fully charged. Refer to Battery Inspection/Test in Engine Electrical.
Step | Action | Values | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Refer to Power Distribution Schematics in Wiring Systems and Engine Controls Schematics . Connector End View References: Refer to Engine Control Module Connector End Views , Power and Grounding Connector End Views , and Electrical Center Identification Views in Wiring Systems. | ||||||||||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |||||||||
2 |
Does the test lamp illuminate on at least one test point of each fuse? | -- | Go to Step 3 | Go to Step 11 | ||||||||
3 |
Does the test lamp illuminate on either test point of the fuse? | -- | Go to Step 4 | Go to Step 38 | ||||||||
4 |
Does the test lamp illuminate on either test point of the fuse? | -- | Go to Step 5 | Go to Step 8 | ||||||||
5 |
Does the test lamp illuminate on either test point of the fuse? | -- | Go to Step 6 | Go to Diagnostic System Check - Engine Electrical in Engine Electrical | ||||||||
6 |
Does the test lamp illuminate on either test point of the fuse? | -- | Go to Step 7 | Go to Diagnostic System Check - Engine Electrical in Engine Electrical | ||||||||
7 |
Notice: Refer to Test Probe Notice in the Preface section. Does the test lamp illuminate? | -- | Go to Step 35 | Go to Diagnostic System Check - Engine Electrical in Engine Electrical | ||||||||
8 |
Notice: Refer to Test Probe Notice in the Preface section. Does the test lamp illuminate? | -- | Go to Step 9 | Go to Step 24 | ||||||||
9 |
Does the test lamp illuminate? | -- | Go to Step 10 | Go to Step 31 | ||||||||
10 |
Does the test lamp illuminate? | -- | Go to Step 32 | Go to Step 35 | ||||||||
11 |
Notice: Refer to Test Probe Notice in the Preface section. Does the test lamp illuminate? | -- | Go to Step 13 | Go to Step 12 | ||||||||
12 | Probe the mounting stud for the positive battery cable at the underhood fuse block with a test lamp that is connected to a good ground Does the test lamp illuminate? | -- | Go to Step 35 | Go to Diagnostic System Check - Engine Electrical in Engine Electrical | ||||||||
13 |
Does the test lamp illuminate? | -- | Go to Step 17 | Go to Step 14 | ||||||||
14 |
Does the test lamp illuminate? | -- | Go to Step 15 | Go to Step 19 | ||||||||
15 |
Does the test lamp illuminate? | -- | Go to Step 16 | Go to Step 31 | ||||||||
16 |
Does the test lamp illuminate? | -- | Go to Step 33 | Go to Step 35 | ||||||||
17 |
Does the test lamp illuminate? | -- | Go to Step 18 | Go to Step 31 | ||||||||
18 |
Does the test lamp illuminate? | -- | Go to Step 33 | Go to Step 35 | ||||||||
19 |
Does the test lamp illuminate | -- | Go to Step 20 | Go to Step 35 | ||||||||
20 |
Notice: Refer to Test Probe Notice in the Preface section. Does the test lamp illuminate? | -- | Go to Step 23 | Go to Step 21 | ||||||||
21 |
Does the resistance measure greater than the specified value? | 5 ohms | Go to Step 34 | Go to Step 22 | ||||||||
22 | Test the main relay coil control bus bar circuit of the underhood fuse block for a high resistance or an open. Refer to Circuit Testing Did you find a condition? | -- | Go to Step 35 | Go to Step 31 | ||||||||
23 |
Does the test lamp illuminate on at least one test point of each fuse? | -- | Go to Step 24 | Go to Step 35 | ||||||||
24 | Measure the resistance from terminal #85 of the relay to terminal #86 with a DMM. Refer to Troubleshooting with a Digital Multimeter in Wiring Systems. Does the resistance measure within the specified range? | 70-110 ohms | Go to Step 25 | Go to Step 36 | ||||||||
25 | Measure the resistance from terminal #30 of the relay to terminal #87 with a DMM. Refer to Troubleshooting with a Digital Multimeter in Wiring Systems. Does the DMM display the specified value? | ∞ ohms | Go to Step 26 | Go to Step 36 | ||||||||
26 | Measure the resistance from terminal #30 of the relay to terminal #85 with a DMM. Refer to Troubleshooting with a Digital Multimeter in Wiring Systems. Does the DMM display the specified value? | ∞ ohms | Go to Step 27 | Go to Step 36 | ||||||||
27 | Measure the resistance from terminal #85 of the relay to terminal #87 with a DMM. Refer to Troubleshooting with a Digital Multimeter in Wiring Systems. Does the DMM display the specified value? | ∞ ohms | Go to Step 28 | Go to Step 36 | ||||||||
28 | Measure the resistance from terminal #86 of the relay to terminal #87 with a DMM. Refer to Troubleshooting with a Digital Multimeter in Wiring Systems. Does the DMM display the specified value? | ∞ ohms | Go to Step 29 | Go to Step 36 | ||||||||
29 |
Does the resistance measure more than the specified value? | 3 ohms | Go to Step 36 | Go to Step 30 | ||||||||
30 | Test for an intermittent and for a poor connection at the main relay location on the underhood fuse block. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find a condition? | -- | Go to Step 35 | Go to Intermittent Conditions | ||||||||
31 | Test for shorted terminals and poor connections at the ECM electrical connector. Refer to Testing for Intermittent Conditions and Poor Connections and Micro-Pack 100W Connectors in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 38 | Go to Step 37 | ||||||||
32 | Repair the short to ground in the relay coil control circuit between the underhood fuse block and the ECM. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 38 | -- | ||||||||
33 | Repair the short to voltage in the relay coil control circuit between the underhood fuse block and the ECM. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 38 | -- | ||||||||
34 | Repair the high resistance or an open in the relay coil control circuit between the underhood fuse block and the ECM. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 38 | -- | ||||||||
35 | Replace the underhood fuse block. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems. Did you complete the replacement? | -- | Go to Step 38 | -- | ||||||||
36 | Replace the main relay. Refer to Relay Replacement . Did you complete the replacement? | -- | Go to Step 38 | -- | ||||||||
37 | Replace the ECM. Refer to Engine Control Module Replacement . Did you complete the replacement? | -- | Go to Step 38 | -- | ||||||||
38 |
Did you correct the condition? | -- | Go to Step 39 | |||||||||
39 | Observe the capture info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||||||
