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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 1026 (Engine Controls - 6.6L (Lb7) (Troubleshooting & Diagnosis) >> Troubleshooting >> Ignition Relay Diagnosis (EDU Relay) >> Circuit Description

Circuit Description

WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The ignition relay is normally an open relay. The relay armature is held in the open position by spring tension. When the ignition switch is turned to the Run or Start position, current will flow through the relay coil. A wire connected to the other end of the relay coil completes the path to ground through the engine control module (ECM). The electromagnetic field created by the relay coil overcomes the spring tension and moves the armature, allowing the contacts to close. The closed relay contacts allow current to flow from the battery to the following fuses:

When the ignition switch is turned to the OFF position, the electromagnetic field collapses. This action allows the spring tension to move the armature away from the relay contacts, which interrupts current flow to the fuses.

If the ignition relay fails to close, the engine will crank but will not run. The class 2 communications will be available with the use of a scan tool.

The ignition relay table assumes that the vehicle battery is fully charged. Refer to Battery Inspection/Test (Non-HP2) or Battery Inspection/Test (HP2) in Engine Electrical.

Ignition Relay Diagnosis (EDU Relay)

Step Action Yes No
Schematic Reference:  Power Distribution Schematics  ,  Ground Distribution Schematics  in Wiring Systems and  Engine Controls Schematics 
Connector End View Reference:  Engine Control Module (ECM) Connector End Views  or  Engine Controls Connector End Views 
1 Did you perform the Diagnostic System Check-Engine Controls? Go to Step 2  Go to Diagnostic System Check - Engine Controls
2
  1. Turn ON the ignition, with the engine OFF.
  2. Remove the underhood junction block cover.
  3. Probe the following fuses with a test lamp that is connected to a good ground.
Does the test lamp illuminate on at least one test point of each fuse?		 Go to Step 3  Go to Step 9 
3
  1. Turn OFF the ignition.
  2. Probe both test points of the EDU fuse with a test lamp that is connected to a good ground. Refer to Probing Electrical Connectors and Troubleshooting with a Test Lamp in Wiring Systems.
Does the test lamp illuminate on either test point of the fuse?		 Go to Step 4  Go to Step 24 
4
  1. Turn OFF the ignition.
  2. Remove the ignition relay from the underhood junction block with the J 43244  Relay Puller Pliers and Relay Replacement (Within an Electrical Center) or Relay Replacement (Attached to Wire Harness) in Wiring Systems.
  3. Probe the coil ground circuit of the ignition relay at the underhood junction block with a test lamp that is connected to battery positive. Refer to Probing Electrical Connectors and Troubleshooting with a Test Lamp in Wiring Systems.
Does the test lamp illuminate?		 Go to Step 5  Go to Step 7 
5
  1. Turn OFF the ignition.
  2. Disconnect the negative battery cable at the battery. Refer to Battery Negative Cable Disconnect/Connect Procedure (Single Battery) or Battery Negative Cable Disconnect/Connect Procedure (Auxiliary Battery) in Engine Electrical.
  3. Disconnect the underhood junction block electrical connector that contains the ground wire for the ignition relay.
  4. Disconnect the engine control module (ECM) electrical connector that contains the ground wire for the ignition relay.
  5. Test the coil ground wire of the ignition relay for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems
Did you find and correct the condition?		 Go to Step 24  Go to Step 6 
6 Test the ignition relay coil ground bus bar circuit of the underhood junction block for a short to ground. Refer to Circuit Testing in Wiring Systems.
Did you find a condition?		 Go to Step 23  Go to Step 20 
7 Test the relay load bus bar circuit of the underhood junction block between the ignition relay and the fuses to the circuit components for a short to battery positive voltage. Refer to Circuit Testing in Wiring Systems.
Did you find a condition?		 Go to Step 23  Go to Step 8 
8
  1. Turn OFF the ignition.
  2. Remove the following fuses from the underhood junction block:
    • The ECM RPV fuse
    • The EDU fuse
  3. Probe all these fuse terminals in the underhood junction block with a test lamp that is connected to a good ground. Refer to Probing Electrical Connectors and Troubleshooting with a Test Lamp in Wiring Systems.
Does the test lamp illuminate at any of the fuse terminals?		 Go to Diagnostic System Check - Engine Electrical in Engine Electrical Go to Step 21 
9
  1. Turn ON the ignition, with the engine OFF.
  2. Probe both test points of the 20-amp ECM B fuse in the underhood junction block with a test lamp that is connected to a good ground. Refer to Probing Electrical Connectors and Troubleshooting with a Test Lamp in Wiring Systems
Does the test lamp illuminate on both test points of the fuse?		 Go to Step 13  Go to Step 10 
10 Does the test lamp illuminate on one test point of the 20-amp ECM B fuse? Go to Step 11  Go to Step 12 
11 Test the battery positive voltage bus bar circuit of the underhood junction block between the 20-amp ECM B fuse and the ignition relay for a short to ground. Refer to Circuit Testing in Wiring Systems.
Did you find a condition?		 Go to Step 23  Go to Step 21 
12
  1. Turn OFF the ignition.
  2. Probe the mounting stud for the battery positive cable at the underhood junction block with a test lamp that is connected to a good ground. Refer to Probing Electrical Connectors and Troubleshooting with a Test Lamp in Wiring Systems.
Does the test lamp illuminate?		 Go to Step 23  Go to Diagnostic System Check - Engine Electrical in Engine Electrical
13
  1. Turn OFF the ignition.
  2. Remove the ignition relay with the J 43244  from the underhood junction block.
  3. Probe the battery positive voltage circuit of the ignition relay at the underhood junction block with a test lamp that is connected to a good ground. Refer to Probing Electrical Connectors and Troubleshooting with a Test Lamp in Wiring Systems.
Does the test lamp illuminate?		 Go to Step 14  Go to Step 23 
14
  1. Turn ON the ignition.
  2. Probe the ignition 1 voltage circuit of the ignition relay at the underhood junction block with a test lamp that is connected to a good ground. Refer to Probing Electrical Connectors and Troubleshooting with a Test Lamp in Wiring Systems.
Does the test lamp illuminate?		 Go to Step 15  Go to Step 23 
15
  1. Turn ON the ignition with the engine OFF.
  2. Probe the coil ground circuit of the ignition relay at the underhood junction block with a test lamp that is connected to a battery positive voltage. Refer to Probing Electrical Connectors and Troubleshooting with a Test Lamp in Wiring Systems.
Does the test lamp illuminate?		 Go to Step 18  Go to Step 16 
16
  1. Turn OFF the ignition.
  2. Disconnect the negative battery cable at the battery. Refer to Battery Negative Cable Disconnect/Connect Procedure (Single Battery) or Battery Negative Cable Disconnect/Connect Procedure (Auxiliary Battery) in Engine Electrical.
  3. Disconnect the underhood junction block electrical connector that contains the ground wire for the ignition relay.
  4. Disconnect the engine control module (ECM) electrical connector that contains the ground wire for the ignition relay.
  5. Test the coil ground wire of the ignition relay for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?		 Go to Step 24  Go to Step 17 
17 Test the ignition relay coil ground bus bar circuit of the underhood junction block for a high resistance or an open. Refer to Circuit Testing in Wiring Systems.
Did you find a condition?		 Go to Step 23  Go to Step 20 
18
  1. Turn OFF the ignition.
  2. Jumper the ignition relay battery positive voltage circuit and the ignition relay load circuit together at the underhood junction block with a 20-amp fused jumper wire. Refer to Using Fused Jumper Wires in Wiring Systems.
  3. Probe the following fuses with a test lamp that is connected to a good ground:
Does the test lamp illuminate on at least one test point of each fuse?		 Go to Step 19  Go to Step 23 
19 Test for an intermittent and for a poor connection at the underhood junction block ignition relay connector location. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
Did you find a condition?		 Go to Step 23  Go to Step 21 
20 Test for an intermittent and for a poor connection at the ECM connectors. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
Did you find a condition?		 Go to Step 24  Go to Step 22 
21 Replace the ignition relay. Refer to Relay Replacement (Within an Electrical Center) or Relay Replacement (Attached to Wire Harness) in Wiring Systems.
Did you complete the replacement?		 Go to Step 24  -
22 Replace the ECM. Refer to Engine Control Module (ECM) Replacement .
Did you complete the replacement?		 Go to Step 24  -
23 Replace the underhood junction block assembly. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems.
Did you complete the replacement?		 Go to Step 24  -
24
  1. Replace any open fuses.
  2. Turn the ignition OFF for 30 seconds.
  3. Attempt to start the engine.
Does the engine start and run?		 Go to Step 25  Go to Engine Cranks but Does Not Run 
25
  1. Clear the DTCs with a scan tool.
  2. Operate the vehicle for 5 minutes.
Does a DTC set during this ignition cycle?		 Go to Diagnostic Trouble Code (DTC) List System OK
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