GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 GMC C Sierra - 2WD 🢒 Engine 🢒 Engine Controls - 6.6L (LLY) 🢒 Ignition Relay Diagnosis

Circuit Description

The EDU/IGN 1 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:

    • The ECM RPV fuse
    • The EDU fuse

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 EDU/IGN 1 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 EDU/IGN 1 relay table assumes that the vehicle battery is fully charged. Refer to Battery Inspection/Test in Engine Electrical.

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 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 fuse block cover.
  3. Probe the following fuses with a test lamp that is connected to a good ground.
    4. • The ECM RPV fuse
    • The EDU fuse

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 EDU/IGN 1 relay from the underhood fuse block with theJ 43244 Relay Puller Pliers and Relay Replacement in Wiring Systems.
  3. Probe the coil ground circuit of the EDU/IGN 1 relay at the underhood fuse 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 Disconnection and Connection in Engine Electrical.
  3. Disconnect the underhood fuse block electrical connector that contains the ground wire for the EDU/IGN 1 relay.
  4. Disconnect the engine control module (ECM) electrical connector that contains the ground wire for the EDU/IGN 1 relay.
  5. Test the coil ground wire of the EDU/IGN 1 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 EDU/IGN 1 relay coil ground bus bar circuit of the underhood fuse 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 fuse block between the EDU/IGN 1 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 fuse block:
    3. • The ECM RPV fuse
    • The EDU fuse
  3. Probe both fuse terminals for each fuse in the underhood fuse 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 fuse 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 fuse block between the 20-amp ECM B fuse and the EDU/IGN 1 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 fuse 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 EDU/IGN 1 relay with the J 43244 from the underhood fuse block.
  3. Probe the battery positive voltage circuit of the EDU/IGN 1 relay at the underhood fuse 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 EDU/IGN 1 relay at the underhood fuse 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 EDU/IGN 1 relay at the underhood fuse 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 Disconnection and Connection in Engine Electrical.
  3. Disconnect the underhood fuse block electrical connector that contains the ground wire for the EDU/IGN 1 relay.
  4. Disconnect the engine control module (ECM) electrical connector that contains the ground wire for the EDU/IGN 1 relay.
  5. Test the coil ground wire of the EDU/IGN 1 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 EDU/IGN 1 relay coil ground bus bar circuit of the underhood fuse 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 EDU/IGN 1 relay battery positive voltage circuit and the EDU/IGN 1 relay load circuit together at the underhood fuse 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:
    4. • The ECM RPV fuse
    • The EDU fuse

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 fuse block EDU/IGN 1 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 EDU/IGN 1 relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

Go to Step 24

--

22

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

Go to Step 24

--

23

Replace the underhood fuse 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) Types

System OK