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For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2004 🢒 CTS 🢒 Engine 🢒 Engine Electrical 🢒 Main Relay Circuit Diagnosis

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 powertrain control module (PCM) supplies the ground path to the relay coil control circuit, via an internal integrated circuit called an output driver module (ODM). When the PCM 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 and relays:

    • TOS fuse
    • PCM fuse
    • ODD INJ/Coil fuse
    • Pre 02/CAM fuse
    • Post 02 fuse
    • Even INJ/Coil fuse
    • Low speed fan relay
    • S/P fan relay

When the ignition switch is turned to the OFF position, the main relay will continue to be energized for 20 seconds. Once the power is interrupted to the output driver module in the PCM, 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 PCM class 2 communications will be available with the use of a scan tool.

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: Power Distribution Schematics in Wiring Systems and Engine Controls Schematics

Connector End View References: Power and Grounding Connector End Views , Electrical Center Identification Views in Wiring Systems, and Powertrain Control Module 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. • TOS fuse
    • PCM fuse
    • Odd INJ/Coil fuse
    • Pre 02/CAM fuse
    • Post 02 fuse
    • Even INJ/Coil fuse

Refer to Troubleshooting with a Test Lamp in Wiring Systems.

Does the test lamp illuminate on at least one test point of each fuse?

--

Go to Step 3

Go to Step 17

3
  1. Turn OFF the ignition.
  2. Probe both test points of the PCM fuse in the underhood fuse block with a test lamp that is connected to a good ground.

Does the test lamp illuminate on either test point of the fuse?

--

Go to Step 4

Go to Step 47

4
  1. Turn OFF the ignition.
  2. Remove the main relay from the underhood fuse block with the J 43244 . Refer to Relay Replacement in Wiring Systems.
  3. Probe both the test points of the PCM fuse with a test lamp that is connected to a good ground.

Does the test lamp illuminate on either test point of the fuse?

--

Go to Step 5

Go to Step 14

5
  1. Turn OFF the ignition.
  2. Remove the TOS fuse from the underhood fuse block.
  3. Probe both test points of the PCM fuse with a test lamp that is connected to a good ground.

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
  1. Turn OFF the ignition.
  2. Remove the Odd INJ/Coil fuse from the underhood fuse block.
  3. Probe both test points of the PCM fuse with a test lamp that is connected to a good ground.

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
  1. Turn OFF the ignition.
  2. Remove the Pre O2/CAM fuse from the underhood fuse block.
  3. Probe both test points of the PCM fuse with a test lamp that is connected to a good ground.

Does the test lamp illuminate on either test point of the fuse?

--

Go to Step 8

Go to Diagnostic System Check - Engine Electrical in Engine Electrical

8
  1. Turn OFF the ignition.
  2. Remove the Post O2 fuse from the underhood fuse block.
  3. Probe both test points of the PCM fuse with a test lamp that is connected to a good ground.

Does the test lamp illuminate on either test point of the fuse?

--

Go to Step 9

Go to Diagnostic System Check - Engine Electrical in Engine Electrical

9
  1. Turn OFF the ignition.
  2. Remove the Even INJ/Coil fuse from the underhood fuse block.
  3. Probe both test points of the PCM fuse with a test lamp that is connected to a good ground.

Does the test lamp illuminate on either test point of the fuse?

--

Go to Step 10

Go to Diagnostic System Check - Engine Electrical in Engine Electrical

10
  1. Turn OFF the ignition.
  2. Remove the low speed fan relay from the underhood fuse block with theJ 43244 Relay Puller Pliers. Refer to Relay Replacement in Wiring Systems.
  3. Probe both test points of the PCM fuse with a test lamp that is connected to a good ground.

Does the test lamp illuminate on either test point of the fuse?

--

Go to Step 11

Go to Diagnostic System Check - Engine Electrical in Engine Electrical

11
  1. Turn OFF the ignition.
  2. Remove the high speed fan relay from the underhood fuse block.
  3. Probe both test points of the PCM fuse with a test lamp that is connected to a good ground.

Does the test lamp illuminate on either test point of the fuse?

--

Go to Step 12

Go to Diagnostic System Check - Engine Electrical in Engine Electrical

12
  1. Turn OFF the ignition.
  2. Remove the S/P fan relay from the underhood fuse block.
  3. Probe both test points of the PCM fuse with a test lamp that is connected to a good ground.

Does the test lamp illuminate on either test point of the fuse?

--

Go to Step 13

Go to Diagnostic System Check - Engine Electrical in Engine Electrical

13
  1. Turn OFF the ignition.
  2. Remove the PCM fuse from the underhood fuse block.

    3. Notice: Refer to Test Probe Notice in the Preface section.

  3. Probe the main relay load circuit bus bar terminal in the underhood fuse block with a test lamp that is connected to a good ground. Refer to Probing Electrical Connectors in Wiring Systems.

Does the test lamp illuminate?

--

Go to Step 41

Go to Diagnostic System Check - Engine Electrical in Engine Electrical

14
  1. Turn OFF the ignition.

    2. Notice: Refer to Test Probe Notice in the Preface section.

  2. Probe the main relay coil control circuit terminal in the underhood fuse block with a test lamp that is connected to battery positive voltage. Refer to Probing Electrical Connectors in Wiring Systems.

Does the test lamp illuminate?

--

Go to Step 15

Go to Step 30

15
  1. Turn OFF the ignition.
  2. Disconnect the powertrain control module (PCM) electrical connector that contains the relay coil control circuit. Refer to Powertrain Control Module Replacement .
  3. Probe the main relay coil control circuit terminal in the underhood fuse block with a test lamp this is connected to battery positive voltage.

Does the test lamp illuminate?

--

Go to Step 16

Go to Step 37

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 main relay coil control circuit. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems.
  4. Probe the main relay coil control circuit at the underhood fuse block electrical connector with a test lamp that is connected to battery positive voltage.

Does the test lamp illuminate?

--

Go to Step 38

Go to Step 41

17
  1. Turn OFF the ignition.
  2. Remove the main relay from the underhood fuse block with the J 43244 . Refer to Relay Replacement in Wiring Systems.

    3. Notice: Refer to Test Probe Notice in the Preface section.

  3. Probe the battery positive voltage terminal for the relay coil in the underhood fuse block with a test lamp that is connected to a good ground. Refer to Probing Electrical Connectors in Wiring Systems.

Does the test lamp illuminate?

--

Go to Step 19

Go to Step 18

18

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 41

Go to Diagnostic System Check - Engine Electrical in Engine Electrical

19
  1. Turn OFF the ignition.
  2. Probe the relay coil control circuit terminal in the underhood fuse block with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 23

Go to Step 20

20
  1. Turn ON the ignition, with the engine OFF.
  2. Probe the relay coil control circuit terminal in the underhood fuse block with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 21

Go to Step 25

21
  1. Turn OFF the ignition.
  2. Disconnect the PCM electrical connector that contains the relay coil control circuit. Refer to Powertrain Control Module Replacement .
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the main relay coil control circuit at the underhood fuse block with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 22

Go to Step 37

22
  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 main relay coil control circuit. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems.
  4. Disconnect the main relay coil connector circuit terminal from the underhood fuse block electrical connector. Refer to Pull to Seat Connectors in Wiring Systems.
  5. Connect the electrical connector that contains the main relay coil control circuit to the underhood fuse block.
  6. Connect the negative battery cable at the battery.
  7. Turn ON the ignition, with the engine OFF.
  8. Probe the main relay coil control circuit terminal at the underhood fuse block electrical connector with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 39

Go to Step 41

23
  1. Turn OFF the ignition.
  2. Disconnect the PCM electrical connector that contains the relay coil control circuit. Refer to Powertrain Control Module Replacement .
  3. Probe the main relay coil control circuit terminal in the underhood fuse block with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 24

Go to Step 37

24
  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 main relay coil control circuit. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems.
  4. Connect the negative battery cable at the battery.
  5. Probe the main relay coil connector circuit at the underhood fuse block electrical connector with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 39

Go to Step 41

25
  1. Turn OFF the ignition.
  2. Probe the battery positive voltage terminal for the main relay armature at the underhood fuse block with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 26

Go to Step 41

26
  1. Turn ON the ignition, with the engine OFF.

    2. Notice: Refer to Test Probe Notice in the Preface section.

  2. Probe the relay coil control circuit terminal at the underhood fuse block with a test lamp that is connected to battery positive voltage.

Does the test lamp illuminate?

--

Go to Step 29

Go to Step 27

27
  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 main relay coil control circuit. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems.
  4. Disconnect the PCM electrical connector that contains the main relay coil control circuit. Refer to Powertrain Control Module Replacement .
  5. Measure the resistance of the relay coil control circuit from the underhood fuse block electrical connector to the PCM electrical connector with a DMM. Refer to Troubleshooting with a Digital Multimeter in Wiring Systems.

Does the resistance measure greater than the specified value?

5 ohms

Go to Step 40

Go to Step 28

28

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 in Wiring Systems.

Did you find a condition?

--

Go to Step 41

Go to Step 37

29
  1. Turn OFF the ignition.
  2. Connect a 20-amp fused jumper wire between the battery positive voltage circuit terminal of the underhood fuse block and the relay load circuit terminal of the underhood fuse block. 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 TOS fuse
    • The PCM fuse
    • The Odd INJ/Coil fuse
    • The Pre O2/CAM fuse
    • The Post O2 fuse
    • The Even INJ/Coil fuse

Does the test lamp illuminate on at least one test point of each fuse?

--

Go to Step 30

Go to Step 41

30

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 31

Go to Step 42

31

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 32

Go to Step 42

32

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 33

Go to Step 42

33

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 34

Go to Step 42

34

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 35

Go to Step 42

35
  1. Connect a 20-amp fused jumper wire from the battery positive cable at the battery to relay terminal #85. Refer to Using Fused Jumper Wires in Wiring Systems.
  2. Connect a jumper wire from the negative battery cable to the relay terminal #86.
  3. Measure the resistance from terminal #30 of the relay to terminal #87 with a DMM.

Does the resistance measure more than the specified value?

3 ohms

Go to Step 42

Go to Step 36

36

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 41

Go to Intermittent Conditions

37

Test for shorted terminals and poor connections at the PCM 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 47

Go to Step 46

38

Repair the short to ground in the relay coil control circuit between the underhood fuse block and the PCM. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 47

--

39

Repair the short to voltage in the relay coil control circuit between the underhood fuse block and the PCM. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 47

--

40

Repair the high resistance or an open in the relay coil control circuit between the underhood fuse block and the PCM. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 47

--

41

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 47

--

42

Replace the main relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

--

Go to Step 47

--

43

Replace the low speed fan relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

--

Go to Step 47

--

44

Replace the high speed fan relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

--

Go to Step 47

--

45

Replace the S/P fan relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

--

Go to Step 47

--

46

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 47

--

47
  1. Reassemble the vehicle as necessary.
  2. Replace any open fuses.
  3. Clear any DTCs with a scan tool.
  4. Start the engine.
  5. Operate the vehicle in order to verify the repair.

Did you correct the condition?

--

Go to Step 48

Go to Engine Cranks but Does Not Run

48

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK