For 1990-2009 cars

Refer to

#### Cell 21: Power and Ground

.

### Circuit Description

The main relay is controlled by the powertrain control module (PCM). The PCM provides a ground for the relay's coil anytime the ignition switch is ON. The main relay supplies ignition positive voltage to many engine control systems and components. The following components receive power from the main relay:

• | The fuel pump relay

• | The fuel injectors

• | The EVAP control system solenoids

• | The idle air control (IAC) valve

• | The powertrain control module (PCM)

### Diagnostic Aids

Check for any of the following conditions:

• | Check the resistance of the main relay. The resistance across terminal 1 and terminal 2 is 79-95 ohms at 20°C (68°F). The resistance across terminal 3 and terminal 4 is infinite.

• | The main relay's electrical contacts may be pitted or sticking. Replace the main relay if tapping gently on the relay or wiggling the relay causes a change in the relay's operation.

• | The performance of the main relay may be affected by temperature. Check the main relay after sitting outside overnight and after running the engine 30 minutes.

• | Check for a faulty electrical connection to the PCM.

An intermittent malfunction may be caused by a fault in the main relay electrical circuit. Inspect the wiring harness and components for any of the following conditions:

• | Backed out terminals.

• | Improper mating of terminals.

• | Broken electrical connector locks.

• | Improperly formed or damaged terminals.

• | Faulty terminal to wire connections.

• | Physical damage to the wiring harness.

• | A broken wire inside the insulation.

• | Corrosion of electrical connections, splices, or terminals.

Use the following relay cavity table in order to locate the correct cavities to probe during diagnosis. The table layout corresponds to the cavity layout of the relay block.

Relay Cavity Identification
Switch Power
Switch Load
Coil Power | Coil Ground

### Test Description

The numbers below refer to the step numbers in the Diagnostic Table.

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. This step will verify whether the PCM is not providing a ground path for the main relay circuit. When terminal 2 of the main relay is grounded, the main relay turns ON.

3. This step checks for ignition positive voltage at the PCM. The PCM will not operate the main relay if there is no power to terminal C3-20.

4. This step checks for an open in the PCM electrical grounds. The PCM will not operate the main relay if there are no electrical grounds.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Check for an open in the following fuses:

• | IG-COIL fuse

• | FI fuse

Was an open fuse found?

--

Go to Step 13

Go to Step 3

3 |

1. Turn ON the ignition, leaving the engine OFF.
2. Remove and reinstall the main relay several times while listening for a clicking sound from the relay.

Did the main relay click when being reinstalled into the relay box?

--

Go to Diagnostic Aids

Go to Step 4

4 |

1. Turn OFF the ignition.
2. Remove the main relay from the relay box.
3. Turn ON the ignition, leaving the engine OFF.
4. Probe terminal 4 (relay box side) using a test lamp connected to ground. Refer to the Relay Cavity Identification table in Diagnostic Aids.

Does the test lamp illuminate?

--

Go to Step 5

Go to Step 7

5 | Probe terminal 1 of the main relay (relay box side) using a test lamp connected to ground. Does the test lamp illuminate? | -- | Go to Step 6 | Go to Step 7
6 |

1. Turn OFF the ignition.
2. Probe terminal 2 of the main relay using a test lamp connected to battery positive voltage.
3. Turn ON the ignition, leaving the engine OFF.
4. Observe the test lamp.

Does the test lamp illuminate?

--

Go to Step 14

Go to Step 8

7 | Repair the open in the ignition positive voltage circuit between the FI fuse and the main relay. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 16 | --
8 |

1. Check for an open in the main relay control circuit between the main relay and the PCM.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 16

Go to Step 9

9 | Backprobe PCM electrical connector terminal C3-20 with a test lamp connected to ground. Does the test lamp illuminate? | -- | Go to Step 11 | Go to Step 10
10 | Repair the open in the ignition positive voltage circuit between the IG-COIL fuse and the PCM. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 16 | --
11 |

1. Disconnect the PCM electrical connector C1.
2. Probe the PCM ground terminals C1-1, C1-13, and C1-26 (harness side), with a test lamp connected to B+.

Does the test lamp illuminate at each ground terminal?

--

Go to Step 15

Go to Step 12

12 | Repair all opens in the PCM ground circuits. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 16 | --
13 | Repair the short to ground in the IG-COIL fuse circuit or the FI fuse circuit.. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 16 | --
14 | Replace the main relay. Refer to Main Relay Replacement . Is the action complete? | -- | Go to Step 16 | --
15 | Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | Go to Step 16 | --
16 | Operate the vehicle within the conditions under which the original concern was noted. Does the system operate properly with no DTCs? | -- | System OK | Go to the Applicable Diagnostic Table