For 1990-2009 cars
Automatic Transmission Components
Engine Controls Schematics
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

The A/C Request Circuit -- When the HVAC controls are in any position that enables the A/C compressor, and the A/C compressor refrigerant switch is closed (low and high), the A/C Request circuit is connected to B+. The ECM energizes the A/C compressor control circuit when a high signal voltage is detected on the A/C Request circuit. The ECM will de-energize the A/C compressor control circuit during the WOT maneuvers, when the ECT exceeds a predetermined threshold or when the A/C Request circuit signal voltage goes low (either by a pressure switch opening, or the HVAC system is in a non-A/C mode).

The A/C Load Signal -- The ECM also receives an input on the A/C load signal circuit. This is accomplished through a pressure switch connected to B+ which is normally open. When the refrigerant high side pressure reaches approximately 1,200 kPa, the switch closes sending a B+ signal to the ECM. The ECM will then raise the idle speed by approximately 50 RPM in order to help compensate for these conditions.

The A/C Compressor Control Circuit -- The ignition voltage is supplied directly to the A/C Compressor Control relay coil. The ECM controls the relay by grounding the control circuit via an internal switch called a driver. The primary function of the driver is to supply the ground for the component being controlled. When the ECM is commanding a component on, the voltage of the control circuit should be low (near 0 volts). When the ECM is commanding the control circuit to a component off, the voltage potential of the circuit should be high (near battery voltage).

The relay controls the high current flow to the A/C Compressor Clutch. This allows the ECM driver to only have to handle the relatively low current used by the relay.

### Diagnostic Aids

An intermittent may be caused by the following:

• | Corrosion

• | A mis-routed harness

• | A rubbed through wire insulation

• | A broken wire inside the insulation

Refer to Symptoms for Intermittents.

### Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

1. Listen for an audible click when the relay operates. Repeat the commands as necessary.

2. This check can detect a partially shorted coil which would cause an excessive current flow. Leaving the circuit energized for 2 minutes allows the coil to warm up. When warm the coil may open (amps drop to 0), or short (goes above 0.75 amp).

3. It is important to identify and test the relay coil terminals in order to avoid an improper diagnosis.

4. If no trouble is found in the control circuit or the connection at the ECM, the ECM may be malfunctioning, however, this is an extremely unlikely failure.

#### A/C Compressor Control Circuit Diagnosis

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 |

1. Turn on the ignition with the engine off.
2. Use a scan tool in order to command the relay on and off.

Does the relay turn on and off with each command?

--

Go to Step 3

Go to Step 5

3 |

1. Turn off the ignition.
2. Disconnect the ECM.
3. Turn on the ignition on with the engine off.
4. Use DVM J 39200 on 10 amp scale in order to measure the current from the relay control circuit in the ECM harness connector to ground for 2 minutes.

Does the current draw a measure within the range shown?

0.1 to 0.75A

Refer to Diagnostic Aids

Go to Step 4

4 |

1. Turn off the ignition.
2. Disconnect the relay.
3. Use a DVM J 39200 in order to measure the resistance from the relay control circuit in the ECM harness connector to ground.

The DVM should display infinite resistance.

Does the DVM display an infinite resistance?

--

Go to Step 11

Go to Step 9

5 |

1. Turn off the ignition.
2. Disconnect the relay.
3. Connect the test light J 35616-200 between the relay coil terminals in the harness connector.
4. Turn on the ignition.
5. Command the relay on and off with a scan tool.

Does the test light turn on and off with each command?

--

Go to Step 8

Go to Step 6

6 | With the test light connected to ground, probe the ignition feed circuit in the relay harness connector. The test light should illuminate. Does the test light illuminate? | -- | Go to Step 7 | Go to Step 10
7 |

1. Turn off the ignition.
2. Reconnect the relay.
3. Disconnect the ECM.
4. Turn on the ignition with the engine off.
5. Probe the relay control circuit in the ECM harness connector with a fused jumper wire connected to ground.

Does the relay operate?

--

Go to Step 12

Go to Step 9

8 | Check the connections at the relay. Was a problem found and corrected? | -- | Go to Step 14 | Go to Step 11
9 | Repair the relay control circuit. Is the action complete? | -- | Go to Step 14 | --
10 | Repair the ignition feed circuit to the relay. Is the action complete? | -- | Go to Step 14 | --
11 | Replace the relay. Is the action complete? | -- | Go to Step 14 | --
12 | Check for any poor connections or poor terminal tension at the ECM harness connector. Refer to Section 8A, Troubleshooting Procedures, Checking Terminal Contact in Engine Control Module Diagnosis for the proper procedure. Was a problem found and corrected? | -- | Go to Step 14 | Go to Step 13
13 | Important: This vehicle is equipped with a Theft Deterrent System which interfaces with the Engine Control Module (ECM). Program the new ECM with the frequency code of the theft deterrent module currently on the vehicle. Replace the ECM. Refer to Engine Control Module Replacement/Programming .

Is the action complete?

--

Go to Step 14

--

14 |

1. Turn on the ignition with the engine off.
2. Command the relay on and off with the scan tool.

Does the relay turn on and off with each command?

--

The System is OK

Go to Step 2