Air Conditioning (A/C) Compressor Control Circuit Diagnosis Manual A/C
Circuit Description
The VCM control of the A/C clutch improves idle quality and performance by performing the following conditions:
| • | Delaying clutch engagement until the idle speed is increased |
| • | Releasing the clutch when the idle speed is too low |
| • | Providing additional fuel at the instant the clutch is applied |
The additional fuel smooths the cycling of the compressor.
Turning the air conditioning ON supplies battery voltage through the pressure switches to the VCM. When the VCM receives the voltage on the A/C request signal, the A/C enable relay circuit is grounded. The A/C compressor clutch then engages.
Test Description
The numbers below refer to step numbers in the diagnostic table.
-
Checks for low refrigerant as the cause for no A/C.
-
The A/C request signal circuit consists of the HVAC Control module, the A/C high pressure cutout switch and related wiring.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
|
Important: Before using this table confirm that the A/C system has an adequate state of charge. Does the A/C system have an adequate state of charge? | -- | Go to A/C System Diagnosis in HVAC | ||
2 |
Does the A/C clutch engage within the specified time? | 60 sec | Go to Diagnostic Aids | |
3 |
Does the scan tool indicate a YES? | -- | ||
4 |
Does the scan tool indicate a CLOSED switch? | -- | ||
5 |
Does the scan tool indicate a CLOSED switch? | -- | ||
6 | Probe pin B of the harness connector with a test lamp connected to B+. Is the test lamp ON? | -- | ||
7 | Replace the A/C Evaporator/Cycling Cutoff switch. Is the action complete? | -- | -- | |
8 |
Caution: Avoid contact with moving parts and hot surfaces while working around a running engine in order to prevent physical injury. Is the test lamp ON? | -- | ||
9 | Probe the A/C clutch ground circuit at the A/C clutch harness connector with a test lamp connected to B+. Is the test lamp ON? | -- | ||
10 | Repair the A/C clutch ground circuit. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
11 | Check for a poor connection at the A/C compressor clutch harness connector. Did you find a problem? | -- | ||
12 | Repair the A/C compressor clutch harness connector. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
13 | Replace the A/C compressor clutch. Refer to HVAC. Is the action complete? | -- | -- | |
14 |
Is the test lamp ON? | -- | ||
Check for an open in the A/C request signal circuit between the HVAC control module and the VCM. Did you find a problem? | -- | Go to A/C System Diagnosis in HVAC | ||
16 |
Does the A/C relay click? | -- | ||
17 |
Is the test lamp ON? | -- | ||
18 | Repair the open in the A/C relay ignition positive voltage circuit. If the fuse is blown repair the short to ground on the circuit. A blown fuse or lack of voltage to the fuse may cause other symptoms or DTCs to be present. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
19 |
Is the test lamp ON? | -- | ||
20 | Check for an open in the circuit between the UBEC A/C relay control terminal and the VCM C3 connector pin 1. Did you find a problem? | -- | ||
21 | Repair the open in the circuit between the UBEC A/C relay control terminal and the VCM C3 connector pin 1. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
22 |
Is the test lamp ON? | -- | ||
23 | Repair the open in the A/C relay battery feed circuit. If the fuse is blown repair the short to ground on the circuit. A blown fuse or lack of voltage to the fuse may cause other symptoms or DTCs to be present. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
24 |
Does the A/C clutch engage? | -- | ||
25 | Replace the A/C relay. Refer to A/C Relay Repair and Replacement in HVAC. Is the action complete? | -- | -- | |
26 | Repair the open or short to ground in the compressor clutch control circuit. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
27 | Check for an open in the A/C cycling signal circuit. Did you find a problem? | -- | ||
28 | Check for a poor connection at the VCM. Did you find a problem? | -- | ||
29 | Repair the circuit as necessary. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
30 |
Is the action complete? | -- | -- | |
31 |
Does the A/C clutch engage within the specified time? | 60 sec | ||
32 | Use a scan tool in order to select the Capture Info and Review Capture Info functions. Are any DTCs displayed that have not been diagnosed? | -- | Go to applicable DTC table | System OK |
Air Conditioning (A/C) Compressor Control Circuit Diagnosis Auto A/C
Circuit Description
The VCM control of the A/C clutch improves idle quality and performance by performing the following conditions:
| • | Delaying clutch engagement until the idle speed is increased |
| • | Releasing the clutch when the idle speed is too low |
| • | Providing additional fuel at the instant the clutch is applied |
The additional fuel smooths the cycling of the compressor.
Turning the air conditioning ON supplies battery voltage through the pressure switches to the VCM. When the VCM receives the voltage on the A/C request signal, the A/C enable relay circuit is grounded. The A/C compressor clutch then engages.
Test Description
The numbers below refer to step numbers in the diagnostic table.
-
Checks for low refrigerant as the cause for no A/C.
-
The A/C request signal circuit consists of the HVAC Control module, the A/C high pressure cutout switch and related wiring.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
|
Important: Before using this table confirm that the A/C system has an adequate state of charge. Does the A/C system have an adequate state of charge? | -- | Go to A/C System Diagnosis in HVAC | ||
2 |
Does the A/C clutch engage within the specified time? | 60 sec | Go to Diagnostic Aids | |
3 |
Does the scan tool indicate a YES? | -- | ||
4 |
Does the scan tool indicate a CLOSED switch? | -- | ||
5 |
Does the scan tool indicate a CLOSED switch? | -- | ||
6 | Probe pin B of the harness connector with a test lamp connected to B+. Is the test lamp ON? | -- | ||
7 | Replace the A/C Evaporator/Cycling Cutoff switch. Is the action complete? | -- | -- | |
8 |
Caution: Avoid contact with moving parts and hot surfaces while working around a running engine in order to prevent physical injury. Is the test lamp ON? | -- | ||
9 | Probe the A/C clutch ground circuit at the A/C clutch harness connector with a test lamp connected to B+. Is the test lamp ON? | -- | ||
10 | Repair the A/C clutch ground circuit. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
11 | Check for a poor connection at the A/C compressor clutch harness connector. Did you find a problem? | -- | ||
12 | Repair the A/C compressor clutch harness connector. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
13 | Replace the A/C compressor clutch. Refer to HVAC. Is the action complete? | -- | -- | |
14 |
Is the test lamp ON? | -- | ||
Check for an open in the A/C request signal circuit between the HVAC control module and the VCM. Did you find a problem? | -- | Go to A/C System Diagnosis in HVAC | ||
16 |
Does the A/C relay click? | -- | ||
17 |
Is the test lamp ON? | -- | ||
18 | Repair the open in the A/C relay ignition positive voltage circuit. If the fuse is blown repair the short to ground on the circuit. A blown fuse or lack of voltage to the fuse may cause other symptoms or DTCs to be present. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
19 |
Is the test lamp ON? | -- | ||
20 | Check for an open in the circuit between the UBEC A/C relay control terminal and the VCM C3 connector pin 1. Did you find a problem? | -- | ||
21 | Repair the open in the circuit between the UBEC A/C relay control terminal and the VCM C3 connector pin 1. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
22 |
Is the test lamp ON? | -- | ||
23 | Repair the open in the A/C relay battery feed circuit. If the fuse is blown repair the short to ground on the circuit. A blown fuse or lack of voltage to the fuse may cause other symptoms or DTCs to be present. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
24 |
Does the A/C clutch engage? | -- | ||
25 | Replace the A/C relay. Refer to A/C Relay Repair and Replacement in HVAC. Is the action complete? | -- | -- | |
26 | Repair the open or short to ground in the compressor clutch control circuit. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
27 | Check for an open in the A/C cycling signal circuit. Did you find a problem? | -- | ||
28 | Check for a poor connection at the VCM. Did you find a problem? | -- | ||
29 | Repair the circuit as necessary. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
30 |
Is the action complete? | -- | -- | |
31 |
Does the A/C clutch engage within the specified time? | 60 sec | ||
32 | Use a scan tool in order to select the Capture Info and Review Capture Info functions. Are any DTCs displayed that have not been diagnosed? | -- | Go to applicable DTC table | System OK |
