Fuel System > Engine Control System > P0646 A/C Clutch Relay Control Circuit Low

General Description

The A/C clutch relay is activated if the A/C switch is operated while the blower is running and system operation is enabled by the PCM. When A/C is requested, the Engine Control Module(PCM) provides a ground path to the A/C clutch relay control circuit. When the relay circuit is grounded, the A/C clutch relay is energized. The PCM delays grounding the relay circuit for a short time, so the PCM can adjust the engine idle speed for the additional load. The PCM will temporarily de-energized the A/C clutch relay for one or more of the following conditions:

-	Full acceleration when the throttle is at WOT.(Wide Open Throttle)

-	Risk of overheating: Engine coolant temp. exceeds threshold value

-	A/C system pressure exceeds threshold value

-	Engine starting

DTC Description

PCM monitors voltage through A/C compressor relay. Checking voltage every 10 sec. under detecting condition, if the voltage lower than the specified value is detected for more than 5 sec., PCM sets P0646.

DTC Detecting Condition

Item

Detecting Condition

Possible cause

DTC Strategy

•	Detects circuit short to low voltage

•	Poor connection

•	Open or short to ground in A/C relay circuit

•	Faulty A/C relay

•	Faulty PCM

Enable Conditions

•	After 0.5 sec under conditions below

•	No DTC exists

•	Engine running

•	11V ≤ Battery voltage ≤ 16V

Threshold value

•	Open or short to ground

Diagnosis Time

•	Continuous (More than 5 seconds failure for every 10 seconds test)

MIL On Condition

•	DTC only (NO MIL ON)

Specification

Coil Resistance

70Ω ~ 120Ω

Schematic Diagram

Monitor DTC Status

Check DTC Status

(1)	Connect scantool to Data Link Connector(DLC).

(2)

IG "ON".

(3)	Select "DTC" button, and then Press "DTC Status" to check DTC's information from the DTCs menu.

(4)	Read "DTC Status" parameter.

(5)

Is parameter displayed "Present fault"?

	▶ Go to "Terminal and connector inspection" procedure.
	▶ Fault is intermittent caused by poor contact in the sensor's and/or PCM's connector or was repaired and PCM memory was not cleared. Thoroughly check connectors for looseness, poor connection, ending, corrosion, contamination, deterioration, or damage. Repair or replace as necessary and go to "Verification of Vehicle Repair" procedure.

Terminal And Connector Inspection

1.	Many malfunctions in the electrical system are caused by poor harness and terminal condition. Faults can also be caused by interference from other electrical systems, and mechanical or chemical damage.

2.	Thoroughly check connectors for looseness, poor connection, bending, corrosion, contamination, deterioration, or damage.

Has a problem been found?

	▶ Repair as necessary and go to "Verification of Vehicle Repair" procedure
	▶ Go to " Power Circuit Inspection " procedure.

Power Circuit Inspection

■ Check voltage

IG "OFF".

2.	Disconnect A/C relay.

3.	IG "ON" and ENG "OFF"

Measure voltage between battery power supply terminals of A/C relay harness connector and chassis ground.

Specification : Approx. B+

Is the measured voltage normal?

	▶ Go to "Control Circuit Inspection" procedure.
	▶ Check the fuse between Battery and A/C relay. ▶ Check Chassis ground for looseness. ▶ Repair open or short to ground in power harness and go to "Verification of Vehicle Repair" procedure.

Control Circuit Inspection

■ Check short in harness

IG "OFF".

2.	Disconnect A/C relay and PCM connector.

Measure the resistance between control terminal of A/C relay harness connector and chassis ground.

Specification : Infinite

Is the measured resistance within specification ?

	▶ Go to "Check open in harness" as follows.
	▶ Repair short in coil control harness and go to "Verification of Vehicle Repair" procedure.

■ Check open in harness

IG "OFF".

2.	Disconnect A/C relay and PCM connector.

Measure the resistance between control terminal of A/C relay harness connector and A/C relay control terminal of PCM harness connector.

Specification : Approx. below 1Ω

Is the measured resistance within specification ?

	▶ Go to "Component Inspection" procedure.
	▶ Repair open in coil control harness and go to "Verification of Vehicle Repair" procedure.

Component Inspection

■ Check A/C relay

IG "OFF"

2.	Disconnect A/C Relay

3.	Measure resistance between battery power supply and power supply to A/C Compressor terminals of A/C relay. (Measurement "A")

Measure resistance between battery power supply and A/C relay control terminals of A/C relay.(Measurement "B")

Specification :

Terminal	Power approval
Battery power supply - Power supply to A/C Compressor	NO
Battery power supply - A/C relay control	YES (About 70 ~ 120Ω)

Is the measured resistance within specification ?

	▶ Substitute with a known - good PCM and check for proper operation. If the problem is corrected, replace PCM and go to "Verification of Vehicle Repair" procedure.
	▶ Substitute with a known - good A/C relay and check for proper operation. If the problem is corrected, replace A/C relay and go to "Verification of Vehicle Repair" procedure.

There is a memory reset function on scantool that can erase optional parts automatically detected and memorized by PCM. After testing PCM on the vehicle, use this function to reuse the PCM on the others.

Verification Of Vehicle Repair

After a repair, it is essential to verify that the fault has been corrected.

1.	Connect scantool and select "DTC" button.

2.	Press "DTC Status" button and confirm that "DTC Readiness Flag" indicates "Completed". If not, drive the vehicle within conditions noted in the freeze frame data or enable conditions

3.	Read "DTC Status" parameter

Is parameter displayed "History(Not Present) fault"?

	▶ System performing to specification at this time. Clear the DTC
	▶ Go to the applicable troubleshooting procedure.