Code No. 12 Air Flow Sensor System

OPERATION

Power is supplied to the air flow sensor (terminal No. 4) from the engine control relay (terminal No. 1) and earthed to the engine-A/T-ECU (terminal No. 16) from the air flow sensor (terminal No. 5).

A power voltage of 5 V is applied to the air flow sensor output terminal (terminal No. 3) from the engine-A/T-ECU (terminal No. 65).

An air flow sensor filter reset signal is inputted to the air flow sensor (terminal No. 7) from the engine-A/T-ECU (terminal No. 19).

FUNCTION

Air flow sensor outputs a pulse signal proportional to the intake air flow rate.

The engine-A/T-ECU determines the basic injection timing of the injector using the pulse signal output from the air flow sensor and the engine speed signal.

When the throttle position sensor output is low, the engine-A/T-ECU causes the power transistor in the unit to be ON to send an air flow sensor filter reset signal to the air flow sensor. In response to the reset signal, the air flow sensor resets the filter circuit to improve the ability of the air flow sensor to measure the air flow rate in the area where the intake air flow rate is low.

TROUBLE JUDGMENT

Check Condition

Engine speed of 500 r/min or more.

Judgment Criterion

Sensor output frequency of 3.3 Hz or less for 4 seconds.

PROBABLE CAUSE

Failed air flow sensor

Open/short circuit in air flow sensor circuit or loose connector contact

Failed engine-A/T-ECU

DIAGNOSIS PROCEDURE

STEP 1. MUT-II/III data list

Refer to Data list reference table .

Item 12: Air flow sensor

Q: Is the check result normal?

YES: Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points

).
NO: Go to Step 2

STEP 2. Connector check: B-08 air flow sensor connector

Q: Is the check result normal?

YES: Go to Step 3

.
NO: Repair.

STEP 3. Perform voltage measurement at B-08 air flow sensor connector.

Disconnect connector, and measure at harness side.

Ignition switch: ON

Voltage between terminal No. 3 and earth.

OK: 4.9 - 5.1 V

Q: Is the check result normal?

YES: Go to Step 9

.
NO: Go to Step 4

STEP 4. Perform voltage measurement at C-111 engine-A/T-ECU connector.

Measure engine-A/T-ECU terminal voltage.

Ignition switch: ON

Voltage between terminal No. 65 and earth.

OK: 4.9 - 5.1 V

Q: Is the check result normal?

YES: Go to Step 5

.
NO: Go to Step 6

STEP 5. Connector check: C-111 engine-A/T-ECU connector

Q: Is the check result normal?

YES:

Check and repair harness between B-08 (terminal No. 3) air flow sensor connector and C-111 (terminal No. 65) engine-A/T-ECU connector.

Check output line for open circuit.

NO: Repair.

STEP 6. Connector check: C-111 engine-A/T-ECU connector

Q: Is the check result normal?

YES: Go to Step 7

.
NO: Repair.

STEP 7. Check harness between B-08 (terminal No. 3) air flow sensor connector and C-111 (terminal No. 65) engine-A/T-ECU connector.

Check output line for short circuit.

Q: Is the check result normal?

YES: Go to Step 8

.
NO: Repair.

STEP 8. MUT-II/III data list

Refer to Data list reference table .

Item 12: Air flow sensor

Q: Is the check result normal?

YES: Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points

).
NO: Replace engine-A/T-ECU.

STEP 9. Perform voltage measurement at B-08 air flow sensor connector.

Disconnect connector, and measure at harness side.

Ignition switch: ON

Voltage between terminal No. 4 and earth.

OK: System voltage

Q: Is the check result normal?

YES: Go to Step 11

.
NO: Go to Step 10

STEP 10. Connector check: B-17X engine control relay connector

Q: Is the check result normal?

YES:

Check and repair harness between B-08 (terminal No. 4) air flow sensor connector and B-17X (terminal No.1) engine control relay connector.

Check power supply line for open/short circuit.

NO: Repair.

STEP 11. Check at B-08 air flow sensor connector.

Disconnect and measure at harness side.

Resistance between terminal No. 5 and earth.

OK: 2 Ω or less

Q: Is the check result normal?

YES: Go to Step 14

.
NO: Go to Step 12

STEP 12. Connector check: C-112 engine-A/T-ECU connector

Q: Is the check result normal?

YES: Go to Step 13

.
NO: Repair.

STEP 13. Check harness between B-08 (terminal No. 5) air flow sensor connector and C-112 (terminal No. 16) engine-A/T-ECU connector.

NOTE: Before checking harness, check intermediate connector C-16, and repair if necessary.

Check earthing line for open circuit and damage.

Q: Is the check result normal?

YES: Go to Step 8

.
NO: Repair.

STEP 14. Perform voltage measurement at B-08 air flow sensor connector.

Use special tool test harness (MB991709) to connect connector, and measure at pick-up harness.

Ignition switch: ON

Voltage between terminal No. 7 and earth.

OK: 7 - 8 V

Q: Is the check result normal?

YES: Go to Step 17

.
NO: Go to Step 15

STEP 15. Connector check: C-112 engine-A/T-ECU connector

Q: Is the check result normal?

YES: Go to Step 16

.
NO: Repair.

STEP 16. Check harness between B-08 (terminal No. 7) air flow sensor connector and C-112 (terminal No. 19) engine-A/T-ECU connector.

Check reset signal line for short circuit.

Q: Is the check result normal?

YES: Replace air flow sensor.
NO: Repair.

STEP 17. Perform voltage measurement at B-08 air flow sensor connector.

Use special tool test harness (MB991709) to connect connector, and measure at pick-up harness.

Selector lever position: P

Voltage between terminal No. 7 and earth.

OK:

0 - 1 V (Engine: Idling)

6 - 9 V (Engine: 3,000 r/min)

Q: Is the check result normal?

YES: Go to Step 20

.
NO: Go to Step 18

STEP 18. Connector check: C-112 engine-A/T-ECU connector

Q: Is the check result normal?

YES: Go to Step 19

.
NO: Repair.

STEP 19. Check harness between B-08 (terminal No. 7) air flow sensor connector and C-112 (terminal No. 19) engine-A/T-ECU connector.

Check reset signal line for open circuit and damage.

Q: Is the check result normal?

YES: Go to Step 8

.
NO: Repair.

STEP 20. Output wave pattern measurement at B-08 air flow sensor connector (Use oscilloscope).

Use special tool test harness (MB991709) to connect connector, and measure at pick-up harness.

Engine: Idling

Selector lever position: P

Voltage between terminal No. 3 and earth.

OK: Wave patterns should be displayed on inspection procedure using an oscilloscope (Refer to

) and noise should not be displayed in the wave pattern.

Q: Is the check result normal?

YES: Go to Step 8

.
NO: Go to Step 21

STEP 21. Replace air flow sensor.

After replacing the air flow sensor, re-check the trouble symptoms.

Q: Is the check result normal?

YES: Go to Step 22

.
NO: Check end.

STEP 22. Connector check: B-17X engine control relay connector

Q: Is the check result normal?

YES: Go to Step 23

.
NO: Repair.

STEP 23. Check harness between B-08 (terminal No. 4) air flow sensor connector and B-17X (terminal No. 1) engine control relay connector.

Check power supply line for damage.

Q: Is the check result normal?

YES: Go to Step 24

.
NO: Repair.

STEP 24. Connector check: C-111 engine-A/T-ECU connector

Q: Is the check result normal?

YES: Go to Step 25

.
NO: Repair.

STEP 25. Check harness between B-08 (terminal No. 3) air flow sensor connector and C-111 (terminal No. 65) engine-A/T-ECU connector.

Check output line for damage.

Q: Is the check result normal?

YES: Go to Step 26

.
NO: Repair.

STEP 26. Connector check: C-112 engine-A/T-ECU connector

Q: Is the check result normal?

YES:

Check intermediate connector C-16, and repair if necessary. If intermediate connector is normal, check and repair harness between B-08 (terminal No. 5) air flow sensor connector and C-127 (terminal No. 16) engine-A/T-ECU connector.

Check earthing line for damage.

No: Repair.


Code No. 12 Air Flow Sensor System

OPERATION Power is supplied to the air flow sensor (terminal No. 4) from the engine control relay (terminal No. 1) and earthed to the engine-A/T-ECU (terminal No. 16) from the air flow sensor (terminal No. 5). A power voltage of 5 V is applied to the air flow sensor output terminal (terminal No. 3) from the engine-A/T-ECU (terminal No. 65). An air flow sensor filter reset signal is inputted to the air flow sensor (terminal No. 7) from the engine-A/T-ECU (terminal No. 19).
FUNCTION Air flow sensor outputs a pulse signal proportional to the intake air flow rate. The engine-A/T-ECU determines the basic injection timing of the injector using the pulse signal output from the air flow sensor and the engine speed signal. When the throttle position sensor output is low, the engine-A/T-ECU causes the power transistor in the unit to be ON to send an air flow sensor filter reset signal to the air flow sensor. In response to the reset signal, the air flow sensor resets the filter circuit to improve the ability of the air flow sensor to measure the air flow rate in the area where the intake air flow rate is low.
TROUBLE JUDGMENT Check Condition Engine speed of 500 r/min or more. Judgment Criterion Sensor output frequency of 3.3 Hz or less for 4 seconds.
PROBABLE CAUSE Failed air flow sensor Open/short circuit in air flow sensor circuit or loose connector contact Failed engine-A/T-ECU
	DIAGNOSIS PROCEDURE

	STEP 1. MUT-II/III data list Refer to Data list reference table . Item 12: Air flow sensor Q: Is the check result normal? YES: Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points ). NO: Go to Step 2 .
	STEP 2. Connector check: B-08 air flow sensor connector
	Q: Is the check result normal? YES: Go to Step 3 . NO: Repair.
	STEP 3. Perform voltage measurement at B-08 air flow sensor connector.
	Disconnect connector, and measure at harness side. Ignition switch: ON Voltage between terminal No. 3 and earth. OK: 4.9 - 5.1 V Q: Is the check result normal? YES: Go to Step 9 . NO: Go to Step 4 .
	STEP 4. Perform voltage measurement at C-111 engine-A/T-ECU connector.
	Measure engine-A/T-ECU terminal voltage. Ignition switch: ON Voltage between terminal No. 65 and earth. OK: 4.9 - 5.1 V Q: Is the check result normal? YES: Go to Step 5 . NO: Go to Step 6 .
	STEP 5. Connector check: C-111 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Check and repair harness between B-08 (terminal No. 3) air flow sensor connector and C-111 (terminal No. 65) engine-A/T-ECU connector. Check output line for open circuit. NO: Repair.
	STEP 6. Connector check: C-111 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Go to Step 7 . NO: Repair.
	STEP 7. Check harness between B-08 (terminal No. 3) air flow sensor connector and C-111 (terminal No. 65) engine-A/T-ECU connector.
	Check output line for short circuit. Q: Is the check result normal? YES: Go to Step 8 . NO: Repair.
	STEP 8. MUT-II/III data list Refer to Data list reference table . Item 12: Air flow sensor Q: Is the check result normal? YES: Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points ). NO: Replace engine-A/T-ECU.
	STEP 9. Perform voltage measurement at B-08 air flow sensor connector.
	Disconnect connector, and measure at harness side. Ignition switch: ON Voltage between terminal No. 4 and earth. OK: System voltage Q: Is the check result normal? YES: Go to Step 11 . NO: Go to Step 10 .
	STEP 10. Connector check: B-17X engine control relay connector
	Q: Is the check result normal? YES: Check and repair harness between B-08 (terminal No. 4) air flow sensor connector and B-17X (terminal No.1) engine control relay connector. Check power supply line for open/short circuit. NO: Repair.
	STEP 11. Check at B-08 air flow sensor connector.
	Disconnect and measure at harness side. Resistance between terminal No. 5 and earth. OK: 2 Ω or less Q: Is the check result normal? YES: Go to Step 14 . NO: Go to Step 12 .
	STEP 12. Connector check: C-112 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Go to Step 13 . NO: Repair.
	STEP 13. Check harness between B-08 (terminal No. 5) air flow sensor connector and C-112 (terminal No. 16) engine-A/T-ECU connector.
	NOTE: Before checking harness, check intermediate connector C-16, and repair if necessary. Check earthing line for open circuit and damage. Q: Is the check result normal? YES: Go to Step 8 . NO: Repair.
	STEP 14. Perform voltage measurement at B-08 air flow sensor connector.
	Use special tool test harness (MB991709) to connect connector, and measure at pick-up harness. Ignition switch: ON Voltage between terminal No. 7 and earth. OK: 7 - 8 V Q: Is the check result normal? YES: Go to Step 17 . NO: Go to Step 15 .
	STEP 15. Connector check: C-112 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Go to Step 16 . NO: Repair.
	STEP 16. Check harness between B-08 (terminal No. 7) air flow sensor connector and C-112 (terminal No. 19) engine-A/T-ECU connector.
	Check reset signal line for short circuit. Q: Is the check result normal? YES: Replace air flow sensor. NO: Repair.
	STEP 17. Perform voltage measurement at B-08 air flow sensor connector.
	Use special tool test harness (MB991709) to connect connector, and measure at pick-up harness. Selector lever position: P Voltage between terminal No. 7 and earth. OK: 0 - 1 V (Engine: Idling) 6 - 9 V (Engine: 3,000 r/min) Q: Is the check result normal? YES: Go to Step 20 . NO: Go to Step 18 .
	STEP 18. Connector check: C-112 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Go to Step 19 . NO: Repair.
	STEP 19. Check harness between B-08 (terminal No. 7) air flow sensor connector and C-112 (terminal No. 19) engine-A/T-ECU connector.
	Check reset signal line for open circuit and damage. Q: Is the check result normal? YES: Go to Step 8 . NO: Repair.
	STEP 20. Output wave pattern measurement at B-08 air flow sensor connector (Use oscilloscope).
	Use special tool test harness (MB991709) to connect connector, and measure at pick-up harness. Engine: Idling Selector lever position: P Voltage between terminal No. 3 and earth. OK: Wave patterns should be displayed on inspection procedure using an oscilloscope (Refer to ) and noise should not be displayed in the wave pattern. Q: Is the check result normal? YES: Go to Step 8 . NO: Go to Step 21 .
	STEP 21. Replace air flow sensor. After replacing the air flow sensor, re-check the trouble symptoms. Q: Is the check result normal? YES: Go to Step 22 . NO: Check end.
	STEP 22. Connector check: B-17X engine control relay connector
	Q: Is the check result normal? YES: Go to Step 23 . NO: Repair.
	STEP 23. Check harness between B-08 (terminal No. 4) air flow sensor connector and B-17X (terminal No. 1) engine control relay connector.
	Check power supply line for damage. Q: Is the check result normal? YES: Go to Step 24 . NO: Repair.
	STEP 24. Connector check: C-111 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Go to Step 25 . NO: Repair.
	STEP 25. Check harness between B-08 (terminal No. 3) air flow sensor connector and C-111 (terminal No. 65) engine-A/T-ECU connector.
	Check output line for damage. Q: Is the check result normal? YES: Go to Step 26 . NO: Repair.
	STEP 26. Connector check: C-112 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Check intermediate connector C-16, and repair if necessary. If intermediate connector is normal, check and repair harness between B-08 (terminal No. 5) air flow sensor connector and C-127 (terminal No. 16) engine-A/T-ECU connector. Check earthing line for damage. No: Repair.