Inspection Procedure 37: Mixture Adjusting Screw (Variable Resistor) System

OPERATION

A power voltage of 5 V is applied to the mixture adjusting screw (terminal No. 3) from the engine-A/T-ECU (terminal No. 46).

The power voltage is earthed to the engine-A/T-ECU (terminal No. 57) from the mixture adjusting screw (terminal No. 1).

The sensor signal is inputted to the engine-A/T-ECU (terminal No. 71) from the mixture adjusting screw output terminal (terminal No. 2).

FUNCTION

The mixture adjusting screw is a variable resistor for manually adjusting the idling fuel mixture.
When the shaft of the mixture adjusting screw is turned, the resistance between the output terminal and the earth terminal changes. Because of this, the output voltage also changes in accordance with the turning of the shaft.
The engine-A/T-ECU controls the injectors to produce a richer idling fuel mixture in accordance with the increase in the output voltage.

PROBABLE CAUSE

Failed mixture adjusting screw

Open/short circuit in mixture adjusting screw 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 17: Mixture adjusting screw

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-10 mixture adjusting screw connector

Q: Is the check result normal?

YES: Go to Step 3

.
NO: Repair.

STEP 3. Check mixture adjusting screw itself.

Check mixture adjusting screw itself (Refer to ).

Q: Is the check result normal?

YES: Go to Step 4

.
NO: Replace mixture adjusting screw.

STEP 4. Perform voltage measurement at B-10 mixture adjusting screw 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 10

.
NO: Go to Step 5

STEP 5. 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. 46 and earth.

OK: 4.9 - 5.1 V

Q: Is the check result normal?

YES: Go to Step 6

.
NO: Go to Step 7

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

Q: Is the check result normal?

YES:

Check and repair harness between B-10 (terminal No. 3) mixture adjusting screw connector and C-111 (terminal No. 46) engine-A/T-ECU connector.

Check power supply line for open circuit.

NO: Repair.

STEP 7. Connector check: C-111 engine-A/T-ECU connector

Q: Is the check result normal?

YES: Go to Step 8

.
NO: Repair.

STEP 8. Check harness between B-10 (terminal No. 3) mixture adjusting screw connector and C-111 (terminal No. 46) engine-A/T-ECU connector.

Check power supply line for short circuit.

Q: Is the check result normal?

YES: Go to Step 9

.
NO: Repair.

STEP 9. MUT-II/III Data List

Refer to Data list reference table .

Item 17: Mixture adjusting screw

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 10. Perform resistance measurement at B-10 mixture adjusting screw connector.

Disconnect connector and measure at harness side.

Resistance between terminal No. 1 and earth.

OK: 2 Ω or less

Q: Is the check result normal?

YES: Go to Step 13

.
NO: Go to Step 11

STEP 11. Connector check: C-111 engine-A/T-ECU connector

Q: Is the check result normal?

YES: Go to Step 12

.
NO: Repair.

STEP 12. Check harness between B-10 (terminal No. 1) mixture adjusting screw connector and C-111 (terminal No. 57) engine-A/T-ECU connector.

Check earthing line for open circuit and damage.

Q: Is the check result normal?

YES: Go to Step 9

.
NO: Repair.

STEP 13. Perform voltage measurement at B-10 mixture adjusting screw connector.

Use special tool test harness (MB991536) to connect connector, and measure at pick-up harness.

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 15

.
NO: Go to Step 14

STEP 14. Connector check: C-111 engine-A/T-ECU connector

Q: Is the check result normal?

YES:

Check and repair harness between B-10 (terminal No. 3) mixture adjusting screw connector and C-111 (terminal No. 46) engine-A/T-ECU connector.

Check power supply line for damage.

NO: Repair.

STEP 15. Perform voltage measurement at B-10 mixture adjusting screw connector.

Use special tool test harness (MB991536) to connect connector, and measure at pick-up harness.

Ignition switch: ON

Voltage between terminal No. 1 and earth.

OK: 0.5 V or less

Q: Is the check result normal?

YES: Go to Step 17

.
NO: Go to Step 16

STEP 16. Connector check: C-111 engine-A/T-ECU connector

Q: Is the check result normal?

YES:

Check and repair harness between B-10 (terminal No. 1) mixture adjusting screw connector and C-111 (terminal No. 57) engine-A/T-ECU connector.

Check earthing line for damage.

NO: Repair.

STEP 17. Perform voltage measurement at B-10 mixture adjusting screw connector

Use special tool test harness (MB991536) to connect connector, and measure at pick-up harness.

Ignition switch: ON

Voltage between terminal No. 2 and earth.

OK:

Accelerator pedal fully released: 0.536 - 0.735 V

Accelerator pedal fully depressed: 4.5 - 5.0 V

Q: Is the check result normal?

YES: Go to Step 19

.
NO: Go to Step 18

STEP 18. Connector check: C-110 engine-A/T-ECU connector

Q: Is the check result normal?

YES:

Check and repair harness between B-10 (terminal No. 2) mixture adjusting screw connector and C-110 (terminal No. 71) engine-A/T-ECU connector.

Check output line for short circuit and damage.

NO: Repair.

STEP 19. Perform voltage measurement at C-110 engine-A/T-ECU connector

Measure engine-A/T-ECU terminal voltage.

Ignition switch: ON

Voltage between terminal No. 71 and earth.

OK:

Accelerator pedal fully released: 0.536 - 0.735 V

Accelerator pedal fully depressed: 4.5 - 5.0 V

Q: Is the check result normal?

YES: Go to Step 21

.
NO: Go to Step 20

STEP 20. Connector check: C-110 engine-A/T-ECU connector

Q: Is the check result normal?

YES:

Check and repair harness between B-10 (terminal No. 2) mixture adjusting screw connector and C-110 (terminal No. 71) engine-A/T-ECU connector.

Check output line for open circuit and damage.

NO: Repair.

STEP 21. Connector check: C-110 engine-A/T-ECU connector

Q: Is the check result normal?

YES: Go to Step 9

.
NO: Repair.


Inspection Procedure 37: Mixture Adjusting Screw (Variable Resistor) System

OPERATION A power voltage of 5 V is applied to the mixture adjusting screw (terminal No. 3) from the engine-A/T-ECU (terminal No. 46). The power voltage is earthed to the engine-A/T-ECU (terminal No. 57) from the mixture adjusting screw (terminal No. 1). The sensor signal is inputted to the engine-A/T-ECU (terminal No. 71) from the mixture adjusting screw output terminal (terminal No. 2).
FUNCTION The mixture adjusting screw is a variable resistor for manually adjusting the idling fuel mixture. When the shaft of the mixture adjusting screw is turned, the resistance between the output terminal and the earth terminal changes. Because of this, the output voltage also changes in accordance with the turning of the shaft. The engine-A/T-ECU controls the injectors to produce a richer idling fuel mixture in accordance with the increase in the output voltage.
PROBABLE CAUSE Failed mixture adjusting screw Open/short circuit in mixture adjusting screw 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 17: Mixture adjusting screw 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-10 mixture adjusting screw connector
	Q: Is the check result normal? YES: Go to Step 3 . NO: Repair.
	STEP 3. Check mixture adjusting screw itself. Check mixture adjusting screw itself (Refer to ). Q: Is the check result normal? YES: Go to Step 4 . NO: Replace mixture adjusting screw.
	STEP 4. Perform voltage measurement at B-10 mixture adjusting screw 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 10 . NO: Go to Step 5 .
	STEP 5. 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. 46 and earth. OK: 4.9 - 5.1 V Q: Is the check result normal? YES: Go to Step 6 . NO: Go to Step 7 .
	STEP 6. Connector check: C-111 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Check and repair harness between B-10 (terminal No. 3) mixture adjusting screw connector and C-111 (terminal No. 46) engine-A/T-ECU connector. Check power supply line for open circuit. NO: Repair.
	STEP 7. Connector check: C-111 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Go to Step 8 . NO: Repair.
	STEP 8. Check harness between B-10 (terminal No. 3) mixture adjusting screw connector and C-111 (terminal No. 46) engine-A/T-ECU connector. Check power supply line for short circuit.
	Q: Is the check result normal? YES: Go to Step 9 . NO: Repair.
	STEP 9. MUT-II/III Data List Refer to Data list reference table . Item 17: Mixture adjusting screw 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 10. Perform resistance measurement at B-10 mixture adjusting screw connector.
	Disconnect connector and measure at harness side. Resistance between terminal No. 1 and earth. OK: 2 Ω or less Q: Is the check result normal? YES: Go to Step 13 . NO: Go to Step 11 .
	STEP 11. Connector check: C-111 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Go to Step 12 . NO: Repair.
	STEP 12. Check harness between B-10 (terminal No. 1) mixture adjusting screw connector and C-111 (terminal No. 57) engine-A/T-ECU connector.
	Check earthing line for open circuit and damage. Q: Is the check result normal? YES: Go to Step 9 . NO: Repair.
	STEP 13. Perform voltage measurement at B-10 mixture adjusting screw connector.
	Use special tool test harness (MB991536) to connect connector, and measure at pick-up harness. 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 15 . NO: Go to Step 14 .
	STEP 14. Connector check: C-111 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Check and repair harness between B-10 (terminal No. 3) mixture adjusting screw connector and C-111 (terminal No. 46) engine-A/T-ECU connector. Check power supply line for damage. NO: Repair.
	STEP 15. Perform voltage measurement at B-10 mixture adjusting screw connector.
	Use special tool test harness (MB991536) to connect connector, and measure at pick-up harness. Ignition switch: ON Voltage between terminal No. 1 and earth. OK: 0.5 V or less Q: Is the check result normal? YES: Go to Step 17 . NO: Go to Step 16 .
	STEP 16. Connector check: C-111 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Check and repair harness between B-10 (terminal No. 1) mixture adjusting screw connector and C-111 (terminal No. 57) engine-A/T-ECU connector. Check earthing line for damage. NO: Repair.
	STEP 17. Perform voltage measurement at B-10 mixture adjusting screw connector
	Use special tool test harness (MB991536) to connect connector, and measure at pick-up harness. Ignition switch: ON Voltage between terminal No. 2 and earth. OK: Accelerator pedal fully released: 0.536 - 0.735 V Accelerator pedal fully depressed: 4.5 - 5.0 V Q: Is the check result normal? YES: Go to Step 19 . NO: Go to Step 18 .
	STEP 18. Connector check: C-110 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Check and repair harness between B-10 (terminal No. 2) mixture adjusting screw connector and C-110 (terminal No. 71) engine-A/T-ECU connector. Check output line for short circuit and damage. NO: Repair.
	STEP 19. Perform voltage measurement at C-110 engine-A/T-ECU connector
	Measure engine-A/T-ECU terminal voltage. Ignition switch: ON Voltage between terminal No. 71 and earth. OK: Accelerator pedal fully released: 0.536 - 0.735 V Accelerator pedal fully depressed: 4.5 - 5.0 V Q: Is the check result normal? YES: Go to Step 21 . NO: Go to Step 20 .
	STEP 20. Connector check: C-110 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Check and repair harness between B-10 (terminal No. 2) mixture adjusting screw connector and C-110 (terminal No. 71) engine-A/T-ECU connector. Check output line for open circuit and damage. NO: Repair.
	STEP 21. Connector check: C-110 engine-A/T-ECU connector
	Q: Is the check result normal? YES: Go to Step 9 . NO: Repair.