Inspection Procedure 30: Mixture adjusting screw (variable resistor) system <Vehicles without catalytic converter>

OPERATION

A power voltage of 5 V is applied to the mixture adjusting screw (terminal No. 3) from the engine-ECU <M/T> or engine-A/T-ECU <A/T> (terminal No. 97).
The power voltage is earthed to the engine-ECU <M/T> or engine-A/T-ECU <A/T> (terminal No. 96) from the mixture adjusting screw (terminal No. 1).
The sensor signal is inputted to the engine-ECU <M/T> or engine-A/T-ECU <A/T> (terminal No. 117) 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-ECU <M/T> or engine-A/T-ECU <A/T> controls the injectors to produce a richer idling fuel mixture in accordance with the increase in the output voltage.

PROBABLE CAUSES

Failed mixture adjusting screw
Open/short circuit in mixture adjusting screw or loose connector contact
Failed engine-ECU <M/T> or engine-A/T-ECU <A/T>

DIAGNOSIS PROCEDURE

STEP 1. Connector check: B-47 mixture adjusting screw connector

Q. Is the check result normal?

	Go to Step 2 .

	Repair or replace the connector.

STEP 2. Check mixture adjusting screw itself.

Check mixture adjusting screw itself (Refer to ).

Q. Is the check result normal?

	Go to Step 3 .

	Replace the mixture adjusting screw.

STEP 3. Perform voltage measurement at B-47 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?

	Go to Step 8 .

	Go to Step 4 .

STEP 4. Perform voltage measurement at D-246 engine-ECU <M/T> connector or D-217 engine-A/T-ECU <A/T> connector.

Measure engine-ECU <M/T> or engine-A/T-ECU <A/T> terminal voltage.
Ignition switch: ON
Voltage between terminal No. 97 and earth.

OK: 4.9 - 5.1 V

Q. Is the check result normal?

	Go to Step 5 .

	Go to Step 6 .

STEP 5. Connector check: D-246 engine-ECU <M/T> connector or D-217 engine-A/T-ECU <A/T> connector

Q. Is the check result normal?

	Check intermediate connector D-111, and repair if necessary. If intermediate connector is normal, check and repair harness between B-47 (terminal No. 3) mixture adjusting screw connector and D-246 (terminal No. 97) engine-ECU <M/T> connector or D-217 (terminal No. 97) engine-A/T-ECU <A/T> connector. Check power supply line for open circuit.

	Repair or replace the connector.

STEP 6. Connector check: D-246 engine-ECU <M/T> connector or D-217 engine-A/T-ECU <A/T> connector

Q. Is the check result normal?

	Go to Step 7 .

	Repair or replace the connector.

STEP 7. Check harness between B-47 (terminal No. 3) mixture adjusting screw connector and D-246 (terminal No. 97) engine-ECU <M/T> connector or D-217 (terminal No. 97) engine-A/T-ECU <A/T> connector.

note

Before checking harness, check intermediate connector D-111, and repair if necessary.

Check power supply line for short circuit.

Q. Is the check result normal?

	Replace the engine-ECU <M/T> or engine-A/T-ECU <A/T>.

	Repair the damage harness wire.

STEP 8. Perform resistance measurement at B-47 mixture adjusting screw connector.

Disconnect connector and measure at harness side.
Resistance between terminal No. 1 and earth.

OK: Continuity (2 Ω or less)

Q. Is the check result normal?

	Go to Step 11 .

	Go to Step 9 .

STEP 9. Connector check: D-246 engine-ECU <M/T> connector or D-217 engine-A/T-ECU <A/T> connector

Q. Is the check result normal?

	Go to Step 10 .

	Repair or replace the connector.

STEP 10. Check harness between B-47 (terminal No. 1) mixture adjusting screw connector and D-246 (terminal No. 96) engine-ECU <M/T> connector or D-217 (terminal No. 96) engine-A/T-ECU <A/T> connector.

note

Before checking harness, check intermediate connector D-111, and repair if necessary.

Check earthing line for open circuit and damage.

Q. Is the check result normal?

	Replace the engine-ECU <M/T> or engine-A/T-ECU <A/T>.

	Repair the damage harness wire.

STEP 11. Perform voltage measurement at B-47 mixture adjusting screw connector.

Measure mixture adjusting screw terminal voltage.
Ignition switch: ON
Voltage between terminal No. 3 and earth.

OK: 4.9 - 5.1 V

Q. Is the check result normal?

	Go to Step 13 .

	Go to Step 12 .

STEP 12. Connector check: D-246 engine-ECU <M/T> connector or D-217 engine-A/T-ECU <A/T> connector

Q. Is the check result normal?

	Check intermediate connector D-111, and repair if necessary. If intermediate connector is normal, check and repair harness between B-47 (terminal No. 3) mixture adjusting screw connector and D-246 (terminal No. 97) engine-ECU <M/T> connector or D-217 (terminal No. 97) engine-A/T-ECU <A/T> connector. Check power supply line for damage.

	Repair or replace the connector.

STEP 13. Perform voltage measurement at B-47 mixture adjusting screw connector

Measure mixture adjusting screw terminal voltage.
Ignition switch: ON
Voltage between terminal No. 2 and earth.

OK: 1.0 - 4.0 V

Q. Is the check result normal?

	Go to Step 15 .

	Go to Step 14 .

STEP 14. Connector check: D-246 engine-ECU <M/T> connector or D-217 engine-A/T-ECU <A/T> connector

Q. Is the check result normal?

	Check and repair harness between B-47 (terminal No. 2) mixture adjusting screw connector and D-246 (terminal No. 117) engine-ECU <M/T> connector or D-217 (terminal No. 117) engine-A/T-ECU <A/T> connector. Check output line for short circuit.

	Repair or replace the connector.

STEP 15. Perform voltage measurement at D-246 engine-ECU <M/T> connector or D-217 engine-A/T-ECU <A/T> connector

Measure engine-ECU <M/T> or engine-A/T-ECU <A/T> terminal voltage.
Ignition switch: ON
Voltage between terminal No. 117 and earth.

OK: 1.0 - 4.0 V

Q. Is the check result normal?

	Go to Step 17 .

	Go to Step 16 .

STEP 16. Connector check: D-246 engine-ECU <M/T> connector or D-217 engine-A/T-ECU <A/T> connector

Q. Is the check result normal?

	Check and repair harness between B-47 (terminal No. 2) mixture adjusting screw connector and D-246 (terminal No. 117) engine-ECU <M/T> connector or D-217 (terminal No. 117) engine-A/T-ECU <A/T> connector. Check output line for open circuit and damage.

	Repair or replace the connector.

STEP 17. Connector check: D-246 engine-ECU <M/T> connector or D-217 engine-A/T-ECU <A/T> connector

Q. Is the check result normal?

	Replace the engine-ECU <M/T> or engine-A/T-ECU <A/T>.

	Repair or replace the connector.