DESCRIPTION
The Volvo Computerized Engine Control System is used on Volvo 4-cylinder engines. It consists of a computer controlled electronic ignition system and Bosch LH-Jetronic 2.2 fuel system with integrated Constant Idle Speed system.
The computer controlled ignition system consists of Volvo breakerless distributor, an electronic control unit (ECU), ignition coil, ignition switch, throttle switch and knock sensor. The distributor contains no vacuum or centrifugal advance mechanisms. The rotor and armature are a single unit. (Scheme 4)
Exploded View of Volvo Electronic Distributor Distributor uses a Hall Effect sending unit. Scheme 4
A Hall Effect sending unit in distributor provides engine speed signals to ECU. The distributor also consists of necessary connecting wiring and a distributor cap.
The fuel system includes an ECU, ignition coil, ignition switch, system relay, fuel system components, A/C switch, airflow meter, air control valve, starter motor, throttle switch, coolant temperature sensor, oxygen sensor and connecting wiring and hoses.
Note. Only those components directly used in the Constant Idle Speed system will be covered in this section. For information on fuel injection system, refer to appropriate FUEL INJECTION article.
Volvo Computer Controlled Ignition System. Scheme 5
COMPUTER CONTROLLED IGNITION
As engine turns distributor shaft, shutter blades of rotor/armature assembly pass through Hall Effect sending unit's pick-up coil. A signal is then sent to the ECU.
In addition to receiving engine speed information from distributor, the ECU receives engine load information from intake manifold. Speed and vacuum signals are monitored by the ECU to control spark advance and dwell.
A throttle switch provides throttle information. Under normal conditions, throttle position signal is sent to the ECU.
The manifold vacuum signal is also processed by the control unit to control ignition timing. When throttle is closed, (during deceleration and idle) the circuit is closed, and no signal is sent to the ECU.
The ECU analyzes all information, and interrupts signal to ignition coil primary circuit. This causes a high voltage surge in secondary circuit, firing spark plugs at proper time.
The knock sensor detects engine detonation. The ECU receives a signal from the knock sensor during engine detonation and then retards timing. Timing is retarded 2° every 20 milliseconds until maximum retard of 6° is obtained, or detonation stops. Timing is then advanced in 1° increments every 20 milliseconds, until base timing is achieved or detonation begins again.
Electronic Control Unit
The ECU receives and processes information from the engine coolant temperature sensor, throttle switch and distributor. After receiving information on coolant temperature, throttle position and engine speed, the control unit sends electrical impulses to air control valve to regulate idle speed.
Air Control Valve
The air control valve consists of a small electric motor mounted between connecting hoses at air flow throttle valve. The electric motor rotates clockwise or counterclockwise, depending on electrical signals received from the ECU.
The control valve is rotated to increase or decrease amount of air which is taken from in front of throttle valve, by-passed around throttle valve and injected into intake manifold (behind throttle valve).
Throttle Switch
Throttle switch is a small electrical switch activated by throttle lever. The switch provides control unit with idle and full throttle position information.
Coolant Temperature Sensor
A coolant temperature sensor provides an electrical impulse to the ECU when coolant temperatures are low. The control unit then activates air control valve to provide more air (to increase engine speed).
A/C Switch
When A/C is activated, control unit increases idle speed to compensate for additional load caused by the A/C compressor.
Bosch Electronic Fuel Injection System With Integral Constant Idle Speed Control System. Scheme 6
ENGINE DOES NOT START
Check fuse No. 12 in fuse block and fuel pump fuse at rear of engine compartment. Check ignition system, starting system, charging system, and all electrical connectors for proper connections, routing and function. Check air intake system and fuel system. Check computer system and fuel pump relays.
ENGINE HARD TO START (COLD & WARM)
Check starting and charging systems. Check electrical and fuel connectors for proper connections and routing. Check air intake system and fuel system. Check air temperature sensor.
POOR OR NO IDLE
Check air intake system and fuel system. Check airflow meter.
EXCESSIVE FUEL CONSUMPTION
Check fuel system for excessive pressure and leaks. Check throttle valve switch and mixture adjustment.
POOR PERFORMANCE (LOW TOP SPEED)
Check air intake system, fuel pressure, and fuel pump. Check throttle valve switch and mixture adjustment.
ERRATIC IDLE
Check air intake system and fuel injectors. Check air control valve hose for proper connection and blockage. Check throttle valve adjustment and attaching screws.
IDLE SPEED TOO HIGH
Check throttle valve and throttle valve switch. Check idle speed. Check air temperature sensor.
IDLE SPEED TOO LOW (ENGINE COLD)
Check air temperature sensor and air control valve. Check idle speed control system.
JERKING, POOR ACCELERATION
Check acceleration enrichment.
DIAGNOSIS & TESTING
Note. Only components directly related to Constant Idle Speed system will be covered in this article. Information not available for ignition system testing. Refer to appropriate article in FUEL SYSTEM section for fuel system components.
| CAUTION | Before testing components, ensure battery is properly charged, all wires are okay, and that connections are secure. Inspect distributor cap and rotor for cracks and carbon tracking. Turn ignition off when connecting test equipment, connecting or disconnecting ECU, or when replacing parts. |
ELECTRONIC CONTROL UNIT FOR THE CONSTANT IDLE SPEED SYSTEM
- Turn ignition off and remove passenger kick panel. Push latch to side and disconnect electrical connector from control unit by pulling connector down from top. Remove screw in end of connector cover and separate connector block from cover. NOTE: Always check terminals through holes in side of connector. Terminal numbers are stamped on side of connector.
- To check ground connections, connect ohmmeter between terminal "11" and ground, then terminal "25" and ground. Resistance should be zero ohms. If not, check ground points on intake manifold.
- To check power supply, turn ignition on. Connect voltmeter between terminal "18" and ground. Battery voltage should be registered. If not, check wire for short or open. Turn ignition off.
- To check system relay, connect voltmeter between terminal "9" and ground. Using a jumper wire, jumper terminal "21" to ground. Relay should energize and battery voltage should be indicated. If not, check for short or ground. If wiring is okay, replace system relay.
| Component | Test Terminals (1) | Test Conditions | Value |
|---|---|---|---|
| ECU power supply from ignition switch | 18 and ground | Ignition ON | Battery voltage |
| ECU power supply from main relay | 9 and ground | Ignition OFF, terminal 21 grounded | Battery voltage |
| ECU ground connections | 5 and ground, 11 and ground, 19 and ground, 11 and 19, 25 and ground | Ignition OFF | Continuity |
| Fuel pump power supply from main relay | N/A | Ignition ON, terminal 17 grounded | Fuel pumps should operate |
| Fuel injectors | 13 (at ECU connector) and 87/2 (at main relay) | Ignition OFF | 4.0 ohms |
| Coolant temperature sensor | 2 and ground | Ignition OFF | See COOLANT TEMPERATURE SENSOR RESISTANCE table |
| Throttle switch idle contacts | 3 and ground | Throttle closed | Continuity |
| " | " | Throttle open | No continuity |
| Throttle switch full-throttle contacts | 12 and ground | Throttle fully open | Continuity |
| " | " | Throttle less than fully open | No continuity |
| Air mass meter power supply | 5 at air mass meter connector (orange wire) and ground | Ignition ON, terminal 21 grounded | Battery voltage |
| Air mass meter ground | 1 at air mass meter connector (black wire) and ground | Ignition OFF | Continuity |
| Air mass meter hot wire | 6 and 7 | Ignition OFF | 3.5-4.0 ohms |
| Air mass meter idle mixture potentiometer | 6 and 14 | Basic check | 0-1000 ohms (varies with idle mixture adjustment) |
| Engine RPM signal | 1 and ground | Ignition ON | Battery voltage |
| Oxygen sensor wiring harness | 20 and ground | Oxygen sensor connected | No continuity |
| " | " | Separate connector and connect green wire to ground | Continuity |
| Lambda-sond test connector | 22 and ground | Ignition OFF | No continuity |
| Idle speed test connector | 12 and ground | Ignition OFF | No continuity |
| Air control valve | 10 (at ECU connector) 87/2 (at main relay) | Ignition OFF | Approximately 20 ohms |
| " | 23 (at ECU connector) 87/2 (at main relay) | Ignition OFF | Approximately 20 ohms |
| AC microswitch | 16 and ground | AC switch OFF | No continuity |
| " | " | AC switch ON | Resistance varies as knob turns |
| (1) At ECU connector unless otherwise specified. | |||
| (1) | At ECU connector unless otherwise specified. |
LH SYSTEM ELECTRICAL TEST
- Connect ohmmeter between ground and terminal "3" at ECU connector. With ignition off, depress accelerator pedal. Zero resistance should be present at idle, and infinite resistance should be measured at all other throttle positions.
- Connect ohmmeter between ground and terminal "12" at ECU connector. Repeat step 1). Infinite resistance should be measured at idle, and zero resistance should be measured at all other throttle positions.
- If resistance readings are not as specified in steps 1) and 2), check throttle switch ground at intake manifold. If ground connection is okay, replace throttle switch.
Connect ohmmeter between ground and terminal "16" at ECU connector. As A/C knob is rotated, resistance should decrease. If not, replace A/C switch.
AIR MASS METER CIRCUIT
- Turn ignition off. Ensure resistance across ECU terminals No. 6 and 7 is 3.7 ohms. If not, check circuits or replace airflow meter. check resistance across ECU terminals No. 6 and 14. It should be zero to 1000 ohms. If resistance is not as specified, adjust CO level. See TUNE-UP article in TUNE-UP section.
- If resistance is still out of specifications after adjusting CO or if CO cannot be adjusted, the circuit is defective. Turn CO adjustment screw to full counterclockwise position.
- If resistance is not at or near 1000 ohms, a short circuit is indicated. Turn CO adjustment screw to full clockwise position. If resistance is not at or near zero ohms, an open circuit is indicated.
Using a thermometer, record coolant temperature. Connect ohmmeter between ground and terminal "2" at ECU connector. If reading is not as specified, replace sensor. See COOLANT TEMPERATURE SENSOR RESISTANCE table. Extremely high resistance value indicates open circuit and zero resistance indicates short circuit.
| Temperature | (Ohms) |
|---|---|
| 68°F (20°C) | 2280-2720 |
| 175°F (80°C) | 290-364 |
COOLANT TEMPERATURE SENSOR RESISTANCE
Connect ohmmeter between fuel pump relay terminal "87" and ECU connecter terminal "10". Then connect ohmmeter between same fuel pump relay terminal and ECU connecter terminal "23". Resistance should be about 20 ohms for both connections.
- Before adjusting switch, open throttle valve slightly and listen. If a "click" is heard when throttle valve is opened, throttle switch is properly adjusted. If sound is not heard, adjust switch.
- To adjust throttle switch, loosen switch retaining screws and turn switch clockwise. Turn switch counterclockwise until switch touches contact stop. Verify that throttle valve is not open. Tighten retaining screws and repeat step 1).
See also:
• COOLANT TEMPERATURE SENSOR RESISTANCE