Fuel Injection System - Bosch Afc Mercury Tracer pre-I

DESCRIPTION

Note. For specifications on Throttle Position Sensor (TPS), idle speed and mixture, and fuel pump output volume, see appropriate article in TUNE-UP PROCEDURES section.

The Bosch AFC (L-Jetronic) fuel injection system is used on all models. However, variations may exist between model applications. This article covers the Bosch AFC system in general. For computer control information, see the appropriate article in the ENGINE PERFORMANCE section.

The Bosch Airflow Controlled (AFC) fuel injection system is an electronically controlled system operated by incoming airflow. Some vehicles are equipped with a potentiometer to measure incoming air flow, while other vehicles use a hot wire type airflow sensor.

The AFC fuel injection system also contains a feedback system which measures oxygen content of exhaust gases and maintains the air/fuel ratio at about 14.7:1.

The fuel injection system consists of an electric fuel pump, fuel pressure regulator, fuel damper, fuel injectors, cold start injector, Electronic Control Unit (ECU), and airflow meter. In addition, an air temperature sensor, throttle position sensor, coolant temperature sensor, oxygen sensor, catalytic converter, auxiliary air valve, idle speed control valve, throttle body, and electrical relays are used.

Note. Not all models use all components.

ELECTRIC FUEL PUMP(S)

Fuel under pressure from electric fuel pump flows through a fuel damper, fuel filter, injector fuel rail and fuel pressure regulator. Fuel pump(s) may be located on frame rail, in fuel tank or both. Electrical power for fuel pump operation during cranking mode is provided from starter relay via the fuel pump relay (if equipped) and ECU.

FUEL PRESSURE REGULATOR

The pressure regulator is a sealed unit which is divided by a diaphragm into 2 chambers (fuel and spring chambers). The fuel chamber receives fuel through the inlet side from the injector fuel rail. The spring chamber is connected to intake manifold vacuum.

At idle, intake manifold vacuum is high. The diaphragm is pulled down by intake manifold vacuum. Any excessive fuel is returned to the fuel tank. As the throttle is depressed, intake manifold vacuum decreases. The regulator spring overcomes manifold vacuum increasing fuel pressure.

FUEL INJECTORS

A fuel rail links the fuel pressure regulator with the fuel injectors. Each cylinder is provided with a solenoid-operated injector which sprays fuel toward the back of each intake valve.

ELECTRONIC CONTROL UNIT (ECU)

All components of the control system are electrically connected to the ECU. The ECU is a pre-programmed computer which receives and interprets data from various sensors to calculate the amount of fuel required by the engine to maintain efficiency with minimum exhaust emissions. The oxygen sensor informs the ECU of oxygen content of exhaust gases and the ECU constantly adjusts the air/fuel ratio by controlling the injector "on" time.

An automatic function of the ECU is to provide fuel enrichment whenever engine is cranked, regardless of engine temperature. This is activated by a direct electrical connection from the starter circuit to the ECU (most models). The ECU is a sealed unit, and no service is required.

Hot Wire Type

The airflow meter continually measures temperature, amount, density, and speed of air entering engine intake system. The meter consists of a platinum wire filament located within intake air stream.

The wire filament is kept at a constant temperature above that of air entering engine regardless of composition of air entering engine. The airflow meter sends a temperature related signal to be processed by the ECU. (Scheme 1)

Typical Hot Wire Airflow Meter. Scheme 1

Potentiometer Type

This airflow meter uses a movable vane connected to a potentiometer. As air entering the engine moves the vane, the potentiometer is moved informing the ECU on the amount of air entering the engine. Some potentiometer airflow meters use an air temperature sensor located inside the airflow meter air passage. (Scheme 2)

Typical Potentiometer Airflow Meter. Scheme 2

AIR TEMPERATURE SENSOR

The air temperature sensor is an integral component of the airflow meter which converts temperature of incoming air into electrical signals. These signals are received by the ECU and processed to adjust the amount of fuel delivered by the injectors. The air temperature sensor is not serviceable.

THROTTLE POSITION SENSOR (TPS)

A contact-type TPS is installed on the throttle chamber. It converts throttle position into electrical signals to inform ECU of throttle position. Signals are sent to ECU when throttle is fully open or at idle. (Scheme 3) Some models send a specific signal to ECU, depending on throttle angle. The open contacts prevent loss of power during sudden acceleration/deceleration by signaling ECU of the required fuel enrichment.

Typical Contact-Type Throttle Position Sensor. Scheme 3

COOLANT TEMPERATURE SENSOR

This sensor provides ECU with engine temperature information relating to warm-up enrichment operation. Some models use a dual-sensor element which also signals the ignition computer (if equipped).

During warm-up period after a cold engine start, additional fuel is required to maintain engine performance. As engine temperature increases, the ECU decreases fuel enrichment until engine reaches normal operating temperature.

ELECTRICAL RELAYS

The various relays used with the electronic controls of the AFC injection system control power to injectors, fuel pump, ECU, and cold start system. The electrical relays may consist of one component for all relays or a combination of individual relays.

AUXILIARY AIR VALVE

Most models with Bosch AFC fuel injection use an Auxiliary Air Valve (AAV) to shorten engine warm-up time. The AAV supplies additional air into the intake system which increases engine RPM during a cold start.

The AAV consists of an electrically heated bi-metallic strip, movable disc, and air by-pass channel. The heater coil on the bi-metallic strip is energized by the fuel pump relay. Control of the valve is based upon engine temperature; the air by-pass channel is open when engine is cold and gradually closes as temperature rises. At predetermined temperatures, air by-pass channel is blocked and additional airflow stops. (Scheme 4)

Auxiliary Air Valve (Typical). Scheme 4

COLD START INJECTOR

Most models use a cold start injector which delivers additional fuel, and a start injector time switch which controls operation of the cold start injector. The start injector time switch limits cold start injection to 1-12 seconds, depending upon engine coolant temperature. When engine coolant temperature rises above a specified point, bi-metallic contact breaks ground circuit of cold start injector and cold start enrichment is by-passed.

INERTIA SWITCH

When impact forces exceed the magnetic force that retains the ball, it triggers the contact that opens the electric circuit. When this circuit is opened, the fuel pumps stop.

If the inertia switch is tripped, the engine stops running and cannot be restarted until it is reset. On Merkur XR4Ti, to reset switch, open liftgate and fold floor covering back for access to spare tire well. Press button on top of the inertia switch. (Scheme 5)

On Scorpio, open liftgate and press reset switch located near liftgate lock striker. On Tracer 3 and 5 door models, open liftgate and reset switch located on left side of spare tire well. On Tracer station wagons, switch is located in vehicle jack storage compartment.

Note. DO NOT reset the inertia switch until the complete fuel system has been inspected for leaks.

Merkur XR4Ti Inertia Switch (Typical). Scheme 5

ADJUSTMENTS

Note. For specifications on Throttle Position Sensor (TPS), idle speed and mixture, and fuel pump output volume, see appropriate article in the TUNE-UP section.

TESTING

Note. For testing of fuel system components not covered in this article, refer to appropriate article in the ENGINE PERFORMANCE section.

DO NOT attempt to test ECU, as permanent damage could result. It is possible to check wires for continuity. The ECU should only by judged faulty after compression is checked, ignition system has been tested and found problem-free, and all other fuel injection components have been thoroughly tested (including wiring).

Note. AFC electrical systems can be checked by using Electronic Fuel Injection testers prescribed by the manufacturer. Instructions for use of testers must be followed carefully to prevent damage to system.

FUEL INJECTORS & RESISTORS

1. Connect tachometer to engine. Start engine and run at idle. Remove harness connector from injectors one at a time. Engine idle speed should drop 100-300 RPM as each injector is disconnected. If engine idle speed does not drop, check the wiring connector, injector resistance or injection signal from the computer.
2. Disconnect electrical connector from each injector. Measure injector resistance. See «INJECTOR RESISTANCE SPECIFICATIONS»(/mercury/tracer/pre-i-1988-1989/remont/testing-diagnostics/#fuel-injection-system-bosch-afc) table. If injector is not to specification, replace injector. INJECTOR RESISTANCE SPECIFICATIONS Application Ohms Merkur XR4Ti 2-2.7 Scorpio 16-18 Tracer Built Before 11-2-87 1-3 Built After 11-2-87 11-15
3. On Tracer, check injector resistors. Disconnect resistor block. Measure resistance between terminal "B" and terminal No. 1, 2, 3 and 4. If reading is not 6 ohms, replace resistor block.

FUEL PRESSURE

Merkur XR4Ti

1. Relieve fuel system pressure. Connect Fuel Pressure Gauge (T85L-9974-A) and Adapter Hose (T85L-9974-A) to Schrader valve on fuel manifold. Start engine. If the engine will not start, remove fuel pump relay from fuse panel. Connect a jumper wire from relay terminals No. 87 to No. 30. (Scheme 6) With engine idling, fuel pressure should be 35-45 psi (2.5-3.2 kg/cm 2 ).
2. If fuel pressure is not within specification, check for plugged fuel filter, restricted fuel line, leaking fuel pressure regulator or faulty fuel pump. Turn engine off. Fuel system should hold at least 30 psi (2.1 kg/cm 2 ).

Identification of Typical Relay Terminals. Scheme 6

Scorpio

1. Relieve fuel system pressure. Disconnect fuel pressure regulator fuel return line. Connect Fuel Pressure Gauge (T80L-9974-B) to Schrader vavle on fuel rail.
2. Connect a jumper wire to Self-Test connector Black/Red wire. Turn ignition switch to "RUN" position. Connect other end of jumper wire to ground for one minute.
3. Fuel pressure should be 43.5 psi (3.0 kg/cm 2 ). Immediately after fuel pump shutdown, fuel pressure should be 30 psi (2.0 kg/cm 2 ). If fuel pressure is not to specification, check fuel pressure regulator, restricted fuel lines, leaking fuel injectors or malfunctioning fuel pump.

Tracer

1. Relieve fuel system pressure. Connect Fuel Pressure Gauge (T80L-9974-A) and Adapter Hose (T85L-9974-A) to the schrader valve on the fuel supply manifold.
2. Start the engine. If engine does not start, remove fuel pump relay from circuit protection panel, and connect a jumper wire from relay terminal 87 in the panel to the alternator ouput terminal.
3. With pump running, check reading on pressure gauge. If pressure reaches 28.4-31.3 psi (20.0-22.0 Kg/cm 2 ) at idle with vacuum, 35.6-40.5 psi (25.0-28.5 Kg/cm 2 ) at idle without vacuum, pressure is okay. Go to step 4). If pressure is low, remove fuel pressure gauge. Connect fuel pressure gauge to fuel pump outlet connection, located under rear seat. With fuel pump check connector jumpered, turn ignition on. If fuel pressure is now to specification, check fuel lines for restrictions. If fuel pressure is still low, check fuel filter.
4. Insert test volume hose in a graduated container and open the flow control valve for ten seconds. If pump delivers 73.2 cubic inches/minute @ 36.3 psi (1.2 cubic liters/minute @ 25.5 Kg/cm 2 ), volume is okay. If volume is low, check fuel delivery system.
5. Disconnect jumper wire to stop pumps and observe pressure gauge. If pressure holds at 30 psi. (20.7 Kg/cm 2 ), system is okay. If pressure drops, check for leaks. If no leaks are found, check individual pumps. Install fuel pump relay.

Note. Maximum pressure is 64.0-85.3 psi (45.0-60.0 Kg/cm 2 ).

On models with air temperature sensor, turn ignition off. Disconnect electrical connector at airflow meter (break-out box terminals No. 25 and 46 on Merkur XR4Ti) and connect ohmmeter between terminals No. 6 and 27. If readings are not to specification, replace airflow meter.

TEMPERATURE SENSOR RESISTANCE

See appropriate article in ENGINE PERFORMANCE section.

Disconnect connector from temperature sensor. Using an ohmmeter, measure resistance across sensor terminals. Refer to the SCORPIO AIR TEMPERATURE SENSOR RESISTANCE table.

SCORPIO AIR TEMPERATURE SENSOR RESISTANCE

Disconnect airflow meter connector. Using an ohmmeter, check resistance across meter terminals. (Scheme 8) Also, refer to the TRACER AIRFLOW METER RESISTANCE table.

Airflow Meter Terminals (Typical). Scheme 7

TRACER AIRFLOW METER RESISTANCE

Ensure engine is cold, then start engine. Pinch rubber hose between air valve and throttle chamber. Engine speed should decrease. After engine reaches operating temperature, pinch hose again. Engine speed should not decrease more than 50 RPM. If valve does not operate as outlined, replace auxiliary air valve.

Turn ignition on. Connect a vacuum pump to pressure sensor. Leave connector connected to sensor. Connect a voltmeter between Light Green wire and ground. With no vacuum applied to sensor, voltage should be zero. Apply 30 in. Hg to sensor. Voltmeter should indicate 3.5-4.5 volts.

Note. For all other models, refer to appropriate article in 1988 COMPUTERIZED ENGINE CONTROLS section.

1. Check wiring and controller for damage or deterioration. Repair as necessary. Connect test light to idle speed actuator between Red wire and ground. Turn ignition on. If light is on, go to next step. If light is off, repair open in power feed circuit.
2. Connect jumper wire from actuator Gray wire to ground and turn ignition on. If speed controller operates, go to next step. If not, replace idle speed controller.
3. Using an ohmmeter, check for continuity between wire at ECA pin No. 21 and Gray wire at controller. If no continuity, repair wiring. If continuity is present, idle speed controller circuit is okay.

Note. For all other models, refer to appropriate article in the ENGINE PERFORMANCE section.

All Models

Disconnect coolant temperature sensor connector. Using an ohmmeter, measure resistance between sensor terminals. Refer to the COOLANT TEMPERATURE SENSOR RESISTANCE table.

COOLANT TEMPERATURE SENSOR RESISTANCE

WIRING DIAGRAMS

Note. Tracer Wiring Diagram applies only to vehicles built before 11/2/87.

Merkur EEC-IV Wiring Diagram. Scheme 8

Scheme 9

Tracer EEC System Wiring Diagram (Prior to 11-02-87). Scheme 10

See also:
• INJECTOR RESISTANCE SPECIFICATIONS