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Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

All References to ARM = “Alfa Romeo Workshop Manual - Spider” Publication PA 3748

Bosch L-Jetronic Fuel Injection System (ARM 04-3)

Basics of Bosch L-Jetronic Fuel Injection System
An electric fuel pump provides pressurized fuel, controlled by a pressure regulator, to the four fuel injectors located in the intake manifold (an additional injector functions only during cold starts). Each injector contains a tiny electromagnetic valve which can be opened and closed with a simple electronic signal. A series of sensors feeds data to a “brain” (Electronic Control Unit or ECU). The ECU “studies” the data and determines the correct duration of electrical pulses to the injectors. The sensors are spread around the engine compartment, and locations vary from car to car. All L-Jetronic cars use the sensors listed below.

Initial Troubleshooting
1. Check fuel pump fuse (8 amp) located behind passenger seat and pump operation.
2. Check that voltage to Ignition Electronic Control Unit (ICU) pin 10 is min. 12.2V.
3. Check Injection Electronic Control Unit (ECU) grounds at pins 5,16,17.
4. For cold starting problems, check cold start injector and thermo-time switch.

Bill Unger, 1987 Spider Veloce

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Injection Electronic Control Unit (ECU) (S1) (aka. electronic control module, ECM)

Function: Controls fuel injection with electric pulses to the fuel injectors.
Location & Operation: Under shelf behind passenger seat. Assembles data from the remote sensors to determine fuel injection timing and duration.
WARNING: May be damaged by using a regular analog VOM meter. Use only a high-impedence digital volt/ohm meter (DVOM).
Test: Disconnect the harness and test the components through the pin connectors, not from the components to the ECU (ARM 04-23). Test continuity from ECU pins 5,16,17 to ground at auxiliary air device mounting and from ECM pin 23 to ground at lambda sensor.

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Fuel Injectors (S3)

Function: Sprays fuel into combustion chamber.
Location & Operation: Under air intake manifold. Controlled by electrical impulses from injection ECU which open and close electromagnetic valves.
Failure: Causes poor or no engine operation.
Test: Listen to each injector with a mechanic’s stethoscope. Check electrical pulse with plug-in solenoid neon test lamp.

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Airflow Meter (S5) (aka. volume air flow sensor, air mass sensor)

Function: Measures intake air flow and controls fuel pump.
Location & Operation: On top of air filter box. A floating flap valve that opens in proportion to the intake air flow, the flap angle transfers to a potentiometer fastened to the throttle shaft, sending a voltage signal to the injection ECU.
Failure: Potentiometer can become worn, giving false readings and causing power loss, backfiring, stalling. More common is an air leak in the intake air system, allowing for “un-measured” air to enter the engine.
Test: 1) Turn ignition key to “on” and listen for fuel pump operation when opening flap valve. 2) Unrestricted travel and changing resistance across the terminals as the flap is moved.


Air Flow Meter
Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Ambient Temperature Sensor (S5) (aka. air intake temperature)

Function: Measures intake air temperature (air density).
Location & Operation: In the air meter, a NTC thermistor (temperature sensitive resistor) sends a signal to the injection ECU. Air density and air flow are used to calculate air mass and control fuel injection duration. Cooler air is denser, and requires a richer mixture than warm air.
Failure: Causes poor fuel mileage.
Test: For resistance rise & voltage drop at ECU pin 27 (ARM 04-23) when using a hot air blower versus ambient temperature (cold=high voltage, low ohms / hot=low voltage, high ohms / @20°C = 2.6 kohms, 2.6 VDC). Or use an ohmmeter to measure the resistance across the two pins of the temperature sensor.

Temperature F° Resistance Ohms
14 7,000-12,000
68 2,000-3,000
122 700-1,000
176 200-400
185+ Less than 200

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Idle Speed CO Adjustment (S5)

Function: Controls fuel delivery.
Location & Operation: An adjustment screw under a sealing plug on the airflow meter. While the total air intake flow remains the same, the flow rate through the flap valve changes, thus changing the angle setting and consequently the fuel injection duration.

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Auxiliary Air Device (S9) (aka. idle speed control)

Function: Assures cold engine smooth running by increasing air flow.
Location & Operation: On RH camshaft cover with a bimetallic strip sensitive to cylinder head temperature that controls a bypass (leak) on the throttle valve.
Test: For engine speed change by pinching output hose at cold (speed slows) and operating temperatures (no change).

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Cold Start Injector (S4) (aka. cold start solenoid)

Function: Enriches the intake fuel mix during cold engine starts (“choke”).
Location & Operation: Intake manifold, injects an additional momentary fuel spray. It is not controlled by the ECU, only the thermo-time switch.
Test: For 12 VDC at terminal 45. For fuel spray (if below 95°F/50°C) when cranking engine.

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Thermo-Time Switch (S8)

Function: Allows cold start injector to operate during engine starting.
Location & Operation: Below injectors near coolant temperature sensor, a bimetallic strip opens over 35°C (95°F) and opens the circuit to the cold start injector. A second heating circuit warms the switch during cranking so the cold start injector will shut off and prevent flooding during prolonged efforts to start the engine. Does not send signal to ECU.
Failure: If it fails to energize the cold-start injector, it will cause difficult starting (but not affect running). If it fails to shut off after startup, the cold-start injector continues to spray fuel and causes an extremely rich fuel mixture.
Test: Remove the cold start injector from the intake manifold and place the nozzle in a container while cranking the engine. If the temperature of the engine is below 95 degrees, the cold-start injector should spray. If it does not spray, connect a test light to each of the two terminals of the injector and crank the engine. If one terminal activates the light but the other does not, replace the cold-start injector. If both terminals activate the light, inspect the wiring between the cold-start injector and the thermo-time switch. If the wiring is OK, replace the switch. If neither terminal lights up the light, repair the wiring harness between the starter solenoid and the cold-start injector. At the ECU, check continuity at cold (momentarily closed) and operating (open) temperatures between terminal 46 and ground and for 12 VDC at terminal 47.

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Engine Coolant NTC (negative temperature coefficient) -Thermistor (S7) (aka. engine coolant temperature sensor, ECT)

Function: Enriches the intake fuel mix during cold engine starts.
Location & Operation: Below injectors near thermo-time switch, a thermistor sends engine coolant temperature to the injector ECU.
Failure: Causes rich running, difficulty starting.
Test: For continuity at terminal 49 to ground at auxiliary air device mount. Test for resistance rise & voltage drop at ECU pin 13 (ARM 04-23) in very hot water versus ambient temperature (cold=high voltage, low ohms / hot=low voltage, high ohms / @20°C = 2.2-2.7 kohms, 3.44 VDC). Or use an ohmmeter to measure the resistance across the two pins of the temperature sensor.
Temperature F° Resistance Ohms
14 7,000-12,000
68 2,000-3,000
122 700-1,000
176 200-400
185+ Less than 200


Engine Coolant Temperature Sensor
Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Fuel Cut Off (S6) (aka. throttle valve switch, engine load-range switch, fuel-load switch, throttle position switch, TPS)

Function: Shuts off fuel flow during deceleration.
Location & Operation: A micro switch closes when the throttle valve fully closes against its stop limit and sends a signal to the injector ECU to shut off the injectors until the engine slows to a predetermined speed.
Failure: Causes poor acceleration from idle, high fuel consumption.
Test: For continuity when open (on throttle=closed circuit) and closed (off throttle=open circuit), unplug connector and check across terminals 2 and 18 (ARM 04-15).

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Open Throttle Switch (S6) (aka. throttle valve switch, engine load-range switch, fuel-load switch, wide-open-throttle, WOT, or throttle position switch, TPS)

Function: Allows maximum fuel flow during acceleration.
Location & Operation: A micro switch closes at full throttle and sends a signal to the injector ECU to increase fuel flow and switch off the lambda sensor circuit.
Failure: Causes poor acceleration from idle or high fuel consumption.
Test: For continuity when open (on throttle=open circuit) and closed (WOT=closed circuit), unplug connector and check across terminals 3 and 18 (ARM 04-15). With engine running, short across terminals 3 and 18 momentarily, and engine speed should increase.


Throttle Position Switch
Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Bypass for Idle Speed Adjustment

Function: Controls idle speed.
Location & Operation: Adjustment screw leaks additional air past the throttle valve when fully closed. Does not change CO value.

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Exhaust Gas Lambda Sensor (S10) (aka. O2 or oxygen sensor)

Function: Minimizes exhaust emissions by adjusting fuel mix.
Location & Operation: Under car in exhaust pipe. Beginning at 600°F, a platinum sponge coated ceramic capsule measures O2 differential between exhaust and atmosphere The electrical potential across the capsule sends a signal to the ignition ECU. Too much oxygen indicates lean running; too little means the mixture is rich. The ECU uses the feedback to constantly adjust mixture.
Failure: Causes a rich fuel mixture.
WARNING: May be damaged by dropping, exposing to antifreeze, using silicone sealers on the threads, excessive oil or fuel in exhaust. Must be replaced in event of blown head gasket. Regular analog VOM meters will destroy it. Use only a high-impedence digital volt/ohm meter (DVOM).
Test: For 0.5-1.0 VDC at ECM pin 24 (ARM 04-23) when tip of sensor is passed from dark blue to light blue portion of flame using propane torch. For continuity from ECM pin 23 to ground at the sensor.


O2 Sensor
Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Altitude Compensation System (S16)

Function: Reduces fuel injection duration in proportion to the altitude.
Location & Operation: Behind passenger seat next to injection ECU, a sensing valve measures atmospheric pressure and sends a signal to the injection ECU.
Test: For resistance between terminals (ARM 04-15).

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Variable Valve Timing Device (VVT) (S15)

Function: Controls intake valve timing according to load.
Location & Operation: On front of RH camshaft cover, a solenoid controls a hydraulic actuator causing the camshaft to rotate ~10° and change valve timing.

Spider Series III -- Bosch L-Jetronic Fuel Injection System Basics

Bill Unger
registered 2008-10-12
Florida Alfa Club *

Engine Revolution Sensor

Function: Counts engine revolutions.
Location & Operation: Magnetic flywheel sensor in the bell housing sends signal to the ignition ECU.
Failure: Causes engine to stop.
Test: For continuity (closed circuit) when passing a magnet near sensors or housing.


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