Starting in 1974, VW introduced “Air Flow Controlled” (A.F.C.) Electronic Fuel Injection, a simplified version of “Manifold Pressure Controlled” (M.P.C.) injection used on earlier vehicles. Because the A.F.C. injection measures the intake air flow directly, the fuel mixture remains constant regardless of changes in exhaust back-pressure (a common result of add-on emission control devices such as air pumps & catalytic converters). A.F.C. injection was first incorporated on Type II (Bus) vehicles as of the 1975 Model Year, and remained in use until the introduction of the water-cooled Vanagon and Digijet Injection in 1983.
Basic operation of the A.F.C. Injection System is as follows:
Basic operation of the A.F.C. Injection System is as follows:
US 1076 or VW 1318 with Adapter VW 1318/17, Pressure Gauge
SUN 105 CO Tester
SUN 120 239 CO Adapter
US 4487 Evaporative System Leak Tester (Slack Tube)
US 1115 L.E.D. Test Light (Leads must be connected Red to Positive & Black to Negative) – (Standard 12V Test Light Optional)
1 Liter Graduated Container
The control unit is an electronic computer. It receives incoming signals regarding air volume, engine revolutions, temperature and throttle position. From this information the computer calculates the correct injection time for the injectors.
The air flow sensor measures the amount of air entering the intake manifold and provides the control unit (E.C.U.) with voltage signals.
Intake air opens the air flow sensor flap which actuates the potentiometer to determine the voltage signal. This signal and the engine speed information supplied by terminal No. 1 of the ignition coil is used for the determination of fuel injector opening duration.
A compensation flap connected to the air sensor dampens sudden movements of the air sensor flap due to oscillations of the intake air.
The air flow sensor also has a set of contacts which controls the fuel pump. This circuit prevents the engine from flooding in the event of a leaking injector, by allowing the fuel pump to operate only when the engine is running or during cranking.
An intake air temperature sensor is mounted in the air flow sensor housing of most vehicles. This temperature sensor is an NTC resistor like the cylinder head temperature sensor, and is connected between terminals 6 & 27 of the air flow sensor. The signal it supplies the E.C.U. is used to modify the fuel injection rate depending on intake air temperature.
Some earlier production vehicles equipped with intake air pre-heating do not use an intake air temperature sensor. The air flow sensor used on these vehicles does not have terminal 27
The cylinder head temperature sensor is screwed into the left side cylinder head, and is connected to the control unit by a single wire. This sensor is critical to good engine performance, particularly during starting and warm-up.
The sensor resistance decreases as engine temperature increases. This input is used by the control unit (E.C.U.) to provide a richer mixture during cold start and warm-up, by lengthening the injector opening duration. The lower the temperature, the more fuel is provided to the engine.
A temperature sensor spacer P/N 022 133 079 is available for cars which run well on initial start-up and when fully warm, but have performance problems only during warm-up.
This spacer does not affect resistance values when the engine is cold or when it is fully warm, but it does delay the change in resistance during warm-up. The spacer temporarily insulates the temperature sensor housing from the heat of the cylinder head.
The spacer does not improve hard starting, or performance problems after the engine is at operating temperature.
The double relay, or sometimes referred to as “dual relay,” supplies current from the battery to the control unit and the injectors whenever the ignition key is on.
Whenever the starter is engaged, the relay activates the fuel pump and supplies current to the cold start system.
After the engine is started, contacts in the intake air sensor control the relay so the fuel pump will run constantly during normal engine operation.
The relay is located in the left side of the engine compartment.
Additional Reading:
The roller-cell fuel pump is driven by a permanent magnet electric motor and is located near the fuel tank.
Steel rollers are held in “cut-outs” on the rotor. Centrifugal force seals the rollers against the walls of the pressure chambers as the rotor spins. Fuel that is trapped between the rollers is then forced out the delivery port. The pump is designed to be both cooled and lubricated by the fuel flowing through it.
The pump delivers several timesthe amount of fuel required to operate the engine at any time. Excess fuel is diverted back to the tank via the fuel pressure regulator.
The system pressure regulator maintains a constant fuel pressure to all injectors by regulating the quantity of fuel returned to the fuel tank.
The regulator is connected to the intake manifold. It responds to manifold vacuum fluctuations, and thereby compensates for engine-load changes.
When the engine is shut off, the regulator closes and seals to maintain residual fuel pressure in the injector lines for improved hot-start capability.
A.F.C. fuel injectors are electronically controlled on/off valves. A solenoid actuates a needle valve allowing fuel to be forced through the injector nozzle. All four injectors open at the same time and inject fuel directly into the intake manifold near the intake valve. Injection quantity is controlled by the amount of time the injectors stay open. Injector opening time is regulated by the E.C.U., based on inputs from the various engine sensors.
The injectors work best with about 3 – 5 volts. A resistor pack reduces the 12 volts available from the power relay whenever the engine is running. The resistors can only be replaced as a complete unit.
The control unit (E.C.U.) opens/closes the injectors by controlling the ground circuit.
Note: On California vehicles from 1979 model year, the resistors are built into the fuel injection control unit and cannot be checked.
Please Note the A.F.C. injectors used on air-cooled engines produced up to 1983 are not interchangeable with Digijet/Digifant Injectors. A.F.C. injectors may be visually identified by their upper injector body color, which is blue.
The full throttle switch is a micro-switch which mounts to the throttle valve housing. An arm on the throttle shaft actuates the switch as full throttle is approached, sending a signal to the control unit (E.C.U.). The control unit will then increase the duration signal to the injectors, enriching the air/fuel mixture during full throttle (heavy load) operation.
The control unit will also eliminate the oxygen sensor influence during full throttle operation on vehicles which are so equipped.
Note: Vehicles produced up to Chassis No. 226 2077 583 used a different throttle switch which incorporated a set of contacts for actuating the E.G.R. system in addition to the full throttle contacts. This type of switch, which has 4 terminals, is mounted to the underside of the throttle valve housing and is directly operated by the throttle shaft.
The cold start valve is centrally located in the intake manifold to provide necessary fuel enrichment to all cylinders during starting.
The electro-magnet coil inside the cold start valve received current from the starter. It is grounded by the thermo-time switch so that it operates for only a short time during starting.
The thermo-time switch controls the opening time of the cold start valve. It is affected by the starter current and engine temperature.
Depending on the engine temperature conducted through the switch housing, the heating coil takes 3 -10 seconds to deflect the bi-metal strip and open the contact points during starting. Opening the contact points interrupts the ground circuit of the cold start valve.
The bi-metal strip and contact points within the thermo-time switch are connected to the ground side of the cold start valve. The heating coil is connected to terminal 50 of the starter circuit. This ensures that the cold start valve operates only for a short time during starting.
The auxiliary air regulator prevents stalling at idle during engine warm-up by supplying additional air to the engine.
Since any engine is less efficient when cold, a “fast-idle is necessary. The auxiliary air regulator controls the amount of air bypassing the closed throttle valve – it is calibrated so there is little difference in the idle speed or a cold or warm engine.
When the engine is cold, the gate valve is open. The heating coil is connected to the fuel pump circuit so that as the engine runs, the bi-metal strip closes the gate valve, gradually cutting off the passage for additional air as the engine warms up.
The deceleration air valve allows air to bypass the throttle plate during deceleration for improved emissions. Two types were used in production. One is electronically controlled, and the other is vacuum controlled.
Automatic Transmission
During deceleration in “Drive” a transmission switch opens the electrical deceleration valve. Vacuum in the intake air distributor will then open the valve allowing air to bypass the closed throttle plate.
Manual Transmission
During deceleration, vacuum in the intake air distributor opens the deceleration valve allowing air to bypass the closed throttle plate.
E.G.R. is a method of emission ontrol in which a portion of the exhaust gas is filtered and then re-introduced into the intake manifold along with the incoming fresh air/fuel charge. The E.G.R. system is disabled at idle and at full throttle operation to ensure smooth idle and peak performance.
Two types of E.G.R. valves were used in production: One type is vacuum operated and controlled by a set of electrical contacts in the throttle switch. The other type is mechanically controlled by an adjustable rod which connects to the throttle valve linkage.
The oxygen sensor is located in the exhaust system upstream of the catalytic converter. It produces varying voltage (up to 1 volt) dependent on the oxygen content of the exhaust gases. This voltage signal is sent to the E.C.U. which will then alter the injection time accordingly. The oxygen sensor operates at all times, except during warm-up or at full throttle.
As of 1979, California vehicles use Hall type transistorized ignition with idle stabilizer. The idle stabilizer has a switching point of 940 RPM. Then idle speed drops below 940 RPM, the stabilizer will compensate by advancing the ignition timing enough to maintain the idle speed at approximately 940 RPM.
Breaker point ignition systems were used on air-cooled engines with the exception of 1979 – 1982 California vehicles which used electronic ignition.
The entire circuit including wiring and connectors can be tested through the multi-pin connector at the E.C.U. This test is to ensure that the proper inputs are reaching the E.C.U.
If specifications are not reached, replace air flow sensor and retest. If air sensor is okay, check wiring.
| Terminal Nos. | Specification |
|---|---|
| 6 and 9 | 200 - 400 Ohms |
| 6 and 8 | 130 - 260 Ohms |
| 8 and 9 | 70 - 140 Ohms |
| 6 and 7 | 40 - 300 Ohms |
| 6 and 27 | Max. 2800 Ohms |
| Resistance Values (ohms) | |
|---|---|
| -10°C (14°F) | 7,000 - 11,600 Ohms |
| +20°C (68°F) | 2,100 - 2,900 Ohms |
| +80°C (176°F) | 270 -390 Ohms |
If resistance reading is too high, transfer ground probe to steel shoulder of sensor and re-check resistance:
Note:
The double relay, or sometimes referred to as “dual relay,” supplies current from the battery to the control unit and the injectors whenever the ignition key is on.
Whenever the starter is engaged, the relay activates the fuel pump and supplies current to the cold start system.
After the engine is started, contacts in the intake air sensor control the relay so the fuel pump will run constantly during normal engine operation.
The relay is located in the left side of the engine compartment.
Additional Reading:
The roller-cell fuel pump is driven by a permanent magnet electric motor and is located near the fuel tank.
Steel rollers are held in “cut-outs” on the rotor. Centrifugal force seals the rollers against the walls of the pressure chambers as the rotor spins. Fuel that is trapped between the rollers is then forced out the delivery port. The pump is designed to be both cooled and lubricated by the fuel flowing through it.
The pump delivers several timesthe amount of fuel required to operate the engine at any time. Excess fuel is diverted back to the tank via the fuel pressure regulator.
The system pressure regulator maintains a constant fuel pressure to all injectors by regulating the quantity of fuel returned to the fuel tank.
The regulator is connected to the intake manifold. It responds to manifold vacuum fluctuations, and thereby compensates for engine-load changes.
When the engine is shut off, the regulator closes and seals to maintain residual fuel pressure in the injector lines for improved hot-start capability.
A.F.C. fuel injectors are electronically controlled on/off valves. A solenoid actuates a needle valve allowing fuel to be forced through the injector nozzle. All four injectors open at the same time and inject fuel directly into the intake manifold near the intake valve. Injection quantity is controlled by the amount of time the injectors stay open. Injector opening time is regulated by the E.C.U., based on inputs from the various engine sensors.
The injectors work best with about 3 – 5 volts. A resistor pack reduces the 12 volts available from the power relay whenever the engine is running. The resistors can only be replaced as a complete unit.
The control unit (E.C.U.) opens/closes the injectors by controlling the ground circuit.
Note: On California vehicles from 1979 model year, the resistors are built into the fuel injection control unit and cannot be checked.
Please Note the A.F.C. injectors used on air-cooled engines produced up to 1983 are not interchangeable with Digijet/Digifant Injectors. A.F.C. injectors may be visually identified by their upper injector body color, which is blue.
The full throttle switch is a micro-switch which mounts to the throttle valve housing. An arm on the throttle shaft actuates the switch as full throttle is approached, sending a signal to the control unit (E.C.U.). The control unit will then increase the duration signal to the injectors, enriching the air/fuel mixture during full throttle (heavy load) operation.
The control unit will also eliminate the oxygen sensor influence during full throttle operation on vehicles which are so equipped.
Note: Vehicles produced up to Chassis No. 226 2077 583 used a different throttle switch which incorporated a set of contacts for actuating the E.G.R. system in addition to the full throttle contacts. This type of switch, which has 4 terminals, is mounted to the underside of the throttle valve housing and is directly operated by the throttle shaft.
The cold start valve is centrally located in the intake manifold to provide necessary fuel enrichment to all cylinders during starting.
The electro-magnet coil inside the cold start valve received current from the starter. It is grounded by the thermo-time switch so that it operates for only a short time during starting.
The thermo-time switch controls the opening time of the cold start valve. It is affected by the starter current and engine temperature.
Depending on the engine temperature conducted through the switch housing, the heating coil takes 3 -10 seconds to deflect the bi-metal strip and open the contact points during starting. Opening the contact points interrupts the ground circuit of the cold start valve.
The bi-metal strip and contact points within the thermo-time switch are connected to the ground side of the cold start valve. The heating coil is connected to terminal 50 of the starter circuit. This ensures that the cold start valve operates only for a short time during starting.
The auxiliary air regulator prevents stalling at idle during engine warm-up by supplying additional air to the engine.
Since any engine is less efficient when cold, a “fast-idle is necessary. The auxiliary air regulator controls the amount of air bypassing the closed throttle valve – it is calibrated so there is little difference in the idle speed or a cold or warm engine.
When the engine is cold, the gate valve is open. The heating coil is connected to the fuel pump circuit so that as the engine runs, the bi-metal strip closes the gate valve, gradually cutting off the passage for additional air as the engine warms up.
The deceleration air valve allows air to bypass the throttle plate during deceleration for improved emissions. Two types were used in production. One is electronically controlled, and the other is vacuum controlled.
Automatic Transmission
During deceleration in “Drive” a transmission switch opens the electrical deceleration valve. Vacuum in the intake air distributor will then open the valve allowing air to bypass the closed throttle plate.
Manual Transmission
During deceleration, vacuum in the intake air distributor opens the deceleration valve allowing air to bypass the closed throttle plate.
E.G.R. is a method of emission ontrol in which a portion of the exhaust gas is filtered and then re-introduced into the intake manifold along with the incoming fresh air/fuel charge. The E.G.R. system is disabled at idle and at full throttle operation to ensure smooth idle and peak performance.
Two types of E.G.R. valves were used in production: One type is vacuum operated and controlled by a set of electrical contacts in the throttle switch. The other type is mechanically controlled by an adjustable rod which connects to the throttle valve linkage.
The oxygen sensor is located in the exhaust system upstream of the catalytic converter. It produces varying voltage (up to 1 volt) dependent on the oxygen content of the exhaust gases. This voltage signal is sent to the E.C.U. which will then alter the injection time accordingly. The oxygen sensor operates at all times, except during warm-up or at full throttle.
As of 1979, California vehicles use Hall type transistorized ignition with idle stabilizer. The idle stabilizer has a switching point of 940 RPM. Then idle speed drops below 940 RPM, the stabilizer will compensate by advancing the ignition timing enough to maintain the idle speed at approximately 940 RPM.
Breaker point ignition systems were used on air-cooled engines with the exception of 1979 – 1982 California vehicles which used electronic ignition.
Electro-mechanical sensor which supplies control unit with signals about intake air volumeand temperature.
Location: Attached to air filter housing.
A rotary-gate valve which stabilizes idle speed during warm-up.
Location: Top of engine between crankcase breather valve and #2 intake pipe.
An electro-magnet valve which sprays extra fuel into the engine during starting.
Location: Mounts to leaft rear of intake air distributor, directly adjacent to ignition distributor.
A solid-state electronic computer which controls injectors by processing signals about temperature, air volume and engine speed.
Location: Mounted to luggage compartment floor in right rear corner of engine compartment.
A compound relay which directs battery current to the fuel injection components.
Location: Mounts to upper left side of front panel in the engine compartment.
Filter which removes foreign particles from the fuel system.
Location: Inboard side of left frame member, between fuel tank and fuel pump.
Vacuum modulated diaphragm valve which controls fuel pressure to the injectors.
Location: Lower left side of front panel in engine compartment in front of #3 intake pipe.
An electric pump which supplies fuel to the injectors.
Location: Inboard side of left frame member to rear of fuel tank.
Electro-magnet valves which spray fuel into each intake port.
Location: Mounts to intake manifold at cylinder head.
External cluster of resistors which reduces voltage supplied to injectors.
Location: Mounts to upper left side of front panel in engine compartment.
Solid-state thermistor element which signals control unit about engine temperature.
Location: Screws into left side cylinder head, directly in front of #3 intake port.
Temperature switch which allows cold start valve to work only for a short time during starting.
Location: Directly under #4 intake pipe connection to intake air distributor.