Fuel injectors testing

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^ The injectors are turned on and off by the DME engine control module (ECM). The quantity of fuel delivered to the engine is determined by the amount of time the injectors remain open.

Injectors tests include electrical checks for resistance, power and ground signal from the ECM.

1984 Precision

Injector flow tests require specialized equipment that opens each injector for a precise amount of time to measure fuel delivery. Most authorized Porsche dealers can perform an injector balance test to locate clogged or faulty injectors.

To quickly check if an injector is electrically functioning, start engine and place screwdriver or finger on injector body. A slight vibration or buzzing sound indicates that injector is functioning electrically.

Porsche 911 Speed Switch Testing

Table e. Fuel Injector Resistance Specification

Injector terminals

Resistance at 20°C (68°F)

Harness connector disconnected

2 to 312

^ With engine running, remove injector harness connectors one at a time.

• Engine RPM should drop each time one injector is disconnected.

• If disconnecting one injector does not affect engine RPM, continue testing that injector and harness connector.

— If any faults are found, turn engine off and disconnect electrical connector from suspected injector.

• Using an ohmmeter, check injector resistance. Compare to specification listed in Table e.

Table e. Fuel Injector Resistance Specification

— If injector resistance is within specifications, check that ECM

signal is present at injector connector.

• Connect LED injector tester or back probe injector harness connector using digital voltmeter (select ms-pulse width range if available).

• Operate starter or run engine and check for pulsed voltage signal.

• Repeat for each injector.

— If no signal is present, check for power to injectors.

• There should be battery voltage (+) at red/white wire of each injector connector with ignition key on.

• If voltage is not present, check main/fuel pump relay output at terminal 87.

Motronic engine control module (ECM) pin assignments

DME relay 87 Ground


Injection time signal (cyl. 4-6)

Injection time signal (cyl. 1-3)

cylinder head temp, sensor

Test connector (A)

Tachometer/Up shift indicator

Bridge to ground (California cars)

Air flow meter

Speed sensor

Air flow meter (2)

Air flow meter(4)


Starter (50)

Full throtle switch

Idle speed switch

Ignition coil

DME relay (87) Idle positioner (3) Idle positioner (5)

Air condition on/off

High altitude switch

Speed sensor

Reference mark sensor

Reference mark sensor

Oxygen sensor

Shield-oxygen sensor wire

Air flow meter (1)

Tachometer/up shift indicator

DME relay 85b



If there is positive (+) battery voltage at the injectors but no pulsed response at the voltmeter, check the wire(s) from the ECM to the injectors. If no wiring faults can be found, the pulsed ground signal from the control module may be missing. Check the outputs from the ECM.

If no wiring faults can be found after the above checks, the ECM may be faulty.

ECM harness electrical tests, as well as fuel injector replacement, is covered later in this repair group.

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