Home ReviewsRefund
Home/Porsche/911 Turbo/Porsche 911 Turbo 964 (1988-1994)/Repair manual/Testing & Diagnostics/Engine Controls - System/component Tests
Contents Section: Testing & Diagnostics All sections

Engine Controls - System/component Tests Porsche 911 Turbo 964

Testing & Diagnostics 3 illustrations ~1748 words

INTRODUCTION

Before testing separate components or systems, perform procedures in BASIC TESTING article in the ENGINE PERFORMANCE Section. Since many computer-controlled and monitored components set a trouble code if they malfunction, also perform procedures in G - TESTS W/ CODES article in the ENGINE PERFORMANCE Section.

Note. Testing individual components does not isolate shorts or opens. Perform all voltage tests with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure. Use ohmmeter to isolate wiring harness shorts or opens.

TESTING PRECAUTIONS

  1. ALWAYS turn ignition switch off, or disconnect battery for resistance tests. ALWAYS disconnect the engine speed/reference sensor for compression tests. If this is not done, dangerous high voltages and insulation damage to the following components may result: ignition coil, high-voltage distributor and ignition leads.
  2. NEVER connect a capacitor to ignition coil terminals. NEVER connect ignition coil terminal No. 1 to ground. The ignition coil and DME control unit could be damaged.
  3. NEVER connect battery leads or test light to ignition coil terminal No. 1. The DME control unit will be damaged. NEVER disconnect ignition lead from ignition coil secondary terminal during engine operation.
  4. Voltage flashover from ignition coil secondary terminal to ignition coil primary coil terminals (No. 1 and 15) must be avoided. The DME control unit could be damaged.
  5. When testing, the secondary circuit of the ignition system MUST be suppressed at all times with 4000-ohm resistance (1000-ohm distributor cap suppression resistor; 3000-ohm spark plug lead suppression resistors).
  6. ALWAYS turn ignition switch off or disconnect battery BEFORE disconnecting ignition or DME control unit connectors. Electrostatic discharge may damage DME control unit.
  7. NEVER disconnect battery while engine is running. Reversed battery polarity could damage ignition or DME control unit. DO NOT exceed a charging voltage of 16 volts (including when using a battery charger).

Note. Complete system and component testing information is not available from manufacturer. Only components with informations available are covered in this article.

CHECK ENGINE Light Control Circuit (1991 Models)

  1. Disconnect DME control unit connector. (Scheme 4) Check the CHECK ENGINE light control circuit by grounding DME control unit wiring harness connector terminal No. 22 and turning ignition on.
  2. If CHECK ENGINE light fails to come on, check bulb and wiring harness (circuit) between DME control and instrument cluster. See «WIRING DIAGRAMS»(/porsche/911-turbo/964-1988-1994/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in the ENGINE PERFORMANCE Section. NOTE: If CHECK ENGINE light control circuit is faulty, a fault code may be stored in memory. See «G - TESTS W/ CODES»(/porsche/911-turbo/964-1988-1994/remont/testing-diagnostics/#engine-controls-tests-wcodes) article in the ENGINE PERFORMANCE Section.

Diagnostic Circuit

  1. Locate 19-pin diagnostic connector underneath cover in front passenger's footwell. Use DVOM to check wiring harness (circuit) continuity between pin No. 7 (Orange/Blue wire) of 19-pin diagnostic connector and DME control unit terminal No. 13. (Scheme 4)
  2. Use DVOM to check wiring harness (circuit) continuity between pin No. 8 (Orange/White wire) of 19-pin diagnostic connector and DME control unit terminal No. 55. If circuit continuity does not exist, check wiring harness for open circuits or circuits shorted to ground. If circuit continuity is okay, go to next step.
  3. Use DVOM to check voltage at pin No. 7 (Orange/Blue wire) of 19-pin diagnostic connector. Repeat voltage check at pin No. 8 (Orange/White wire) of 19-pin diagnostic connector. Voltage readings must be greater than 8 volts with ignition on. If voltage is not as specified, check wiring harness for open/short circuits. See «WIRING DIAGRAMS»(/porsche/911-turbo/964-1988-1994/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in the ENGINE PERFORMANCE Section.

Ground Circuits

  1. Use DVOM to check continuity to ground on Digital Motor Electronics (DME) control unit ground circuits at terminals No. 14, 19, 24 and 30. (Scheme 4) See «WIRING DIAGRAMS»(/porsche/911-turbo/964-1988-1994/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in the ENGINE PERFORMANCE Section. Ensure ohmmeter reading is zero ohm.
  2. If reading is not zero ohm, repair open ground circuit. Check DME ground wires on cylinder No. 1 intake pipe and fuel filter mount. Check ground strap between body and transmission/engine. Ensure all electrical connections are securely attached and free of corrosion.

Power Circuits

  1. Disconnect DME control unit connector. Connect DVOM between terminal No. 24 (ground) and terminal No. 18 (battery voltage; keep-alive memory). (Scheme 4) DVOM should indicate battery voltage. If battery voltage is not present, check Red wire back to fuse block and battery, or Brown ground wire. See «WIRING DIAGRAMS»(/porsche/911-turbo/964-1988-1994/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in the ENGINE PERFORMANCE Section.
  2. Connect DVOM between terminal No. 24 (ground) and terminal No. 37 (ignition signal from DME relay). (Scheme 4) Turn ignition on. DVOM should indicate battery voltage. If battery voltage is not present, check Gray/Brown wire back to DME relay, or Brown ground wire. See «WIRING DIAGRAMS»(/porsche/911-turbo/964-1988-1994/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in the ENGINE PERFORMANCE Section. NOTE: On vehicles equipped with anti-theft system, check power supply from ignition lock to terminal No. 86 of DME relay via the anti-theft system control unit. See «WIRING DIAGRAMS»(/porsche/911-turbo/964-1988-1994/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in the ENGINE PERFORMANCE Section.

AIRFLOW SENSOR

  1. Connect DVOM between ground and terminal No. 3 of airflow sensor wiring harness connector. (Scheme 3) Turn ignition on to check voltage supply to sensor. If reading is 4.7-5.3 volts, go to next step. If voltage is not 4.7-5.3 volts, check Blue/Green wire back to DME control unit.
  2. Disconnect air filter hose. Using jumper wires, connect terminal No. 3 of airflow sensor wiring harness connector with terminal No. 3 of sensor, and terminal No. 4 of wiring harness connector with terminal No. 4 of sensor.
  3. Connect DVOM between ground and airflow sensor terminal No. 2. Turn ignition on to check voltage drop across sensor. With airflow sensor plate closed, reading should be 250-260 millivolts.
  4. Using a screwdriver handle, press airflow sensor plate to full throttle position. Reading should gradually increase to about 4.6 volts. If voltage values are incorrect, replace airflow sensor.

Identifying Airflow Sensor Terminals. Scheme 3

Scheme 3: Identifying Airflow Sensor Terminals
Scheme 3: Identifying Airflow Sensor Terminals

DME Relay

  1. Disconnect DME control unit connector. Connect a jumper wire between terminal No. 3 (DME relay ground) and terminal No. 24 (ground). (Scheme 4)
  2. Turn ignition on and ensure fuel pump runs. If fuel pump does not run, check fuses, DME relay, Black/White wire to DME relay, Green/Black wire to fuel pump, or fuel pump. See «WIRING DIAGRAMS»(/porsche/911-turbo/964-1988-1994/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in the ENGINE PERFORMANCE Section.

Engine (Coolant) Temperature Sensor

Disconnect DME control unit connector. Connect DVOM between terminal No. 30 (ground) and terminal No. 45 (engine temperature sensor). (Scheme 4) See ENGINE (COOLANT) TEMPERATURE SENSOR RESISTANCE. If readings are incorrect, check resistances directly on sensor.

Temperature °F (°C)Ohms
32 (0)4400-6800
59-86 (15-30)1400-3600
104 (40)1000-1300
176 (80)250-390
212 (100)160-210

ENGINE (COOLANT) TEMP SENSOR RESISTANCE

Full Throttle Switch

  1. Disconnect DME control unit connector. Connect DVOM between terminal No. 24 (ground) and terminal No. 53 (full throttle switch). (Scheme 4) With throttle valve closed, ohmmeter reading should be infinity.
  2. Open throttle valve at least 2/3 of the way. With throttle valve open, resistance reading should be less than 10 ohms. If ohmmeter readings are incorrect, measure resistance directly at full throttle switch and adjust if necessary. See «ADJUSTMENTS»(/porsche/911-turbo/964-1988-1994/remont/adjustments/#engine-controls-adjustments) article in the ENGINE PERFORMANCE Section.

Idle Speed Switch

  1. Disconnect DME control unit connector. Connect DVOM between terminal No. 24 (ground) and terminal No. 52 (idle speed switch). (Scheme 4) With throttle valve closed, ohmmeter reading should be less than 10 ohms.
  2. Open throttle valve beyond 1-degree opening angle. With throttle valve open, ohmmeter reading should be infinity. If ohmmeter readings are incorrect, measure resistance directly at idle speed switch and adjust if necessary. See «ADJUSTMENTS»(/porsche/911-turbo/964-1988-1994/remont/adjustments/#engine-controls-adjustments) article in the ENGINE PERFORMANCE Section.

Intake Air Temperature Sensor

  1. Disconnect DME control unit connector. Connect DVOM between terminal No. 26 (airflow meter ground) and terminal No. 44 (intake air temperature sensor). (Scheme 4) See «INTAKE AIR TEMPERATURE SENSOR RESISTANCE»(/porsche/911-turbo/964-1988-1994/remont/testing-diagnostics/#engine-controls-systemcomponent-tests).
  2. If readings are incorrect, check resistances directly on sensor by connecting ohmmeter between terminals No. 1 and 4 at intake air temperature (airflow) sensor. (Scheme 3)
Temperature °F (°C)Ohms
32 (0)4400-6800
59-86 (15-30)1400-3600
104 (40)1000-1300

INTAKE AIR TEMP SENSOR RESISTANCE

Knock Sensors

Check knock sensors and brackets for proper installation. Check sensor connectors for loose, corroded or damaged terminals. Check knock sensor circuits/wiring harness. See WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section. If necessary, replace knock sensors.

Note. Specifications and other testing information are not available from manufacturer. If a knock sensor is faulty, ignition timing will be retarded 6 degrees on crankshaft.

Oxygen Sensor

  1. Disconnect oxygen sensor connector. Start and warm engine to normal operating temperature. Connect DVOM between ground and oxygen sensor voltage signal wire. (Scheme 4)
  2. Note voltage on DVOM. Accelerate engine to enrich mixture, and note voltage reading on DVOM. DVOM must indicate a change in voltage signal. Reading will vary between 150-900 millivolts depending on air/fuel mixture.

Scheme 4

Scheme 4
Scheme 4

FUEL DELIVERY

Note. For fuel system pressure testing, see BASIC TESTING article in the ENGINE PERFORMANCE Section.

Fuel Injectors

  1. Disconnect fuel injector connector. Connect DVOM between ground and terminal No. 2 of fuel injector connector. (Scheme 4) Turn ignition on and check voltage supply to fuel injector. If battery voltage is not indicated, check Red/White wire back to fuse/relay block. See «WIRING DIAGRAMS»(/porsche/911-turbo/964-1988-1994/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in the ENGINE PERFORMANCE Section.
  2. If battery voltage is present, use DVOM to measure resistance across injector terminals. If resistance is not approximately 16 ohms, replace injector. Repeat test for each injector.

IDLE CONTROL SYSTEM

Note. Idle speed is not adjustable. For adjustment of idle switch and full throttle switch, see ADJUSTMENTS article in the ENGINE PERFORMANCE Section.

IDLE SPEED POSITIONER

Disconnect idle speed positioner connector. Connect DVOM between ground and terminal No. 2 of idle speed positioner wiring harness connector. Turn ignition on and check supply voltage to positioner. Reading should be battery voltage. If battery voltage is not indicated, check Red/White wire back to fuse/relay block. See WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section.

IGNITION SYSTEM

Note. For ignition system checks, see BASIC TESTING article in the ENGINE PERFORMANCE Section.

Fuel Tank Venting Valve

Disconnect fuel tank venting valve connector. Connect DVOM between ground and terminal No. 1 of fuel tank venting valve wiring harness. (Scheme 4) Turn ignition on and check voltage supply to valve. Reading should be battery voltage. If battery voltage is not indicated, check Red/White wire back to fuse/relay block. See WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section.

Purge Diaphragm Valve

Remove diaphragm valve from vehicle. Blow air through side fitting on valve. Little or no air should pass through valve. Apply vacuum to top fitting of diaphragm valve with a hand-held vacuum pump. Vacuum should hold, and air blown in, through side fitting, should now pass freely out bottom fitting of diaphragm valve.

Scheme 5

Scheme 5: Purge Diaphragm Valve
Scheme 5: Purge Diaphragm Valve

Poses Online

More articles — Engine Performance