Test point

Check Engine A ground signal is fed from the control unit - terminal 21 - to the warning lamp „Check Engine lamp", causing this lamp to come on when an emis-

Fault code 1 45 sion contro1 component fails.

If the „Check Engine" warning lamp fails, this fault is stored in the fault memory.

Check by supplying ground to disconnected DME control unit plug terminal 21 and switching on ignition. The Check Engine lamp must now come on.

To replace a faulty display lamp, always use the specified lamp.

Reading errors using the Check Engine warning lamp is covered on page D24/28 - 31.

Fault, Fault Code Possible Causes, Elimination, Remarks

Test point 24 - 27

Injector valves (V/£2) Using the System Tester 9288, the injector valves may be checked

Fault code 1 51 directly In the Drive links menu item or the Drive link active menu item.

1 54

Selective injection allows each injector valve to be actuated individually. In case of the Drive links test point, a rather weak switching noise of the injector valves is audible. In case of the Drive link active menu item, each individual injector valve may be isolated with the engine running.

Power supply

Disconnect valve plug, connect voltmeter to the injector valve plug terminal - refer to drawing — and ground (engine). Ignition on.

Display: Battery voltage

If no battery voltage is displayed, check according to wiring diagram.

Checking coil resistance of injector valves

Disconnect valve plug. Check coil resistance at injector valve terminal contacts with an ohmmeter.

Test value: approx. 16 Q

Injection output stage

Adjust oscilloscope according to manufacturer's instructions. Connect a Bosch test lead (1 684 463 093) between injector valve and plug. Connect oscilloscope according to manufacturer's instructions with the test lead.

Caution:

Make sure the tester leads are not grounded in any way.

Fault, Fault Code Possible Causes, Elimination, Remarks

Start engine. If the injection output stage operates correctly or if the tester connections are correct, respectively, the following signal must be displayed:

ti signal

L

ti

Possible Causes, Elimination, Remarks

Test point 28 Fuel pressure

The left-hand fuel collection pipe has a test connection at cyl. no. 3. Unscrew the cap nut of the test connection.

Caution: The inserted ball seat could fall out!

Connect pressure gauge P 378 or VW 1318 to the test connection. Start engine.

Test pressure 3.3 ±0.2 bar (Idle speed)

In case of hot starting problems:

Check with engine stopped and fuel pump running. For the test, remove fuel pump relay and jumper terminal 87b and terminal 30.

Test pressure 3.8 ±0.2 bar

- Switch off fuel pump. Read off pressure on pressure gauge (nominal 3.8 ±0.2 bar).

- Max. permissible pressure drop with warm engine: 0.5 bar in 30 minutes.

- If the pressure drops below the specified value, proceed as described below:

DME-Diagnosis/Troubleshooting

Printed In Germany - III, 1991

Fault, Fault Code Possible Causes, Elimination, Remarks

Build up pressure again by switching on the fuel pump briefly. Remove return pipe from pressure regulator (fuel pump must not be switched on).

End

Test point 29 Intake System Leaks

Check all connections downstream of the air flow sensor for leaks. Remove upper air cleaner section. Attach Special Tool 9264/4 to the air mass sensor and build up a positive pressure of approx. 0.5 bar in the intake system. Leaks will cause the pressure in the intake system to drop quickly.

Note:

Fault flashing code 123 (oxygen regulator at stop) appears in cars with oxygen regulation and very large leaks in the intake system (e.g. intake pipe gasket).

Test point 30

Ground connection/plug connections (V)

• Ground lead (2x) from wiring harness to engine block.

• Battery ground lead at clutch bell housing (engine ground) and body (body ground, next to battery).

• Fuel pump ground between rear lock crosswall and spare wheel well.

• Are ground connections tight and free from corrosion?

If not, undo, clean and retighten ground connections as specified.

Note:

Never start the engine as long as the ground lead between body and engine is disconnected!

This will destroy the control unit immediately!

Plug connections

Are all plugs connected correctly, not loose and not corroded? The pins and sleeves of the plugs must not be bent!

• One 2-pin plug for each of the 4 injection valves

• The 55-pin plug on the DME control unit (behind the floor plate in the passenger footwell)

The 968 is fitted with a standard control unit. The control unit is coded accordingly depending on the vehicle model.

• A 14-pin plug in the engine compartment next to the brake booster

The type of coding may be retrieved under the Version coding heading in the Actual values menu.

• A 3-pin plug for reference mark/engine

Power to the DME control unit is supplied across ignition lock terminal 15 and terminal 30.

speed sensor and a 3-pin plug for the oxygen sensor behind intake pipe for cylinder no. 4 (remove cover at fuel ring pipe first)

• A 3-pin plug at throttle potentiometer

• A 2-pin plug at tank ventilation solenoid valve (at the brake booster)

• A 2-pin plug at NTC II (The temperature sensor is screwed into the engine block above the left-hand balance shaft near cylinder no. 1)

• A 3-pin Hall sender plug (behind the camshaft drive gear)

• A 2-pin plug for knock sensor I (between intake pipe for cylinder no. 1 and 2)

• A 2-pin plug for knock sensor II (between intake pipe for cylinder 3 and 4)

• A 3-pin plug for the idle stabilizer

Test point 31

Ignition system a) Secondary ignition display

Adjust secondary ignition display at oscilloscope. Connect test leads according to manufacturer's instructions.

Note:

If a fault is displayed for all cylinders, the fault is located in the primary or secondary circuit from the ignition coil to the distributor rotor. If a fault is displayed for one cylinder only, the fault is located after the distributor rotor.

b) Spark plug connector (Q)

Suppression resistor: 3 kQ

Visual inspection for damage, traces of burning etc.

d) Ignition timing test

Ignition timing is tested with the engine at operating temperature:

Using System Tester 9288, ignition timing and engine speed can be read directly in the „Actual values" menu item.

Test values: 968 (with manual transmission)

968 (with Tiptronic)

c) Distributor

Distributor rotor, distributor cap: Suppression resistor: 1 kQ each

Visual inspection for damage, dirt, faulty mounting position.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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