Engine Controls - Basic Testing Audi A6 C4

INTRODUCTION

The following diagnostic steps will help prevent overlooking a simple problem. This is also where to begin diagnosis for a no-start condition. The first step in diagnosing any driveability problem is verifying the customer's complaint with a test drive under the conditions the problem reportedly occurred.

Before entering self-diagnostics, perform a careful and complete visual inspection. Most engine control problems result from mechanical breakdowns, poor electrical connections or damaged/misrouted vacuum hoses. Before condemning the computerized system, perform each test listed in this article.

Note. Perform all voltage tests with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure.

VISUAL INSPECTION

Visually inspect all electrical wiring, looking for chafed, stretched, cut or pinched wiring. Ensure electrical connectors fit tightly and are not corroded. Ensure vacuum hoses are properly routed and are not pinched or cut. See VACUUM DIAGRAMS article to verify routing and connections (if necessary). Inspect air induction system for possible vacuum leaks.

Compression

Check engine mechanical condition with a compression gauge, vacuum gauge, or an engine analyzer. See engine analyzer manual for specific instructions.

COMPRESSION SPECIFICATIONS

Exhaust System Backpressure

The exhaust system can be checked with a vacuum or pressure gauge. Remove oxygen sensor. Connect a 0-5 psi pressure gauge and operate engine at 2500 RPM. If exhaust system backpressure is greater than 2 psi, exhaust system or catalytic converter is plugged.

If a vacuum gauge is used, connect vacuum gauge hose to intake manifold vacuum port and start engine. Observe vacuum gauge. Open throttle part way and hold steady. If vacuum gauge reading slowly drops after stabilizing, check exhaust system for restriction.

FUEL SYSTEM

FUEL PRESSURE RELEASE

Remove fuel pump relay. Start engine. Allow engine to run until it stalls. Turn ignition off. Disconnect negative battery cable. Slight pressure may remain in system. Before disconnecting any fuel system line, cover connector with a clean shop towel. Reinstall fuel pump relay.

FUEL PRESSURE TEST

1. Release fuel pressure. Ensure ignition is off. Slowly remove fuel filler cap. Wrap a cloth around service port on fuel rail. Slowly loosen service port cap. Install fuel pressure gauge at service port. Disconnect differential pressure regulator harness connector.
2. With transmission in Neutral, start engine and let it run. System fuel pressure should be 89-96 psi (6.3-6.7 kg/cm 2 ). If system fuel pressure is lower than specification, perform fuel volume check. See «FUEL VOLUME CHECK»(/audi/a6/c4-1994-1997/remont/testing-diagnostics/#engine-controls-basic-testing__fuel-volume-check) . If fuel volume is okay, replace fuel pressure regulator.
3. If fuel pressure is greater than specification, remove return hose from pressure regulator and place in a container. Repeat test. If system pressure is okay, check for restricted fuel return line. If pressure is incorrect, replace fuel pressure regulator.
4. Turn ignition off. Wait 10 minutes. Residual pressure should be 48 psi (3.4 kg/cm 2 ). After 20 minutes, residual pressure should be 46 psi (3.2 kg/cm 2 ).
5. If system fuel pressure is low, check fuel pump check valve, sensor plate free play, fuel distributor "O" rings and seats. Replace if necessary. If these components are okay, replace fuel pressure regulator.

FUEL VOLUME CHECK

Note. Before testing main fuel pump, check transfer pump.

Transfer Pump Fuel Volume Check

1. Turn ignition off. To check transfer pump (in tank), remove rear seat. Remove fuel sending unit access cover. Disconnect ignition coil secondary wire and jumper to ground.
2. With transmission in Neutral, crank engine for 3-4 seconds. While starter is turning and for a few seconds afterwards, an audible sound should be heard at the pump. If sound is not heard, go to next step. If sound is heard, go to step 5).
3. Remove fuel pump relay from relay panel. Activate fuel pump using Remote Control (VAG 1348/3A). If pump does not operate, disconnect fuel pump wire harness connector. (Scheme 2)
4. Using test light, check voltage between Brown wire and Red/Yellow wire. If voltage is present, replace fuel pump. If voltage is not present, repair open or short circuit in wiring.
5. Remove fuel pump relay. Disconnect and plug transfer pump output hose. Attach a hose to pump outlet connector and place other end of hose in a graduated container. Activate fuel pump for 10 seconds.
6. Minimum fuel flow should be 10 ounces (.3 liter). If fuel flow is low, check fuel tank filter for restriction. If fuel tank filter is okay, replace fuel pump.

Scheme 2

Main Pump Fuel Volume Check

1. Remove fuel pump relay from relay panel. Attach a hose to fuel pump outlet connector and place other end of hose in a graduated container.
2. Using a DVOM between fuel pump connector terminals No. 1 (Green/Yellow wire) and No. 4 (Brown wire), measure voltage being applied to fuel pump when activating with Remote Control (VAG 1348/3A). Activate fuel pump for 30 seconds. Record voltage.
3. Fuel pump volume is relative to voltage applied to fuel pump. Compare volume with values in FUEL PUMP PERFORMANCE table. If volume is not as specified, replace fuel pump.

FUEL PUMP PERFORMANCE

IGNITION CHECKS

Note. Perform spark test before performing other ignition checks.

SPARK TEST

Note. When ignition coil is triggered, spark is supplied to 2 spark plugs at one time. One spark plug fires during compression stroke and the other plug fires during exhaust stroke (waste spark).

Check for a strong Blue spark at coil wire and at each spark plug wire by holding wire terminal no more than 1/4" from chassis ground while cranking engine. Disconnect and inspect all related ignition system connectors and harness. Clean or repair as necessary.

CAMSHAFT POSITION (CMP) SENSOR

1. Disconnect CMP sensor connector (located on rear end of left cylinder head). Connect Test Box (VAG 1598) to PCM with Adaptor Cable (VAG 1598/11). Connect LED Tester (1115) between test box terminals No. 8 and 46. Operate starter for several seconds.
2. If LED flashes at each second engine revolution, go to next step. If LED does not flash at each second engine revolution, check for faulty wiring. If wiring is okay, go to next step.
3. Turn ignition on. Check voltage between test box terminals No. 7 and 45. If 9 volts minimum is not present, replace PCM. If 9 volts minimum is present, check voltage between test box terminals No. 8 and 45. If 4 volts minimum is not present, replace PCM. If 4 volts minimum is present, replace CMP sensor.

CRANKSHAFT POSITION (CKP) SENSOR

Disconnect CKP sensor connector (located on left side of cylinder block). Measure resistance between specified terminals of CKP sensor connector. (Scheme 3) See CKP SENSOR RESISTANCE table. If resistance is not as specified, replace CKP sensor. If resistance is as specified, check wiring between CKP sensor and PCM. If wiring is okay, replace PCM.

CKP SENSOR RESISTANCE

Scheme 3

Ignition Coil Primary Resistance

Disconnect ignition coil 3-pin connector (located in front of intake manifold). Disconnect power output stage 3-pin connector (located on right side of engine compartment firewall). Measure resistance between matching terminals of each connector (terminal No. 1 to 1, 2 to 2, and 3 to 3). (Scheme 4) If resistance is not 0.5-1.0 ohm, replace appropriate coil.

Scheme 4

Ignition Coil Primary Voltage Supply

Turn ignition on. Disconnect ignition coil 3-pin connector (located in front of intake manifold). Connect LED Tester (VAG 1527B) between engine ground and each terminal of ignition coil 3-pin connector, in sequence. (Scheme 4) If LED comes on when terminals are probed, voltage supply is okay. If LED does not come on, check fuse No. 32 and wiring. Replace fuse or repair wiring as necessary.

Ignition Coil Secondary Resistance

Disconnect spark plug wires from ignition coil being tested. Measure resistance between coil towers of ignition coil being tested. If resistance is 9000-14,000 ohms, coil secondary circuit is okay. If resistance is not 9000-14,000 ohms, replace coil.

POWER OUTPUT STAGE

1. Disconnect power output stage 4-pin connector (located on right side of engine compartment firewall). Connect LED Tester (1527B) between vehicle battery positive source and Brown/Red wire (ground) terminal of power output stage 4-pin connector. If LED does not come on, repair Brown/Red wire.
2. If LED comes on, disconnect all fuel injector electrical connectors. Turn ignition on. Connect LED tester between engine ground and remaining 3 terminals of power output stage 4-pin connector, in sequence. Operate starter for several seconds while probing each terminal.
3. If LED comes on, replace ignition coil power output stage. If LED does not come on, check wiring between PCM and power output stage. If wiring is okay, replace PCM.

IDLE SPEED & IGNITION TIMING

Idle speed, ignition timing and ignition timing advance are controlled by the PCM and are not adjustable.

SUMMARY

If no faults were found while performing BASIC DIAGNOSTIC PROCEDURES, proceed to TESTS W/CODES article. If no hard codes are found in self-diagnostics, proceed to TESTS W/O CODES article for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures.

See also:
• TESTS W/CODES
• TESTS W/O CODES
• FUEL VOLUME CHECK