VW Passat & Audi A4 1990-2000

Diagnosis And Testing

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Before beginning any diagnosis and testing procedures, visually inspect the components of the ignition system and engine control systems. Check for the following:



Discharged battery
 
Damaged or loose connections
 
Damaged electrical insulation
 
Poor coil and spark plug connections
 
Ignition module connections
 
Blown fuses
 
Damaged vacuum hoses
 
Damaged spark plugs
 

Because the sensors operate on very low voltages, excessive fluid spill and/or leaks could also effect their electrical circuits. All of the sensor electrical connections must be clean and free of any oil, debris or corrosion.

When attempting to search for ignition related problems, note that the sensor inputs the Engine Control Module (ECM) utilizes to calculate the optimal ignition timing may also affect the engine's performance.

The Engine Control Module (ECM) is equipped with a fault memory. This memory can be interrogated with suitable test equipment such as the VAG 1551 or VAG 1552. Using this test equipment allows the ignition system to be checked without having to locate, disconnect, and probe the sensor's wires saving valuable diagnostic time and preventing damage to the components and their electrical connectors.

Keep in mind even sophisticated diagnostic equipment requires a step-by-step logical approach to complete a repair. For example, if a sensor has failed, or has been disconnected, or its wiring damaged or shorted, the ECM and the diagnostic test equipment will only know that the signal from that particular sensor is not within the expected range of operation. The diagnostic test equipment cannot precisely determine the exact problem. Under these conditions the test equipment may state the signal is not "plausible" or "not present."

The test equipment will also state whether the "fault" is currently present, or in the case of an intermittent problem, state that it is a "sporadic fault." It is now up to the diagnostician to carry out a logical process of elimination to determine the exact cause of the problem.

A good start would be to:

  1. Inspect the wiring. If damaged, repair or replace as necessary.
  2.  
  3. Clean the exterior of all of the electrical connections with a suitable electrical contact cleaner and wipe dry with a clean cloth.
  4.  
  5. Disconnect the electrical connectors, and inspect for damaged or corroded terminals.
  6.  
  7. Clean the terminals with electrical contact cleaner and if available, short blasts of compressed air. If damaged repair or replace as necessary. If undamaged, carefully reconnect them.
  8.  

Because the low operational voltage of the sensors, just the slightest debris or poor contact can have a detrimental effect on the ignition system operation.

Ignition system Precautions

WARNING
A suitable high-impedance digital multimeter that is designed for testing computerized electrical components must be used when checking the electrical components of the ignition system. Failure to use the proper equipment will damage the electrical circuits and their components.

Before proceeding with any type of ignition system testing, be sure to follow these important precautions:



Make sure the ignition switch is OFF before connecting or disconnecting any wiring or test equipment.
 
When cranking the engine without starting, as for a compression test, disconnect the power supply connector (+12V) from the coil to prevent it from receiving voltage. Other methods of disabling the ignition system are not recommended, and may result in electrical system or component damage.
 
Always switch the multimeter to the appropriate measuring range BEFORE making the test connections. Use a high-impedance digital multimeter that is designed for testing computerized electrical components.
 
Do NOT install a standard ignition coil in the system.
 
Do NOT connect a condenser/suppressor or powered test light to the negative terminal (1) of the ignition coil.
 
Do NOT connect any 12-volt test instruments to the positive terminal (No. 15) of the ignition coil. The electronic control unit will be permanently damaged.
 
Do NOT use a standard test light (electric bulb type) on electronic circuits. The high electrical consumption of these test lights can lead to electronic component damage.
 
Do NOT connect a quick-charger to the battery for more than 1 minute, nor exceed 16.5 volts with the booster.
 

SECONDARY SPARK TEST


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Fig. This spark tester looks just like a spark plug, attach the clip to ground and crank the engine to check for spark



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Fig. This spark tester has an adjustable air-gap for measuring spark strength and testing different voltage ignition systems



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Fig. Attach the clip to ground and crank the engine to check for spark



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Fig. This spark tester is the easiest to use just place it on a plug wire and the spark voltage is detected and the bulb on the top will flash with each pulse

The best way to perform this procedure is to use a spark tester (available at most automotive parts stores). Three types of spark testers are commonly available. The Neon Bulb type is connected to the spark plug wire and flashes with each ignition pulse. The Air Gap type must be adjusted to the individual spark plug gap specified for the engine. The last type of spark plug tester looks like a spark plug with a grounding clip on the side, but there is no side electrode for the spark to jump to. The last two types of testers allow the user to not only detect the presence of spark, but also the intensity (orange/yellow is weak, blue is strong).

  1. Disconnect a spark plug wire at the spark plug end.
  2.  
  3. Connect the plug wire to the spark tester and ground the tester to an appropriate location on the engine.
  4.  
  5. Crank the engine and check for spark at the tester.
  6.  
  7. If spark exists at the tester, the ignition system is functioning properly.
  8.  
  9. If the spark is weak or does not exist at the spark plug wire, perform diagnosis of the ignition system using individual component diagnosis procedures.
  10.  

Cylinder Drop Test


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Fig. These pliers are insulated and help protect the user from shock as well as the plug wires from being damaged



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Fig. To perform the cylinder drop test, remove and ground one wire at a time and . . .



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Fig. . . . note the idle speed and idle characteristics of the engine. The cylinder(s) with the least drop is/are the non-contributing cylinder(s)

The cylinder drop test is performed when an engine misfire is evident. This test helps determine which cylinder is not contributing the proper power. The only safe method to perform this test is to ground the plug wires one at a time for each cylinder with the engine running, or on models with sequential fuel injection, disconnect one fuel injector electrical connector at a time.

Perform the test on all cylinders to verify which cylinder(s) is suspect.

  1. Place the transaxle in P if an automatic, or in Neutral if a manual. Engage the emergency brake, and start the engine and let it idle.
  2.  
  3. Ground each spark plug wire one at a time or, on models with sequential fuel injection, disconnect one fuel injector electrical connector at a time.
  4.  


WARNING
Make sure your body is free from touching any part of the car that is metal. The secondary voltage in the ignition system is high and although it cannot kill you, it will shock you and it does hurt.

  1. The engine will sputter, run worse, and possibly nearly stall. If this happens reinstall the plug wire or injector connector and move to the next cylinder. If the engine runs no differently, or the difference is minimal, shut the engine off and inspect the spark plug wire, and spark plug.
  2.  
  3. If no problems are found with the plug wire or spark plug, further sensor diagnosis may be necessary. Perform the test on all cylinders to verify which cylinder(s) is suspect. If the ignition system components are determined to be in proper working order, it may be necessary to check the fuel system and/or determine the mechanical condition of the particular cylinder by:
  4.  
    1. Performing a compression test.
    2.  
    3. Performing a leakdown test, if compressed air and a leakdown tester are available.
    4.  
    5. Checking for any vacuum leaks, such as a leaking intake gasket or injector seal.
    6.  

  5. Be sure to perform the cylinder drop test on all cylinders.
  6.  
  7. If the engine runs no differently on all cylinders, or the difference is minimal on all cylinders:
  8.  
    1. Check the cam timing.
    2.  
    3. If the cam timing is OK, verify the mechanical condition of the engine.
    4.  
    5. Check the fuel system
    6.  


 
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