Basic Troubleshooting Principles
TROUBLESHOOTINGThis troubleshooting section applies to starting and running problems caused by faults in the ignition system, including the Hall sender, the ignition control unit, the power stage, the coil, the distributor cap and rotor, and the spark plug wires.
The ignition system's primary function is to provide a properly timed high-voltage spark. The distributor's basic timing adjustment gives the Hall sender and the electronic system its baseline ignition timing. Depending on the system, the Hall sender signal is monitored by the ignition control unit, the knock sensor control unit, or the Digifant or Motronic control unit. The control unit in turn switches the coil primary circuit, causing the coil to discharge high voltage to the spark plugs through the distributor cap and spark plug wires. In this sensitive electronic ignition system, corroded or loose connections may interfere with any part of this function.
A complete failure of the ignition system to produce spark at the spark plugs is self-evident as an ignition system problem. For other problems such as rough idle, misfiring, or poor starting, the cause is not so clear. Start by testing and evaluating the ignition system, but if no ignition problems are discovered, proceed elsewhere. Inadequate engine compression or a malfunctioning fuel system will cause similar problems.
Checking the fuel system is especially important in Digifant and CIS-E Motronic systems where fuel and ignition control are integrated into one control unit.
Cars with the CIS-E Motronic, Digifant II (some California models only), or Digifant I engine management system feature on-board diagnosis (OBD) circuitry in the control unit to detect faults in the system during normal operation. When activated, the system displays faults as a number code which corresponds to a particular component or function of the system which should be checked.
Service department should be equipped with an adequate analyzer which plugs into a special test connection in the car's wiring harness. It can retrieve and interpret the fault codes, and also perform electronic checks of the knock control reference sensor, the Hall sender, and the knock sensors.
This check of the OBD system is recommended by Volkswagen as the first step in any Motronic or Digifant troubleshooting. It is most certainly recommended for any car that is still protected by Volkswagen's extensive warranty coverage.
Basic Troubleshooting Principles
An engine that starts and runs indicates that the ignition system is fundamentally working-delivering voltage to the spark plugs. A hard-starting or poor-running engine, however, may indicate a problem with how well the spark is delivered. A faulty coil, poor spark plug wires, a worn or cracked distributor cap or rotor, and worn or fouled spark plugs are all causes of reduced spark intensity and inefficient combustion. For cars with catalytic converters, this is an especially serious problem since the poorly burned mixture can overload the catalytic converter. This may cause the converter to overheat, causing plugging or presenting afire hazard.
An engine that has good cranking speed but will not even begin to start may indicate a complete failure of the system to produce spark. A basic check to see if spark is being produced will be the most important first troubleshooting step. If a strong spark is observed, then the failure to start is due to another cause, perhaps incorrect timing or engine mechanical failure.
The TCI-h ignition system contains very sensitive electronic components. To protect the system, and for general safety, the following cautions should be observed during any ignition system troubleshooting, maintenance, or repair work.
CAUTION:
- Do not touch or disconnect any of the high tension cables from the coi4 distributor; or spark plugs while the engine is running or being cranked by the starter
- Connect or disconnect ignition system wires, multiple connectors, and ignition test equipment leads only while the ignition is OFF.
- Before operating the starter without starting the engine (as when making a compression test), disable the ignition system. On all except CIS-E Motronic and Digifant I, disconnect the wire from the center of the distributor cap and positively connect it to ground on the engine. On Motronic, disconnect the Hall sender connector at the distributor. On Digifant I, disconnect the connector from the coil power stage. Cranking the engine with the high-voltage secondary wiring disconnected may damage the ignition.
- Do not connect test instruments with a 12-volt current supply to terminal 15 (+) of the ignition coil. The voltage backflow will damage the ignition control unit. In general, make test connections only as specified by Volkswagen, or as described by the instruments manufacturer.
- Do not connect a condenser to terminal 1, the negative (-) coil terminal. This may cause misfiring or ignition system damage.
- Do not disconnect the battery while the engine is running.
- The ignition systems in use a special distributor rotor with 1000 ± 400 ohms resistance. Take care to use the correct part Do not substitute any other Volkswagen or Bosch part.
- Do not quick-charge the battery (for boost starting) for longer than one minute, and do not exceed 16.5 volts at the battery with the boosting cables attached. Wait at least one minute before boosting the battery a second time.
- Do not tow a vehicle suspected of having a defective ignition system without first disconnecting the ignition control unit. On Digifant I disconnect terminal 1 (green) from the coil.
- Do not wash the engine while it is running, or anytime the ignition is switched ON.
- Disconnect the battery when doing any electric welding on the vehicle.
- Do not try to start the engine of a car which has been heated above 176°F (80°C), (for example, in a paint drying booth) until allowing it to cool to normal temperature.
- Do not conduct ignition system tests with a test lamp that uses a normal incandescent bulb. The high electrical consumption of these test lamps may damage the electronic components.
Test Equipment
Many of the tests of ignition system components require the use of high-impedance test equipment to prevent damage to the electrical components. An LED test light which meets these requirements is Volkswagen special tool no. US 1115, available from an authorized Volkswagen dealer parts department (order no. TU1 115 000 28 ZEL). A multimeter (for voltage, current, and resistance measurements) which meets these requirements is Volkswagen special tool no. US 1119, a digital multimeter (order no. TU1 119 000 00 KTM).
Many tests require checking for voltage, continuity, or resistance at the terminals of the component's harness connectors. The blunt tips of a multimeter's probes can damage the terminals, and cause poor connections. To prevent damage, use flat male connectors to probe the harness connector terminals. A suitable flat connector is Volkswagen part no. N 017 457 02. Volkswagen also has a special tool set, no. V.A.G. 1594, which contains a number of special adaptors for making connections that will not damage connectors during electrical testing.
NOTE: Tool numbers and order numbers are subject to change. Always rely on an authorized Volkswagen dealer parts department for the latest and most accurate information.
Quick-Check of Ignition System
If the engine does not start, the most fundamental step in troubleshooting the ignition system is to determine whether or not the system is creating any kind of spark at the spark plug. If a spark is present, then it is the ignition system's efficiency and accuracy or another system's function that is in question. If no spark is present, then more detailed testing of the ignition system is necessary.
To check for spark, turn the ignition off and remove a connector from one of the spark plugs. Connect it to a known good spark plug, preferably a new plug, and hold the spark plug with insulated pliers so that the outer electrode is grounded on the engine.
CAUTION: Any test set-up other than the one described above may cause damage or inconclusive tests.
While a helper actuates the starter, look for spark in the spark plug gap. A bright blue spark indicates a healthy ignition system. A yellow-orange spark is weaker and indicates that, while spark is present and the system is functioning, it is not operating at peak efficiency. Check the condition of the ignition system components as described in Ignition System Visual Inspection and replace any faulty components.
WARNING: If ignition system failure is not the problem, the engine may start during this test. Be prepared to turn off the ignition immediately. Also, running the engine with a spark plug wire disconnected will damage the catalytic converter.
NOTE: Before checking the ignition system when there is no spark or a weak spark, make sure that the battery is fully charged.
If there is no spark, test for the switching of primary voltage at the ignition coil. Connect a test light between terminal 1 (-) of the ignition coil and ground. While the starter is actuated, the test light should flash, indicating that the coil is receiving primary voltage and is being switched correctly by the ignition system.
If the light does not flash, there is a fault in the system that switches the coil, or in the wiring harness or the ignition switch. Begin further troubleshooting by checking the Hall sender as described in Testing Hall Sender.
If the coil is receiving voltage, or if a strong spark is observed but the engine still will not start, begin further troubleshooting with a close visual inspection and testing of the components that conduct the secondary voltage. See Ignition System Visual Inspection and Testing Coil and Spark Plug Wires. Table b lists more ignition system troubleshooting information.
Table b:
Table b. Ignition System Troubleshooting