Volvo 240/740/760/780/940/960 1990-1998

Diagnosis and Testing



The ignition system operates with a very high output and there are hazardous voltages in the low and high voltage circuits.
Always turn the ignition OFF , before separating connectors.
Never disconnect the battery while the engine is running.
Always disconnect the battery when quick charging the battery.
Never use a boost charger or voltage higher than 16 volts to start the engine.
Always remove the control unit if the vehicle is to be stove or if welding is to be carried out. The control unit must not be exposed to temperatures above 176°F (80°C).
Do not replace a control unit without first correcting the original fault, or the same fault may damage the new control unit.
Do not be hasty in condemning the ECM. This system uses voltages and resistances that are very small. Examine the sensors, wiring and connectors carefully. The sensors operate in more harsh conditions than the ECM which is generally in a more protected location.
Check all ground connections before condemning the ECM.
Use care when working around vehicles equipped with Supplementary Restraint System (SRS), often known as "air bags." Vehicles equipped with SRS are generally recognized by the letters SRS molded into the steering wheel cover. Follow all precautions to avoid personal injury.

Before performing any component testing, check for and, if necessary, repair the following:

Damaged, corroded, contaminated, carbon tracked or worn distributor cap and rotor
Damaged, fouled, improperly seated or gapped spark plug(s)
Damaged or improperly engaged electrical connections, spark plug wires, etc.
Discharged battery
Blown fuses


The best way to perform this procedure is to use a spark tester (available at most automotive parts stores). Two 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. This type of tester allows 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.
  3. Connect the plug wire to the spark tester and ground the tester to an appropriate location on the engine.
  5. Crank the engine and check for spark at the tester.
  7. If spark exists at the tester, the ignition system is functioning properly.
  9. If spark does not exist at the spark plug wire, remove the distributor cap and ensure that the rotor is turning when the engine is cranked.
  11. If the rotor is turning, perform the spark test again using the ignition coil wire.
  13. If spark does not exist at the ignition coil wire, test the ignition coil, and other distributor related components or wiring. Repair or replace components as necessary.