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

Ignition Timing



Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.

Ideally, the air/fuel mixture in the cylinder will be ignited by the spark plug just as the piston passes TDC of the compression stroke. If this happens, the piston will be at the beginning of the power stroke just as the compressed and ignited air/fuel mixture forces the piston down and turns the crankshaft. Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.

The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 10 BTDC, each spark plug must fire 10 degrees before each piston reaches TDC. This only holds true, however, when the engine is at idle speed. As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC.

If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded, or after TDC (ATDC), the piston will have already started on its way down when the fuel is ignited. The piston will be forced down for only a portion of its travel, resulting in poor engine performance and lack of power.

Timing marks or scales can be found on the rim of the crankshaft pulley and the timing cover. The marks on the pulley correspond to the position of the piston in the No. 1 cylinder. A stroboscopic (dynamic) timing light is hooked onto the No. 1 cylinder spark plug wire. Every time the spark plug fires, the timing light flashes. By aiming the light at the timing marks while the engine is running, the exact position of the piston within the cylinder can be easily read (the flash of light makes the mark on the pulley appear to be standing still). Proper timing is indicated when the mark and scale are in specified alignment.

When checking timing with the engine running, take care not to get the timing light wires tangled in the fan blades and/or drive belts.


Although the timing may be checked on the Bosch Motronic system, it is not adjustable. All timing functions are carried out by the ECU. The ignition timing may be checked with a conventional inductive timing light.

Ignition timing on vehicles equipped with the EZ115K, EZ116K, or REX-1 ignition system may be checked with a conventional timing light. The timing, however, cannot be adjusted. If the ignition setting is wrong, use the following procedure:

  1. Check the throttle switch.
  3. Check that the wiring to the crank sensor is correctly connected at the connector in the firewall.
  5. Open the cover of the test connector and connect the cable to terminal 6.
  7. Turn the ignition ON . Select Test Function 1 by pushing button once for more than 1 second and count the number of blinks. Note the number and press again in case there are more fault codes (up to 3). Note the fault codes to begin troubleshooting.

Code 1-1-1 -No fault codes in memory.
Code 1-4-2 -Fault in control unit. Engine runs with safety-retarded ignition timing (approximately 10 degrees).
Code 1-4-3 -Knock sensor faulty. Engine runs with safety-retarded ignition timing (approximately 10 degrees).
Code 1-4-4 -Load signal missing (from fuel system control unit). Control unit selects full-load ignition.
Code 2-1-4 -Engine speed sensor faulty.
Code 2-2-4 -Coolant temperature sensor inoperative.
Code 2-3-4 -Throttle switch for idling faulty. Engine runs with safety-retarded ignition timing (does not apply to REX 1 ignition system).
Code 3-3-4 -Throttle switch in idle position-okay (REX 1 ignition only).

  1. If fault Code 1-1-1 (no fault in memory) appears, check the fuel system.
  3. If the LED does not light when the button is pressed, or no code is blinked out, check the connection at the ECU.