Jeep Wagoneer/Commando/Cherokee 1984-1998

Ignition Timing


Timing is adjustable on 1984-86 vehicles only. On 1987-98 vehicles, the timing is controlled by the Powertrain Control Module (PCM) and is not adjustable.


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. Ignition timing is controlled by turning the distributor body in the engine.

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 beginning its downward motion of the power stroke just as the compressed and ignited air/fuel mixture starts to expand. The expansion of the air/fuel mixture then forces the piston down on the power stroke 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 5° BTDC, the spark plug must fire 5° 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. To do this, the distributor has a means to advance the timing of the spark as the engine speed increases. This is accomplished by input from the electronic ignition control module and other computer sources. If the distributor is equipped with a vacuum advance unit, it is necessary to disconnect the vacuum line from the diaphragm when the ignition timing is being set.

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, after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.

The timing is best checked with a timing light. This device is connected in series with the No. 1 spark plug. The current that fires the spark plug also causes the timing light to flash.

When the engine is running, the timing light is aimed at the timing marks on the engine and crankshaft pulley.


See Figures 1, 2, 3 and 4

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Fig. Fig. 1: Common location of timing marks

Timing is adjustable on 1984-86 vehicles only. On 1987-98 vehicles, the timing is controlled by the Powertrain Control Module (PCM) and is not adjustable.

On 1984-86 models, timing should be checked at each tune-up and any time components are replaced in the ignition system. The timing marks consist of a notch on the rim of the crankshaft pulley and a graduated scale attached to the engine front (timing) cover. A stroboscopic flash (dynamic) timing light must be used, as a static light is too inaccurate for emission controlled engines.

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Fig. Fig. 2: Using a timing light to adjust ignition timing

There are three basic types of timing lights available. The first is a simple neon bulb with two wire connections. One wire connects to the spark plug terminal and the other plugs into the end of the spark plug wire for the No. 1 cylinder, thus connecting the light in series with the spark plug. This type of light is pretty dim and must be held very close to the timing marks to be seen. Sometimes a dark corner has to be sought out to see the flash at all. This type of light is very inexpensive. The second type operates from the vehicle's battery-two alligator clips connect to the battery terminals, while an adapter enables a third clip to be connected to the No. 1 spark plug and wire. This type provides a nice bright flash that you can see even in bright sunlight. It is the type most often seen in professional shops. The third type replaces the battery power source with 110 volt current.

  1. Warm up the engine to normal operating temperature. Stop the engine and connect the timing light to the No. 1 spark plug wire and a tachometer to the Tach terminal on the distributor cap.

Most tachometers require a special clip to attach the meter to the distributor. This clip may or may not be included with the tachometer.

  1. Clean off the timing marks and mark the pulley notch and timing scale with white chalk.
  3. Disconnect and plug the vacuum hose from the vacuum advance unit (if equipped).
  5. Start the engine and aim the timing light at the pointer marks. Be careful not to touch the fan, because it may appear to be standing still. If the pulley notch isn't aligned with the proper timing mark (refer to the Tune-Up Specifications chart), the timing will have to be adjusted.
  7. Loosen the distributor clamp locknut. Turn the distributor slowly to adjust the timing, holding it by the base and not the cap. Turn it counterclockwise to advance timing (toward BTDC), and clockwise to retard (toward TDC or ATDC).
  9. Tighten the locknut. Check the timing again, in case the distributor moved slightly as you tightened it.
  11. Unplug and connect the distributor vacuum line if applicable, and correct the idle speed to that specified in the Tune-Up Specifications chart.
  13. Stop the engine and disconnect the timing light.

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Fig. Fig. 3: 2.5L engine timing marks

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Fig. Fig. 4: 2.8L engine timing marks