GM Full-Size Trucks 1988-1998 Repair Information

Ignition Timing



This procedure does not apply to diesel engines

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 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, this piston will be beginning 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 gases 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 benefit 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 degrees BTDC, the spark plug must fire 5 degrees before that 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.

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 engine is 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.

Timing should be checked at each tune-up and any time the points are adjusted or replaced. It isn't likely to change much with HEI. The timing marks consist of a notch on the rim of the crankshaft pulley or vibration damper 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.

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 closely 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 battery-two alligator clips connect to the battery terminals, while an adapter enables a third clip to be connected between No. 1 spark plug and wire. This type is a bit more expensive, but it 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 115 volt current.

Some timing lights have other features built into them, such as dwell meters, or tachometers. These are convenient, in that they reduce the tangle of wires under the hood when you're working, but may duplicate the functions of tools you already have. One worthwhile feature, which is becoming more of a necessity with higher voltage ignition systems, is an inductive pickup. The inductive pickup clamps around the No. 1 spark plug wire, sensing the surges of high voltage electricity as they are sent to the plug. The advantage is that no mechanical connection is inserted between the wire and the plug, which eliminates false signals to the timing light. A timing light with an inductive pickup should be used on HEI systems.


HEI Systems
  1. Start the engine and allow it to reach operating temperature. Stop the engine and connect the timing light to the No. 1 (left front) spark plug wire, at the plug or at the distributor cap. You can also use the No. 6 wire, if it is more convenient. Numbering is illustrated in this section.

Do not pierce the plug wire insulation with HEI; it will cause a miss. The best method is an inductive pickup timing light.

  1. Clean off the timing marks and mark the pulley or damper notch and timing scale with white chalk.
  3. Disconnect and plug the vacuum line at the distributor on models with a carburetor. This is done to prevent any distributor vacuum advance. On fuel injected models, disengage the timing connector which comes out of the harness conduit next to the distributor, this will put the system in the bypass mode. Check the underhood emission sticker for any other hoses or wires which may need to be disconnected.
  5. Start the engine and adjust the idle speed to that specified on the Underhood Emissions label. With automatic transmission, set the specified idle speed in Park. It will be too high, since it is normally (in most cases) adjusted in Drive. You can disconnect the idle solenoid, if any, to get the speed down. Otherwise, adjust the idle speed screw.

The tachometer connects to the TACH terminal on the distributor and to a ground on models with a carburetor. On models with fuel injection, the tachometer connects to the TACH terminal on the ignition coil. Some tachometers must connect to the TACH terminal and to the positive battery terminal. Some tachometers won't work with HEI.

Never ground the HEI TACH terminal; serious system damage will result.

  1. 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 or damper notch isn't aligned with the proper timing mark (see the Underhood Emissions label), the timing will have to be adjusted.

TDC or Top Dead Center corresponds to 0°B, or BTDC, or Before Top Dead Center may be shown as BEFORE. A, or ATDC, or After Top Dead Center may be shown as AFTER.

  1. Loosen the distributor base clamp locknut. You can buy trick wrenches which make this task a lot easier.
  3. Turn the distributor slowly to adjust the timing, holding it by the body and not the cap. Turn the distributor in the direction of rotor rotation to retard, and against the direction of rotation to advance.
  5. Tighten the locknut. Check the timing again, in case the distributor moved slightly as you tightened it.
  7. Reinstall the distributor vacuum line or the timing connector. Correct the idle speed.
  9. Stop the engine and disconnect the timing light.

Distributor Ignition (DI) Systems

Refer to the underhood label for the proper timing setting.

  1. Engage the parking brake, block the wheels and set the transmission in P.
  3. Disconnect the Ignition Control (IC) system by disengaging the "set timing connector". This is a single wire sealed connector that has a tan with black stripe lead. This wire comes out of the wiring harness below the heater case.
  5. With the ignition switch OFF , connect the timing light pickup lead to the No. 1 spark plug wire.
  7. Start the engine and point the timing light at the timing mark on the balancer or pulley and check the timing.
  9. If the timing is not within specifications refer to the underhood emission sticker, loosen the distributor hold-down bolt. Slowly rotate the distributor until the proper timing setting is achieved.
  11. Tighten the hold-down bolt and recheck the timing.
  13. Turn the ignition OFF , remove the timing light and engage the "set timing" connector.

1996-98 MODELS

The distributor on the 4.3L engine is non-adjustable and any attempt to adjust the distributor could cause cross firing. This procedure applies to the 5.0L, 5.7L and 7.4L engines.

The ignition timing is preset and cannot be adjusted. If the distributor position is moved crossfiring may be induced. To check distributor position the following:

An OBD ll compliant scan tool is required for this procedure

  1. With the ignition OFF install scan tool to the Data Link Connector (DLC).
  3. Start the engine and bring the vehicle to operating temperature.
  5. Monitor cam retard on the scan tool.
  7. If cam retard is between -2° and +2° the distributor is properly adjusted.
  9. If cam retard is not between -2° and +2° the distributor must adjusted.
  11. With the engine OFF loosen the distributor hold-down bolt.
  13. Start the engine and check the cam retard reading. Rotate the distributor counterclockwise to compensate for a negative reading and clockwise to compensate for a positive reading.
  15. Momentarily raise the engine speed to over 1000 RPM and check the cam retard reading.
  17. If the proper reading is not achieved repeat Steps 7 and 8.
  19. When the proper reading has been achieved tighten the distributor hold-down bolt and disconnect the scan tool.