Page 32

1-28

GENERAL INFORMATION AND MAINTENANCE

Fig. 108 Loosen the distributor hold-down fasteners

Fig. 109 II necessary, label and remove the spark plug wires from the distributor cap

Fig. 111 . . . then the rotor from the distributor 4. Remove the 4 coil cover screws and cover. 5. Using a finger or a blunt drift, push the spade terminals up out o tributor cap. 6 Remove all 4 coil screws and lift the coil, coil spring and rubber washer out of the cap coil cavity. 7. Remove the two rotor attaching screws (if equipped) and rotor. 8. Using a new distributor cap and rotor, reverse the above procedu assemble, being sure to clean and lubricate the rubber seal washer with trie lubricant.

the dis,eal esto dielec-

Fuel Injected Engines 1. 2. 3. 4.

Tag and remove the spark plug wires. Loosen the cap retaining fasteners and remove the cap. Remove the rotor from the distributor shaft. Installation is the reverse of removal.

INSPECTION 1. Remove the distributor cap and rotor as described in this sectior 2. Check the cap for wear, electrode cracks or damage. Replace if di fective. 3. Check the rotor for cracks and wear. Replace if defective.

Ignition Timing GENERAL INFORMATION ••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 strok Ignition timing is controlled by turning the distributor in the engine.

8/981P14

Fig. 110 Remove the distributor cap . . .

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 stili 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.

1990 gmc sierra service repair manual  
1990 gmc sierra service repair manual  
Advertisement