advance). Located in the distributor are two pickup coils (fig. 4-35), NOT found in 1978-1979 models. One coil operates during starting, whereas the other coil operates when the engine is running. The starting pickup is easily identified; its distributor connection is larger.

The computer selects either the start or run coil, not the ignition switch. The spark advance is controlled primarily by the spark control computer which receives its signals from the following engine sensors:

1. Coolant Temperature Switch (on the water pump housing) signals that the engine temperature is below 150°.

2. Air Temperature Switch (inside the computer, but not used after 1979) senses the temperature of the incoming fresh air which controls the throttle position advance.

3. Carburetor Switch (on the right side of the carburetor) tells the computer whether the engine is at idle or off idle.

4. Vacuum Transducer (on the computer) signals the computer for more spark advance with higher vacuum and less spark advance with lower vacuum. The computer responds over a period of time rather than suddenly, using a timed countdown delay.

5. Throttle Position Transducer (on the carburetor but eliminated in 1980) signals the computer to advance by indicating the new throttle plate position and the rate of change.

UNIT IGNITION SYSTEM (DELCO-REMY)

This unitized ignition system by Delco-Remy is another breakerless ignition system. It is called unitized because the entire system is built into one unit, the distributor. This distributor contains the ignition coil, the secondary wiring harness and cap, shell, rotor, vacuum advance unit, pickup coil, timer core (which replaces the cam), and electronic module. The distributor operates on an electronically amplified pulse. Vacuum spark advance and mechanical spark advance are applied in the usual way. The moving parts of this system induce a voltage that signals the electronic module to interrupt the primary circuit. The desired voltage is then induced in the secondary windings of the ignition coil and directed to the proper sparkplug by the rotor and the secondary wiring harness and cap.

HIGH-ENERGY IGNITION SYSTEM (DELCO-REMY)

The Delco-Remy High-Energy Ignition (HEI) System is a breakerless, pulse-triggered, transistor-controlled, inductive discharge ignition system. The HEI system and the older Unit Ignition System differ in that the HEI system is a full 12-volt system. The Unit Ignition System also incorporates a resistance wire to limit the voltage to the coil, except during starter motor operation.

The cam and point rubbing block of the conventional ignition system are replaced by the timer core, pickup coil, and electronic module in the HEI system (fig. 4-36). A timer core rotates inside the pickup

Figure 4-35.-Lean burn pickup coils.

Figure 4-36.-High-energy ignition system.