General Information

DESCRIPTION AND OPERATION
Conventional Spark Control Ignition System

A conventional electronic ignition system with vacuum/centrifugal spark control is used on carbureted 1985–88 Chevrolet Sprint and 1989–91 Geo Metro models.

The basic components of this ignition system are the ignition coil, the distributor, the spark plugs, and spark plug wiring. The distributor consists of a signal generator (signal rotor and pick-up coil), igniter, rotor, ignition module, vacuum advancer, and centrifugal advancer.

When the distributor shaft rotates, a fluctuating magnetic field is generated due to changes in the air gap between the pick-up coil and signal rotor. Therefore, an alternating current (AC) voltage is induced in the pick-up coil. This induced AC voltage peaks when a ridge on the signal rotor is adjacent to the ridge on the pick-up coil. When the voltage peaks, the igniter breaks the circuit to ground from the negative side of the coil primary winding. With the circuit broken, the magnetic field in the ignition coil, which has been generated by the electrical current passing through it, collapses. The high voltage induced by the collapsing field is then forced to find a ground through the secondary coil wire, the distributor cap, the rotor, the spark plug wire and finally across the spark plug air gap to the engine block.

Spark timing is mechanically controlled by a vacuum advance system which uses engine manifold vacuum and a centrifugal advance mechanism.

Fig. 1: Conventional electronic ignition circuit diagram

Electronic Ignition System With Knock Controller
Fig. 2: Electronic ignition with knock controller circuit diagram

An electronic ignition system with knock controller is used on turbocharged 1987–88 Chevrolet Sprint vehicles only.

This system functions in much the same manner as a conventional electronic ignition system except it utilizes an igniter with knock controller which electrically controls the ignition timing based on signal received from a knock sensor. The system ensures that optimum ignition timing is always obtained for best engine output and drive train protection. When there is no knocking, the ignition system operates in the same way as an ordinary ignition system. When knocking occurs, the system can retard the ignition timing up to 12°, according to the degree of knocking.

The distributor uses a rotor, signal generator, vacuum advance unit and centrifugal advancer. The vacuum advance unit controls the ignition timing as follows. The throttle body bore and surge tank internal pressure is delivered into the vacuum advance unit through hoses. When the pressure is negative (vacuum), it moves the breaker clockwise (in reverse direction to rotor rotation) to advance the ignition timing. On the other hand, the positive pressure causes the breaker to move counterclockwise to retard the ignition timing.

The igniter with knock controller is located at the left side of the engine compartment. It has two main circuits, one serving as a fully transistorized igniter and the other as a knock controller. Based on the engine speed and signals from the Electronic Control Module (ECM) and knock sensor, the knock controller gives the igniter an instruction to retard the ignition timing so that engine knocking is suppressed.

The controller also has a fail safe circuit built-in. It give an instruction to retard ignition timing by a certain amount if the knock sensor fails or a short or open circuit occurs in the output leads.

The knock sensor is located on the cylinder block at the intake manifold side. It detects engine vibration and converts it into an electrical signal, which is transmitted to the igniter.

Electronic Spark Control (ESC) Ignition System
Fig. 3: Electronic Spark Control (ESC) ignition system circuit diagram

An electronic spark control ignition system is used on 1989–91 Geo Metro Base and XFi models, and all 1992–00 Geo Metro models.

The ignition circuit consists of the battery, distributor, ignition switch spark plugs, primary and secondary wiring. The ESC system is monitored and controlled by the engine control module (ECM). The distributor used in this system consists of a signal generator (signal rotor and pick-up coil), and rotor. The igniter is located in the ECM.

All spark timing changes in the distributor are performed electronically by the ECM. After receiving signals indicating engine speed, manifold vacuum, coolant temperature and other engine functions, the ECM selects the most appropriate timing setting from memory and signals the distributor to change the timing accordingly. No vacuum or mechanical advance mechanisms are used.

The ECM controls a driver to ground, which is connected to negative side from the coil's primary circuit. When this ground is interrupted, the field around the primary coil collapses and a high voltage is induced in the secondary coil. The high voltage induced in the secondary coil is then forced to find a ground through the coil wire, distributor cap, rotor, spark plug wire and across the spark plug air gap to the engine block.

Ignition Coil

IGNITION COIL TESTING
Fig. 1: Measure ignition coil voltage

Fig. 2: Measure ignition coil resistance

  1. Check for spark at each spark plug with a spark plug tester. If no spark is detected, proceed to Step 2. If spark is only detected on some spark plugs, check for a faulty distributor cap or rotor. Also check the spark plugs and wires. Replace as needed.
  2. Check for voltage at the ignition coil positive terminal with a voltmeter. If battery voltage is detected, proceed to Step 3. If battery voltage is not detected, repair the open in the wiring between the battery and the ignition coil.
  3. Disconnect the connector on the negative coil wire. Check ignition coil resistance. If resistance is within specification, proceed to Step 4. If resistance is not within specification, replace the coil.
  4. Measure the resistance between the positive and negative terminals. Resistance should be 1.08–1.32 ohms electronic ignition with knock control and 1.33–1.55 ohms for all others.
  5. Measure the resistance between the positive/negative terminals and the coil high tension wire terminal. Resistance should be 11.6–15.8 kilo-ohms for electronic ignition with knock control and 10.7–14.5 kilo-ohms up to 1993. 22.1–30 kilo-ohms for vehicles 1994–00.
  6. On Conventional and ESC ignitions, check the resistance of the noise filter and condenser with an ohmmeter. If the resistance is 2.0–2.5 ohms for both units, proceed to Step 5. If the resistance is not within specification, replace both units as a set.
  7. On Conventional ignitions, check the continuity of the brown/white wire between the ignition coil negative terminal and the distributor. If continuity exists, replace the pick-up coil and igniter. If continuity does not exist, repair the open in the wire.

Adjustments

PICK-UP COIL AIR GAP ADJUSTMENT
  1. Remove the distributor cap and rotor.
  2. Using a nonmagnetic thickness gauge, measure the air gap between the pole piece tooth and pick-up coil.
  3. The air gap should be 0.009-0.015 in. (0.2-0.4 mm). If the gap is out of specification, adjust it.
  4. Remove the module and loosen the screws securing the pick-up coil. Using a screw driver, move the generator pick-up coil and adjust the gap to specification.
  5. After adjustment, tighten the screws and recheck the air gap. Install the module, rotor, and distributor cap.

Camshaft Position Sensor

TESTING
  1. Disconnect the sensor and check for voltage on the pins. The left terminal or B+ should be 10v to 14v. The center or Vout should be 4v to 5v. The right terminal should be ground.
  2. Reconnect the harness to the sensor.
  3. Crank the engine while probing pin C22-2 at the PCM.
  4. Look for varying voltage of 0v to 1v low and 4v to 5v high.
  5. Remove the sensor and visually inspect the rotor using a mirror.
  6. Replace sensor if the rotor checks out to be good.

Crankshaft Position Sensor

TESTING
  1. Check the sensor for resistance. Between the two terminals should be 360 ohms to 460 ohms.
  2. Check the resistance between each terminal and ground. The readings should be At least 1M ohms
  3. Visually inspect the sensor and the sensor pulley for damage, metal particles in the teeth, or incorrect set up.
  4. Replace or correct any component found that is out of parameters