3.1L engine
General description
1. ECM electronic control unit (or PCM transmission control unit) controls the ignition timing set by the distributor (ignition timing setting), through the electronic ignition timing system EST, which provides improved engine performance, fuel economy and emission control.
2. The EST system consists of an ignition distributor, an HEI unit, an ECM unit and connecting wires. The contacts of the four wires going to the EST block are marked with letters on its body. The four-pin electrical connector of the distributor is indicated by the letters ABCD going from left to right (see illustration 9.1 in chapter 5). These circuits perform the following functions:
- A) Contact "A" - EST system. HEI block enabling circuit. The ECM learns about the actual ignition timing when it receives a tell-tale signal. From this point on, it starts to advance or retard the ignition. If the base advance is set incorrectly, the entire ignition curve will be incorrect.
- b) Contact "IN" - Distributor control signal. With it, data on the number of revolutions and the position of the crankshaft are transmitted to the ECM.
- V) Contact "WITH" - detour. At approximately 400 rpm, the ECM supplies 5 volts to this circuit; this signal activates the HEI unit, which begins to transmit ignition timing commands to the ECM. If there is an open or short to "mass" on the circuit, the system will generate a code 42, after which the engine will run at the original ignition timing plus a small amount of advance built into the HEI unit.
- G) Contact "D" - low control signal "masses". This wire in the distributor is shorted to "mass"; as a result, there is no voltage drop in the circuit, the presence of which could impair the characteristics of the motor. If this circuit is broken, engine performance may be impaired.
3. The ECM receives a control positive signal from the distributor containing data on the number of revolutions and position of the crankshaft. The ECM then determines the correct amount of advance for the given engine operating conditions and sends the appropriate pulse to the valve via the EST.
Examination
4. When shorted to "mass" diagnostic contact "Test" in the electrical contact of the ALDL, the ECM will advance the ignition to the specified value. To check the operation of the EST system, the ignition timing must be checked at 1500 rpm with the diagnostic contact not connected to "weight". Then close the contact "mass" (see section 2). If the ignition timing changes at 1500 rpm, then the EST system is working properly. As a rule, if a malfunction occurs in the EST system itself, DTC 42 will be issued.
Setting the basic ignition timing
Note. For the correct performance of this work, first check the data given on the VECI plate.
5. To set the basic ignition timing, locate and disconnect the electrical contact of the ignition setting (the location of this contact and the color of the insulation of the wire leading to it are shown on the VECI plate).
6. Set the ignition timing according to the VECI plate data. This will store code 42 in the ECM memory. Remember to clear the memory after setting the ignition timing (see section 2).
7. More complete information regarding the operations of checking and replacing elements in HEI / EST electronic systems (high power ignition system) and DIS (distributorless direct ignition system) shown in chapter 5.
3.8L engine
General description
8. The EST system consists of the EST box, PCM control box and connecting wires. The contacts of the wires going to the EST block are marked on its body with letters from "A" before "R". The contacts of the four-pin connector of the crankshaft position sensor are marked from left to right with the letters ABCD. Since diagnostic work requires special equipment, they must be carried out in a specialized repair shop.