Code 42

EST OR BYPASS CIRCUIT FAILURE (EXCHANGE PROGRAM)

Circuit Description: 2.3L "N" CARLINE (PORT) The IDI module sends a reference signal to the ECM, when the engine is cranking. While the engine is under 637 rpm, the IDI module controls the ignition timing. When the engine speed exceeds 637 rpm, the ECM sends a 5 volt signal on the "By-pass" CKT 424 to switch the timing to ECM control. An open, or ground, in the EST or "Bypass" circuit, will set a Code 42, and cause the engine to run on module or "Bypass" timing. This will result in poor performance and poor fuel economy.
Test Description: Step numbers refer to numbers on diagnostic chart.
1. Checks to see if ECM recognizes a problem. If it doesn't set Code 42, at this point, it is an intermittent problem and could be due to a loose connection.
2. With the ECM disconnected, the ohmmeter should be reading less than 500 ohms, which is the normal resistance of the ignition module. If the resistance reading is greater than 500 ohms, and CKT 423 and connections are good, the "IDI" assembly should be exchanged per the attached procedure.
3. If the test light was "ON", when connected from 12 volts to ECM harness terminal "BC7", either CKT 423 is shorted to ground, or the ignition module is faulty.
4. Checks to see if ignition module switches, when the bypass circuit is energized by 12 volts, through the test light. If the ignition module actually switches, the ohmmeter reading should shift to over 8,000 ohms.
5. Disconnecting the ignition module should make the ohmmeter read as if it were monitoring an open circuit (infinite reading). If the ohmmeter has a reading other than infinite, CKT 423 is shorted to ground. If CKT 423 is OK, exchange the "IDI assembly per the attached procedure.

Diagnostic Aids:

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Inspect ECM harness connectors for backed out terminals "BC7" or "BC8" improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.