DTC 31:
THEFT DETERRENT SIGNAL MISSING
When the ignition is turned "ON", the PCM will send a message on circuit 1221 to the BCM asking for permission to start. When the BCM receives this message, it instantly enables the starter and sends a message back to the PCM. The message says that the proper ignition key has been used to turn the ignition and that it is OK for the PCM to enable the fuel injectors to start the vehicle. If the BCM does not receive communications from the PCM when the ignition is switched "ON", then the starter motor will be enabled after a one second delay.
DTC 31 will be set, if the PCM sends 20 messages to the BCM and does not receive a message back saying it is OK to start.
Test Description: Number(s) below refer to step number(s) on the diagnostic chart.
2. If the engine cranks after a one second delay it means the BCM did not see a message from the PCM when the ignition was turned "ON".
3. An open or short to earth on circuit 1221 will disable any communication of serial data.
DTC 46:
NO REFERENCE PULSES WHILE CRANKING
This DTC is intended to help in diagnosing a no-start condition. Any time the distributor is turning, the ignition module should generate the crankshaft reference pulses that the PCM should be receiving. Fuel injection pulses are "timed" from the crankshaft pulses, and without them no injection pulses will occur. The PCM can determine when these crankshaft pulses should be present, but aren't.
As with any engine while being cranked, there is a small amount of intake manifold vacuum. Also while cranking, the battery voltage will be less than 11 volts. If the PCM's MAP sensor input detects manifold vacuum and the ignition voltage input detects less than 11 volts and there are no distributor reference input pulses, a DTC 46 will set.
NOTE: It is possible for the ignition system to provide spark, yet there may not be any distributor reference pulses at the PCM.
DTC 46 will set if all the following conditions are present for more than 2 seconds:
Voltage at PCM ignition voltage input terminal "C1" is below 1 volts, and
Map sensor input detects at least 3 kPa (approx. 1" Hg) engine vacuum, and
No distributor reference input pulses at PCM terminal "D14".
Test Description: Number(s) below refer to the step(s) on the diagnostic chart.
2. If engine starts, the problem is not present now. While the PCM monitors 3 inputs to determine DTC 46, only a lack of distributor reference input signal pulses can set the DTC. If a problem occurred on the MAP or ignition voltage inputs, other problems would be apparent.
NOTE: Even one crankshaft reference pulse while cranking will cause DTC 46 to NOT set.
3. Spark on both wires proves the distributor Hall switch and ignition coil to be OK.
No spark on either lead means the ignition system is suspect of causing the DTC 46. If the ignition system cannot generate any spark, it cannot generate the distributor reference signal either.
4. The voltage should be either "OFF" (less than 1 volt) or "ON" (more than 3 volts). This is the square-wave, digital "ON-OFF" distributor reference signal generated by the ignition module. The voltage could be either reading, depending on where the distributor stopped turning.
5. This is again the distributor reference signal. When the distributor is turning, the signal changes from under 1 volt to over 3 volts, back and forth, eight times per distributor revolution. Since it changes quickly, the voltmeter can indicate only an average voltage. (expected reading - approximately 2 volts.)
6. A loose connection at PCM terminal D14 could cause an intermittent voltage measurement to be seen.
NOTE: Use ST-125 spark checker or equivalent to check for adequate spark. An ST-125 requires about 25,000 volts (25 Kilovolts or 25 kV) to spark. Do not use a spark plug in open air earthed to the engine as an indication of sufficient spark. Only a few kilovolts are required to jump the gap of a spark plug outside of the engine, and that would be an inadequate test of the ignition system.
