Skylark V6-3100 3.1L VIN M SFI (1996)
Powertrain Control Module: Description and Operation
PCM to Ignition Control Module Circuits
3X REFERENCE PCM INPUT - From the ignition control module, the PCM uses this signal to calculate engine RPM and crankshaft position. The
PCM compares pulses on this circuit to any that are on the Reference Low circuit, ignoring any pulses that appear on both. The PCM also uses the pulses
on this circuit to initiate injector pulses. If the PCM receives no pulses on this circuit, the PCM will use the 24X reference pulses to calculate RPM and
crankshaft position. The engine will continue to mn and start normally, but DTC P1374 will be set.
24X REFERENCE PCM INPUT - The 24X reference signal is used to accurately control spark timing at low RPM and allow IC operation during
crank. Below 1200 RPM, the PCM is monitoring the 24X reference signal and using it as the reference for ignition timing advance. When engine speed
exceeds 1200 RPM, the PCM begins using the, 3X reference signal to control spark timing. If the 24X reference signal is not received by the PCM while
the engine is running, a DTC P0336 will be set and 3X reference will be used to control spark advance under 1200 RPM, and Bypass Mode will be in
effect at under 400 RPM. The engine will continue to wn and start normally.
REFERENCE LOW PCM INPUT - This is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground
only through the ignition control module. Although this circuit is electrically connected to the PCM, it is not connected to ground at the PCM. The PCM
compares voltage pulses on the 3X or 24X reference input to those on this circuit, ignoring pulses that appear on both. If the circuit is open, or connected
to ground at the PCM, it may cause poor engine performance and possibly a MIL (Service Engine Soon) with no DTC.
BYPASS SIGNAL PCM OUTPUT - The PCM either allows the ignition control module to keep the spark advance at Bypass Mode 10 BTDC, or the
PCM commands the ignition module to allow the PCM to control the spark advance (IC Mode). The ignition control module determines correct
operating mode based ~n the voltage level that the PCM sends to the ignition control module on the bypass circuit. The PCM provides 5 volts on the
bypass circuit if the PCM is going to control spark timing (IC Mode). If the PCM does not apply 5 volts to the bypass circuit, or if the ignition control
module doesn[prime ]t sense the 5 volts, the ignition control module will control spark timing (Bypass Mode). An open or grounded bypass circuit will
set DTC P1361 and the ignition system will stay at Bypass Mode advance.
IGNITION CONTROL (IC) PCM OUTPUT - The IC output circuitry of the PCM sends out timing pulses to the ignition control module on this
circuit. When in the Bypass Mode, the ignition control module grounds these pulses. When in the IC Mode, these pulses are the ignition timing pulses
used by the ignition control module to energize one of the igniuon coils. Proper sequencing of the 3 ignition coils, i.e.; which coil to fire, is always the
job of the ignition control module. If the IC circuit is grounded when the engine is started, DTC P1361 will set and the ignition system will stay in the
Bypass Mode. If the IC circuit becomes open or grounded during IC Mode operation, DTC P135O or P1361 may set. When this happens, the engine will
quit running but will restart. Upon restart following an ignition cycle, DTC Pi 361 will be set, and the ignition system will operate in Bypass Mode.
KNOCK SENSOR (KS) PCM INPUT - The KS system is comprised of A knock sensor, a KS module, and the PCM. The PCM monitors the knock
sensor signal to determine when engine detonation occurs. When the knock sensor detects detonation, the PCM retards the spark timing (IC) to reduce
detonation. Retarded timing can also be a result of excessive valve lifter, pushrod or other mechanical engine or transaxle noise.
CAMSHAFT POSITION PCM INPUT (CAM SIGNAL) - The PCM uses this signal to determine the position of the cylinder #1 piston during its
intake stroke. This signal is used by the PCM to calculate true Sequential ~ueI Injection (SF1) mode of operation. The PCM compares the number of
CAM pulses to the number of 24X and 3X reference pulses. If the cam signal is lost while the engine is running the fuel injection system will shift to a
calculated sequential fuel injection mode based on the last cam pulse, and the engine will continue to run.
