COMPUTER CONTROLLED IGNITION SYSTEMS

COLUMBIA BASIN COLLEGE

General Motors High Energy Ignition

• First Used in 1975

• Replaced Breaker Points, more precise

• Worked with Mechanical Ignition Timing

• Produces a AC Sine wave

• Module converts into Digital Square Wave

• Works better with varied Engine Speeds

• Lower maintenance

Standard Electronic HEI Ignition Module

NOTE: All Electronic Ignition Systems operate very similar

Computer Controlled Timing(All systems are similar)

• Ignition Module controls basic Ignition System Operation

• Controls Ignition Timing Electronically

• Controls advance and retard functions

• Module receives input from PCM to make more precise timing calculations.

• 7 terminals (instead of HEI 4 terminals)

EST Cranking(Module Mode)

Small amount of advance in module mode.EST is grounded.If bypass fails, stays in module mode.

EST Engine Running

Above 200 rpm, module switches to PCM timing control

COMPUTED TIMING

• Computed timing is controlled by delaying primary operation so many degrees after previous cylinder firing.

• Advances timing for the cylinder next in the firing order.

Diagnostic Codes

• OPEN EST WIRE: – Expects to see EST signal pulled to zero.

– With EST Open, signal will “toggle”

– PCM sees “toggle” as abnormal and sets a code.

– When engine reaches 200 rpm, will not apply bypass voltage.

– Module will stay in “module mode”

– If car is running, will die, then restart

Grounded EST Circuit

• During Cranking, EST volts are at zero so PCM doesn’t recognize a problem.

• When engine reaches rpm value, PCM applies bypass voltage to module.

• Bypass voltage switches module transistor to EST line (which is grounded)

• Sets Code 42 and Engine won’t run• Engine will not run with a grounded EST

Grounded or Open Bypass Line

• While engine is cranking, PCM doesn’t see anything abnormal

• When run rpm is reached PCM applies bypass voltage, but bypass doesn’t switch Ignition module to computer timing.

• Sets code 42 and keeps ignition module in module mode.

• If occurs running down road, car will start running in module mode.

Open in Reference Signal

• PCM receives no reference rpm and engine will run in module mode.

• CHECK ENGINE light will be on steady

• Will set a code “12” or “41”

• Code 12 will not store

• Code 41 will store in memory