The electronic ignition (EI) system produces a high energy secondary spark. This spark is used to ignite
the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel
economy, and control of exhaust emissions. This ignition system uses one coil for each pair of cylinders.
Companion cylinders are a pair of cylinders that are at top dead center (TDC) at the same time. The
cylinder that is at TDC of the compression stroke is called the event cylinder. The cylinder that is at TDC
of the exhaust stroke is called the waste cylinder. When the coil is triggered both companion cylinder
spark plugs fire at the same time, completing a series circuit. Because the lower pressure inside the waste
cylinder offers very little resistance, the event cylinder uses most of the available voltage to produce a
very high energy spark. This is known as waste spark ignition. The EI system consists of the following
Crankshaft Position (CKP) Sensors
The crankshaft position (CKP) sensor has a 4-wire harness connector that plugs into the CKP sensor and
connects to the ignition control module (ICM). The CKP sensor contains 2 hall-effect switches in 1
housing, and shares a magnet between the switches. The magnet and each hall-effect switch are
separated by an air gap. A hall-effect switch is a solid state switching device that produces a digital
ON/OFF pulse when a rotating element passes the sensor pick-up and interrupts the magnetic field of the
sensor. The rotating element is called an interrupter ring or blade. There are two interrupter rings built
into the crankshaft balancer. The outer ring and the outer switch provide the ICM with 18X signals or 18
identical pulses per crankshaft revolution. The inner ring and the inner switch provide the ICM with 3
pulses per revolution, each 1 of different duration. This is called the sync pulse. Each sync pulse
represents a pair of companion cylinders. The ICM supplies a 12-volt and a low reference circuit to the
CKP sensor, which is also shared by the camshaft position (CMP) sensor. The 18X reference pulses are
passed from the CKP sensor to the ICM on the CKP sensor 1 signal circuit. The sync pulses are passed
from the CKP sensor to the ICM on the CKP sensor 2 signal circuit. The ICM uses the 18X and sync
pulses to determine the crankshaft position by counting how many ON-OFF 18X pulses occur during a
sync pulse. With this dual interrupter ring arrangement the ICM can identify the correct pair of cylinders to
fire within as little as 120 degrees of crankshaft rotation.
Crankshaft Balance Interrupter Ring
Each interrupter ring has blades and windows that either block the magnetic field or allow it to close one of
the hall-effect switches. The outer hall-effect switch sends a pulse called the 18X reference signal. The
outer interrupter ring has 18 evenly spaced blades and windows. The 18X reference signal produces 18
ON-OFF pulses per crankshaft revolution. The inner hall-effect switch sends a pulse called the sync
signal. The inner interrupter ring has 3 unevenly spaced blades and windows of different widths. The sync
signal produces 3 different length ON-OFF pulses per crankshaft revolution. When the sync interrupter
ring window is between the magnet and the inner switch, the magnetic field will cause the sync hall-effect
switch to ground the supplied voltage from the ICM. The 18X interrupter ring and the hall-effect switch
Camshaft Position (CMP) Sensor
The camshaft position (CMP) sensor signal is a digital ON/OFF pulse, output once per revolution of the
camshaft. The CMP sensor does not directly affect the operation of the ignition system. The CMP sensor
information is used by the powertrain control module (PCM) to determine the position of the valve train
relative to the CKP. By monitoring the CMP and CKP signals the PCM can accurately time the operation of
the fuel injectors. The CMP sensor shares 12-volt and low reference circuits with the CKP sensor. The
CMP signal circuit is input to the ICM.
Ignition Control Module (ICM) and Ignition Coils
Three dual tower ignition coils are mounted to the ICM, and are serviced individually. The ICM performs
the following functions:
• The ICM supplies a power and low reference circuit to the CMP and CKP sensors.
• The ICM determines the correct direction of the crankshaft rotation, and cuts spark and fuel delivery to
prevent damage from backfiring if reverse rotation is detected.
• The ICM determines the correct coil triggering sequence, based on how many 18 X ON-OFF pulses
occur during a sync pulse. This coil sequencing occurs at start-up, and is remembered by the ICM. After
the engine is running, the ICM will continue to trigger the coils without the CKP sync pulse.
• The ICM inputs 18 X and 3 X reference signals to the PCM.
• The 3 X reference signal is also known as the low resolution engine speed signal. This signal is
generated by the ICM using an internal divide-by-six circuit. This circuit divides the 18 X signal pulses by
6. This divider circuit will not begin operation without a sync pulse present at start-up, and without 18 X
and 3 X reference signals no fuel injection will occur.
Powertrain Control Module (PCM)
The PCM maintains proper spark and fuel injection timing for all driving conditions. Ignition control (IC)
spark timing is the method the PCM uses to control spark advance. To provide optimum driveability and
emissions, the PCM monitors input signals from the following components to calculate ignition spark timing:
• The ignition control module (ICM)
• The throttle position (TP) sensor
• The engine coolant temperature (ECT) sensor
• The mass air flow (MAF) sensor
• The intake air temperature (IAT) sensor
• The vehicle speed sensor (VSS)
• The transmission gear position or range information sensors
• The engine knock sensors (KS)
The following describes the PCM to ICM circuits:
• Low resolution engine speed signal--3 X reference--PCM input--from the ICM. The 3X reference signal is
produced by the ICM. The PCM uses this signal to calculate engine RPM and crankshaft position above
1,200 RPM. The PCM also uses the pulses on this circuit to initiate fuel injector operation. The PCM
compares the number of 3X pulses to the number of 18X and cam pulses. If the number of 3X pulses are
incorrect while the engine is cranking or running, the PCM will set a DTC. The engine will continue to start
and run normally using the 18X reference signal.
• Medium resolution engine speed signal--18 X reference--PCM input--from the ICM. The 18 X reference
signal is used to accurately control spark timing at low RPM and allow ignition control (IC) operation during
cranking. The ICM calculates the 18X reference signal by filtering the CKP sensor 18X pulses when the
engine is running and the CKP sync pulses are being received. Below 1,200 RPM, the PCM is monitoring
the 18X reference signal and using the 18X signal as the reference for ignition timing advance. The PCM
compares the number of 18X pulses to the number of 3X and cam pulses. If the number of 18X pulses are
incorrect while the engine is cranking or running, the PCM will set a DTC. The engine will continue to start
and run normally using the 3X reference signal.
• Camshaft position--PCM input--from the ICM. The PCM uses this signal to determine the position of the
cylinder #1 piston during the pistons power stroke. This signal is used by the PCM to calculate true
sequential fuel injection (SFI) mode of operation. The PCM compares the number of CAM pulses to the
number of 18 X and 3 X reference pulses. If the number of 18 X and 3 X reference pulses occurring
between CAM pulses is incorrect, or if no CAM pulses are received while the engine is running, the PCM
will set a DTC. 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.
The engine can be re-started and will run in the calculated sequential mode as long as the condition is
present with a 1 in 6 chance of being correct.
• Low reference--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 ICM. This circuit assures there is no ground drop
between the PCM and ICM.
• IC timing signal--PCM output--to the ICM. The ICM controls spark timing while the engine is cranking,
this is called bypass mode. Once the PCM receives 3 X reference signals from the ICM, the PCM applies 5
volts to the IC timing signal circuit allowing the ICM to switch spark advance to PCM control.
• IC timing control--PCM output--to the ICM. The IC output circuitry of the PCM sends out timing signals to
the ICM on this circuit. When in the Bypass Mode, the ICM grounds these signals. When in the IC Mode,
the signals are sent to the ICM to control spark timing.
Modes of Operation
The PCM has 2 modes of operation. One is the Bypass mode during which the PCM does not apply 5
volts to the IC timing signal circuit, allowing the ICM to control the triggering of each coil for proper spark
timing. The Bypass mode is used during each of the following conditions:
• Engine running below a desired RPM
• Default mode due to a system failure
The other mode is the IC mode during which the PCM is receiving the 18X and the 3X reference pulses
from the CKP sensor and is supplying 5 volts to the IC timing signal circuit. This allow the PCM to
accurately control spark timing for all driving conditions.
|I have a no start issue when warm on my 3800 series 2. This shed some light on the
system and inter-dependencies. Thanks
|You seriously want us to give you how-to instructions on how to replace the engine? We
really do not have enough space to type all that, even if we wanted to ! Go get a service
|My Buick was running fine, and without warring it started to run very ruff and when I
stopped and shut it down, it would not start again. The spark module is ok,and the coils.
it is getting spark.
Any ideas as to what to check next.
|Check fuel pressure. Scan the PCM for any
trouble codes. If this is the 3800 engine, you
may have a bad crank position sensor. It is
located behind the crank pulley on the engine.
|i have buick regal 3800 ,spark plus no working.
|The spark plugs are not working? What the hell does
that mean? If you mean you do not have spark to any of
the plugs, you could have a bad ignition module,
computer, crank sensor, set.
|My 1994 olds, 3800 turned port 6 cilynder,work fine but when i stop the engine and try to re start
wont crank for a long time about six minutes,any idea?
|If the engine does not crank,then check the battery. If
that is good, check for power on the purple wire at the
start when trying to crank. It should have power. If not,
maybe a bad neutral safety switch.
|Here is a good one. 1990 olds 3.8 with the one piece 3 coil assembly. After about 20 min of
driving it will lose spark on the middle coil 2-5. The coil and module have been replaced twice
with no difference and the pcm was replaced and no result either. Going to try crank sensor next
but it shouldn't be able to kill just one coil I wouldn't think.
|i have a 2003 grand prix gt with the 3800. The car was running for about 3 blocks and than it
stalled. Now there is no spark or fuel. Also the red oil light is on. When hooked to the scanner. it
said operations error.
|No, the crank sensor will not cause the coils to act like
that. Either they all fire or not. If you are SURE you are
only loosing spark on 2-5, make sure you are using a
good ignition module, check the plug wires.
|Possibly a bad crank sensor. Very common on this
|I have an older 3800 also with the one piece 3 coil assembly(type 1). My service manual
instructs me to back-probe the crank sensor four plug harness for a 6-9 volt signal across the
b-c pins then again from b to ground and then d to ground again expecting to see 6-9 volts. I've
tried two different ICM's but neither meet the voltage requirements. One is 4.5 and the other
shows 3.8, yet when taken to parts store and tested both show good. New plugs and wires,
gauged fuel pressure keyed up and running,41 lbs consistent, newer CMP, calibrated TPS,
cleaned EGR and capped off ports to check for change, Cleaned and checked IAC, swapped out
mass air with known good part to detect change yet engine will fire up and run 5-10 sec and die.
Outta ideas plz send help!
|If i remember, you need to do thee tests while touching a
wires to ground repeatedly to simulate the crank sensor
signal. Sounds like you may have a bad ICM ground. The
bracket under it get corroded so the ICM does not have
good enough ground.
|How could I adapt the buick 3.8 sfi system to work on a turbocharged air -cooled flat 6?
|Seriously? How the heck knows. Use all the Buick sensors, wiring, computer etc and good luck!
|Could be a transmission problem. They can act like an
|Hi, my 3.8 series 2 Holden misfires under acceleration in top gear. I've changed the plugs, wires,
and coils. Initially all ok but is now starting to misbehave again. Could this be the DFI? I
wondered if the TP sensor??
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GM 3.8L V6 (3800) Electronic Ignition system Description and Operation
ralph philbrook said:
Thanks for your excellent description of the ignition system. My 3800 1988 Olds "98 Regency" it has begun dying when hot, after running. It starts
again after cooling for awhile. I have replaced the coil pack, ignition module under it, and the crankshaft sensor. Now I am thinking of the computer.
How do I diagnose where the spark is failing from? Thanks.
Reply @ralph philbrook, If you are loosing spark even after replacing all these things, it does sound like the computer is bad. Just be sure you are
loosing spark and not injector pulse. Bad injectors can cause an ECM to malfunction. Test each injector with ohm meter. You should be about 12.5 -
13.5 ohms. Any lower and that inector is bad.
Mark W said:
GM 3800 LN3 Vin "C" - Recently had SES light, the DTC's req'd replacement of O2 Sensor, CMP sensor and the interupter magnet. No more SES light
but engine still runs terrible ("chuggle" / no acceleration). Since SES light is off I don't have any DTC's (or could there still be some visable via "blinky"
light test?) I'm thinking original DTC's caused PCM to go into bypass (limp mode) and that for some reason it's not going back to normal? (I've
disconnected battery and have run out the engine) Perhaps I should look for the PCM-->IC Timing Circuit signal? IF so can you tell me which circuit
(letter on ICM connector) that would be. Any other thoughts / suggestions w/b greatly appreciated.
Reply @Mark W, No, the computer is not in limp mode. Check for a plugged catalytic converter or low fuel pressure.