General Motors Reference
G015
Subject:
Distributorless (direct) ignition system (DIS) service
Symptom: Code 42 or other ignition-related problems, including no-start conditions,
engine misfire, or high HC at idle or 2500 rpm
Source:
GM service and training manuals
This reference includes the following information:
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“Computer-Controlled Coil Ignition (C3I)” on page G26
“Direct Ignition System (DIS)” on page G27
“Coil Testing” on page G29
“Spark Testing” on page G29
“Magnetic Pickup (Reluctance) Crankshaft Sensor Testing” on page G30
“Hall-Effect Crankshaft and Camshaft Sensors” on page G30
“Hall-Effect Sensor Testing and Adjustment” on page G31
“Checking Hall-effect sensors” on page G32
“Power Balance Testing” on page G36
“Broken Spark Plug Insulator” on page G37
“Spark Plug Cable Resistance” on page G37
The first of several GM distributorless, or direct, ignition systems was introduced on some
fuel-injected Buick-built V6 engines in 1984. The abbreviation DIS describes any ignition
system without a distributor. GM also uses the name “direct ignition system” specifically for
the systems used on the 2.0L and 2.5L 4-cylinder and 2.8L V6 engines. Additionally, GM uses
two other names for specific versions of DIS: computer-controlled coil ignition (C3I) and
integrated direct ignition (IDI) There are currently three general versions of DIS used.
Computer-Controlled Coil Ignition (C3I)
The C3I system is used on V-6 engines built by Buick. The basic system contains a coil pack,
an ignition module, and Hall-effect sensors to determine crankshaft position (CKP) and
camshaft position (CMP). There are three different C3I types, each of which is identified by
the shape of the coil pack.
Type 1—This version has all three coils molded into a single pack (Figure G015-1). If one coil
malfunctions, the entire coil pack must be replaced. This system is used on 1984 and later
turbocharged 3.8L V8 engines, on 1985 and later 3.0L V6 engines, and on some 1986 and
later 3.8L V6 engines with sequential fuel injection (SFI).
Type 2—This version is similar to type 1, but each of the three coils is separate and can be
replaced individually (Figure G015-1). It is used on some 1986 and later 3.8L SFI V6 engines.
Type 3—The type 3 coil pack is similar to, and can be interchanged with, a type 1 coil pack.
However, the module is electronically different and the connector plugs are not compatible
with components from other systems. It is used on 1988 and later 3300 and 3800 V6 engines.
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Figure G015-1 Type 1 and Type 2 coil packs
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NOTE:
Use the MT2700 DISkV Probe to measure peak firing voltage in any secondary ignition circuit.
You can also use the DISkV Probe to quickly check spark plug polarity.
Direct Ignition System (DIS)
Although DIS and C3I systems operate similarly, they differ in at least one regard. In DIS, a
reluctance sensor, or magnetic pickup, senses CKP. This sensor installs in the side of the
engine block (Figure G015-2). The 1993–99 3.1L engine (VIN M) and the 1996–99 3.4L
engine (VIN E) use both a reluctance sensor and a Hall-effect CKP sensor.
Figure G015-2 Direct ignition system used on 2.0 and 2.5L 4-cylinder and 2.8L V6 engines
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Integrated Direct Ignition (IDI) System
The IDI system is used on 2.3 and 2.4L DOHC engines (Figure G015-3). The coil packs and
module are in an assembly installed between the camshaft covers, and the coils connect
directly to the spark plugs. Like the direct ignition systems on 4-cylinder and 2.8L V6 engines,
the IDI system uses a magnetic pickup, or reluctance, sensor to sense CKP.
Do not mistake “normal” burn marks and carbon tracks on IDI parts for symptoms of a
problem needing to be repaired. If a burn mark appears on a coil and if wiping the debris off
with a damp cloth leaves a small black mark on the corner of the coil nearest a weld, then
consider that burn mark normal. These normal marks are produced during the assembly
process. The welding that occurs on the metal portion nearest the primary wire connection
often produces a black mark and leaves debris on the coil.
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NOTE:
GM campaigns 90C11 and 90C33 offer improved replacement coils for 1988–90 vehicles with
Quad 4 engines. The new coil has a yellow or grey electrical tab where the module harness
connects to the coil, the old coil has a black tab. The new coil also has shorter pins connecting
the spark plugs through the coil housing. An updated coil housing is also available (white).
Figure G015-3 Integrated direct ignition (IDI) system used on Quad 4 engines
In many vehicles, the IDI system can also produce carbon tracks while operating normally.
These tracks are most often found between the secondary ignition terminals on the bottom of
the coil housing. Carbon tracks of this type can be wiped off easily with a damp cloth and
cannot be felt. High voltage distributed in the coil housing cause these “normal” carbon
tracks. If carbon tracks cannot be easily wiped off, you have a problem worth investigating.
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Coil Testing
The coils or coil packs on GM direct ignitions are tested by removing them from the engine
and checking the resistance of the primary and secondary windings with an ohmmeter.
Resistance specifications are as follows:
Coil
Primary Winding
Secondary Winding
C3I type 1
0.35 to 1.5 ohms
10 to 14 Kohms
C3I type 2
0.35 to 1.5 ohms
5 to 7 Kohms
C3I type 3
0.35 to 1.5 ohms
10 to 14 Kohms
DIS and IDI
0.35 to 1.5 ohms
5 to 7 Kohms
Test resistance with the coils both warm and cold. Resistance readings should not vary by
more than 2 kilohms at high or low temperature.
Spark Testing
After checking the coil resistance, test the secondary kv firing capability of each coil. To test,
connect the coil end of one spark plug cable for a suspect coil to the terminal of an HEI spark
tester (GM part number ST125, or equivalent). Then clip the spark tester to the coil tower to
place the tester in series with both plugs for that coil (Figure G015-4). Start the engine and
look for a good strong spark at the tester. Let the engine idle for 10 minutes with the tester
connected. If the spark remains strong after 10 minutes, the coil is good.
Figure G015-4 Spark tester connection to check coil firing voltage
You can test all coils on the engine simultaneously by using a separate spark tester at each
coil. Be sure to separate or insulate the testers from each other to prevent crossfire.
ST125 is the GM part number for the HEI spark tester. Similar testers are available from other
sources under different part numbers. Be sure to use a tester that is designed for high-energy
ignitions. This tester does not have a center electrode. However, similar testers with center
electrodes are available for lower-voltage ignitions. These should not be used on direct
ignition systems.
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Magnetic Pickup (Reluctance) Crankshaft Sensor Testing
All magnetic pickup, or reluctance-type, sensors will cause a no-start condition if defective.
The CKP sensors of DIS and IDI systems can be tested by disconnecting the 2-wire
connector at the ignition module and measuring either the resistance or the ac voltage signal
with a digital volt-ohmmeter (DVOM). Specifications are as follows:
• DIS sensor—900 to 1200 ohms resistance or at least 500 mVac at normal cranking rpm
• IDI sensor—600 to 900 ohms resistance or at least 500 mVac at normal cranking rpm
Hall-Effect Crankshaft and Camshaft Sensors
The C3I systems on Buick-built V6 engines have Hall-effect CKP and CMP sensors, which
deliver a digital (high/low) square wave signal to the ignition module. Buick-built engines have
a CKP sensor on the front of the engine next to the harmonic balancer (Figure G015-5).
These provide cylinder top dead center (tdc) timing pulses and are the basis for the ignition
tach signal. The 3.8L engine with sequential fuel injection (turbo, non-turbo, and late-model
3800) have a separate CMP sensor to provide a number-1 cylinder signal for ignition and fuel
injection timing (Figure G015-5).
Figure G015-5 Crankshaft and camshaft sensors for 3.8L V6 engines
The 3.0L and 3.3L (3300) engines do not have separate CMP sensors. Instead, these
engines use a combination sensor, mounted on the front cover (Figure G015-6). Combination
sensors provide crankshaft tdc signals plus a synchronizing signal, or sync pulse. The sync
pulse indicates crankshaft cylinder position and takes the place of a CMP sensor signal for
ignition operation. These engines do not have sequential fuel injection, and the sync pulse is
not used as a CMP sensor signal for injection timing.
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Figure G015-6 Crankshaft combination sensor for 3.0 and 3.3L V-6 engines
The “fast-start” C3I system on late-model 3800 engine uses a dual CKP sensor plus a
separate CMP sensor. The outer CKP sensor has an 18-blade interrupter ring and sends an
18-pulse (18X) clock signal to the ignition module. Inner CKP sensors have an unevenly
spaced 3-blade interrupter ring that sends a 3-pulse (3X) cylinder signal to the module. Each
of the three pulses has a different duration (Figure G015-7). The ignition module compares
the variable 3X pulses to the steady 18X clock pulses to determine which coil to fire. It can
identify tdc on the compression stroke of the appropriate cylinder within 120 degrees of
crankshaft rotation. The ignition module divides the 18X signal by 6 and sends it to the ECM
for fuel control. It also buffers the CMP signal and sends the buffered signal to the ECM to
synchronize fuel injection.
Figure G015-7 Crankshaft and camshaft sensor signals for 3800 fast-start V6 engines
Hall-Effect Sensor Testing and Adjustment
A CKP sensor that is damaged because of misalignment or a bent interrupter ring causes
hesitation, sag, stumbling, or dieseling. To help identify a CKP sensor problem, scan rpm
during a road test. An erratic display indicates the ECM is not getting a proper reference
signal from the ignition module, which may be due to a bad CKP sensor. Although CKP
sensors are not adjustable for ignition timing, they must be aligned properly to prevent
damage. Early single-slot sensors are superseded by dual-slot sensors for service
replacement. See Reference G028 for adjustment instructions.
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All C3I systems need a crankshaft signal to start the engine. Therefore, a faulty CKP sensor
causes a no-start condition. If a 3.8L V6 with a C3I system loses the CMP signal while
running, it continues to run until it is shut down. With one exception, however, the engine does
not restart without a CMP signal. Late-model 3800 engines with “fast-start” systems do start if
the CMP sensor circuit is open, or if the CMP sensor is shorted. To help identify a CMP sensor
problem on these engines, disconnect the CMP sensor if the engine does not start. If it then
starts, the sensor probably is shorted.
All 1986 and later 3.8L engines set code 41 if the CMP sensor signal is lost. The 1984–85
engines, however, do not set code 41 to indicate this problem.
Checking Hall-effect sensors
Use these basic tests to check Hall-effect CKP and CMP sensors:
• Check battery (system) supply voltage to the ignition module and to the sensor. Refer to
diagrams for terminal identification and voltage levels.
• Verify that the module and the sensors all have good grounds. Refer to diagrams for
terminal identification.
• If the engine does not start, tap the crank sensor lightly with a screwdriver handle. If the
engine then starts, the sensor may have an intermittent open or short circuit.
Use the Vantage® Meter (MT2400), an oscilloscope, a digital volt-ohmmeter (DVOM), or a
light-emitting diode (LED) test lamp (Borroughs 2000-2, Thexton 125, or equivalent) to check
the sensor signal. Connect test equipment as shown in Figure G015-8. The lab scope of the
MT3000 Counselor II digital oscilloscope is ideal for this purpose. Adjust the scope for a 20millisecond (20 ms) sweep and 5.0 volts per division. The scope display for a good sensor
should be similar to the patterns previously shown in Figure G015-6 and Figure G015-7.
Figure G015-8 C3I module test points
Refer to Figure G015-9 on page G33, Figure G015-10 on page G34, Figure G015-11 on page
G34, Figure G015-12 on page G35, and Figure G015-13 on page G35 to identify sensor
circuits, ignition module terminals, and wire color codes for the following tests.
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1X and 3X Signal Tests
Check these signals with either a DVOM, LED test lamp, or a scope. Leave the sensor
connected and backprobe the correct terminals at the ignition module; then crank the engine.
• A dc voltage reading fluctuates if the sensor is working.
• An LED flashes as the signal voltage goes high and low and actually provides a better
indication than a DVOM.
18X Signal Test
This signal is too fast to check reliably with a DVOM or an LED. A scope provides the most
reliable reading. An alternative method is explained below.
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To check the 18x signal:
1. Set a DVOM on the 20 or 40-volt scale and connect the positive (+) lead to battery positive
(+) voltage.
2. Backprobe the sensor signal terminal at the module with the negative (–) DVOM lead.
3. Switch the ignition on and bump the starter; do not crank the engine steadily:
a. The DVOM reading should be high (above 9 volts) when a blade of the interrupter
ring is in the sensor slot (window closed).
b. The DVOM reading should be low (below 3 volts) when the blade is out of the slot
(window open).
It may take several tries to alternately position a blade in and out of the sensor slot and verify
the high and low signal voltage levels.
Figure G015-9 3.0L ignition and ECM circuits
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Figure G015-10 3.3L (3300) ignition and ECM circuits
Figure G015-11 3.8L non-turbo ignition and ECM circuits
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Figure G015-12 3.8L turbo ignition and ECM circuits
Figure G015-13 3.8L (3800) fast-start ignition and ECM circuits
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Power Balance Testing
Conduct a power balance test on a direct ignition system using short lengths of vacuum hose
and a 12-volt test lamp. The vacuum hose must have a high carbon content so that it
conducts secondary voltage. Install a short piece of hose between each coil terminal and its
spark plug cable or between each spark plug terminal and its cable (Figure G015-14). The
vacuum hose should be long enough to be exposed when the plug cable is connected, but
short enough that they do not touch any metal object.
Figure G015-14 Vacuum hose connection for power balance testing
Access to the coil towers on 2.0L and 2.5L 4-cylinder engines may be difficult. Install the
vacuum hoses at the spark plugs if this is the case.
With a 12-volt test lamp connected to ground, touch each vacuum hose to short the cylinder.
Note the engine speed as you short each cylinder. Then slowly move the test lamp probe
away from the hose and note the strength of the arc between the vacuum hose and the test
lamp probe. With any power balance test, engine speed decreases the least on a cylinder
with a problem. Interpret the strength of the arc between the hose and the lamp as follows:
For vacuum hoses placed at coil towers: Touch the vacuum hoses with the test lamp until you
find little or no rpm drop. Then move the probe away from the hose and note the arc. If the arc
jumps a long distance to the grounded test lamp, suspect an open circuit between the coil and
the plug. Look for an open plug cable or plug or a very wide plug gap. If the arc jumps only a
short distance, suspect a short circuit between the coil and the plug. Look for a shorted cable
or plug, or a fouled, cracked, or carbon tracked spark plug (Figure G015-15).
Figure G015-15 Spark plug insulator damage
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For vacuum hoses placed at spark plugs: Touch the hoses with the test lamp until there is little
or no rpm drop. Then move the probe away from the hose and note the arc. If the arc jumps a
long distance to the test lamp, suspect an open plug or a very wide plug gap. If the arc jumps
only a short distance, suspect a shorted or fouled plug, or a nearly open spark plug cable. If
there is no arc, suspect an open circuit in the plug cable or the coil tower.
If the strength of the arc on a cylinder with little rpm drop appears to be the same as the arc on
cylinders with a normal rpm drop, suspect a fuel or compression problem.
Broken Spark Plug Insulator
The ceramic spark plug insulator may break near the base where it is hard to see, figure 15.
This causes a misfire. You may need to gently tap on the spark plug. If the insulator slides
down, it masks the spark, causing a misfire. Check resistance between the terminal and the
center electrode. Resistance should be 6000 ohms or less.
Spark Plug Cable Resistance
Resistance of all GM distributorless ignition cables should be within the following ranges:
Cable Length
Resistance
16 inches (400 mm) or less
2 to 9 Kohms
16 to 24 inches (400 to 600 mm)
3 to 13 Kohms
24 to 36 inches (600 to 900 mm)
4 to 17 Kohms
more than 36 inches (900 mm)
5 to 20 K ohms
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