In 1908, the inductive-discharge battery ignition was invented by Charles Kettering and, since then, it has been practically the only spark ignition system used on automobiles. For over six decades, the ignition system used breaker-points to open and close the low voltage primary circuit. In the mid-1970s, the breaker-points were replaced by electronic ignition. The electronic ignition of that era still used a distributor, but the distributor cam and points were replaced by a signal generator and a transistorized solid-state module.The next technological advance came in the form of the Distributorless or Direct Ignition System (DIS), a term no longer standard in the industry. According to SAE J-1930, which was implemented in 1993, DIS is referred to as Electronic Ignition (EI). This enhanced EI system began to replace the electronic distributor system, a.k.a. High Energy Ignition (HEI), on GM vehicles in 1984. Replacing the antiquated breaker-point triggered ignition systems with a transistorized system yielded some advantages, but the EI of today provides even more.The modern EI uses even fewer moving parts which allows more compact/remote mounting capabilities. This wasn't possible with distributor-type ignition (DI). But the biggest advantage EI provides is extended coil life through the use of multiple coils. Utilizing multiple coils allows for better coil saturation and longer cool-down times between firings for each individual coil.The longer times between firing is made possible because EI coils fire only once per engine revolution. Whereas a single conventional coil on a DI system fires once for each cylinder per distributor revolution. For example, a V8 engine coil would fire eight times per distributor revolution which translates to 12,000 times per minute at 3000 rpm (3000/2 * 8). Whereas an EI coil on the same V8 would only fire 3000 times per minute at 3000 rpm.EI also led the way toward longer maintenance intervals and eliminated the need for periodic ignition timing adjustments. EI systems are no longer sensitive to timing changes caused by engine wear because the signal generator was moved from the distributor shaft to the crankshaft. Since the sensor reads the engine speed directly from the crankshaft, timing changes caused by timing chain or belt stretch are no longer a factor.Types of EIThere are two types of EI systems: Individual-Coil which uses one coil for each cylinder and Waste-Spark which uses one coil for each pair of cylinders.On Individual-Coil systems, the coil secondary terminal is mounted directly to each spark plug terminal. Each coil has a wiring harness connector attached to two primary circuit terminals. The ignition module or PCM switches each primary circuit separately to discharge and saturate the coil. These ignition systems are primarily used on a few import vehicles, such as those manufactured by Saab.The other system, Waste-Spark, is the most common type of direct ignition and it is used by many import and domestic manufacturers. Waste-Spark is the most common system used because it has proven its reliability in marine and motorcycle applications prior to being used in an automotive application.GM uses four variations of the Waste-Spark system on their engines. The four types are: Computer Controlled Coil Ignition (C3I), Direct Ignition System (DIS), Fast Start, and Integrated Direct Ignition (IDI). These systems have some minor differences, but the system's overall construction is the same. Each system has each end of the coil's secondary winding attached to a spark plug. The system fires both spark plugs as a pair whenever the pistons are at TDC in the corresponding cylinders. However, only one of the two cylinders is at TDC on the compression stroke. The other cylinder is at TDC on the exhaust stroke, so the spark is 'wasted'hence the name Waste-Spark.Waste-spark in a nutshellIn order to understand the Waste-Spark theory, you have to understand a basic electrical circuit. Electricity always travels in a circuit. It leaves its source, performs its job, and returns to its point of origin. The coil secondary winding is the source of the spark for the spark plugs. Therefore, the spark has to return there in order for the circuit to be complete.Since the polarity of the coil's windings is fixed, one spark plug always fires in the forward direction (from the center to the outer electrode) and the other plug always fires backwards or in reverse polarity (from the outer to the center electrode). In order for this to work, the two spark plugs for each coil are connected in a series circuit. The spark travels from the coil through one spark plug wire to the plug. The remaining energy travels through the engine block on V-type engines or through the cylinder head on inline engines and fires another spark, backwards, at the mating cylinder's spark plug. Then, the circuit is completed by the energy returning to the coil via the other spark plug wire. In essence, the second spark plug wire in an EI system replaces the coil's secondary ground lead on a DI system.More powerAs you can imagine it takes considerably more voltage (up to 30% more) to fire a spark plug backwards. This doesn't even include the fact that two plugs are being fired simultaneously from one coil. So where is the additional power coming from?The additional voltage is provided via low resistance coils which allow higher primary current. EI systems are capable of outputting more voltage than a conventional DI system. For example, GM EI systems can output 40+kv.As for firing multiple plugs, the waste-spark theory works because the plug that fires on the compression stroke uses 85-90% of the coil's supplied energy while the plug firing on the exhaust stroke only uses 10-15% of the supplied energy.The exhaust stroke doesn't require much voltage to fire the plug because there is no compression in the cylinder to increase resistance. A typical GM coil is capable of firing a spark across a gap of up to one inch outside the engine, but factor in compression and the pressures of an internal combustion engine, and the distance is reduced to a fraction of an inch.Motor Age diagnostic tipsWhen checking spark plug condition on an engine equipped with EI, pull a plug from each bank on a V-engine or from opposing cylinders on an inline engine. This is necessary to assess the true condition of the plugs. Forward firing plugs (normal polarity) will wear at the center electrode, but backward firing (reverse polarity) plugs will wear at the outer electrode.The wear occurs differently depending on how the spark plug fires because when the electricity jumps a gap, it carries metal particles with it. Thus, the point of origin erodes and the receiving point accumulates the eroded particles.Also, GM coils are generally labeled with the numbers of the cylinders they control. But, if you are ever unsure of which cylinders are paired, rest easy because there is a simple formula that determines the paired cylinders on any vehicle equipped with EI. All you need is the vehicle's firing order.For example, let's look at a GM 3.1L V6 engine. The firing order for the 3.1L engine is: 1-2-3-4-5-6. In order to determine the paired cylinders, divide the firing order in half and place the second group of cylinders under the first group. In this case, the result is: 1-2-3/4-5-6. If you take a number in the first group and look at the number directly beneath it in the second group, you have your paired cylinders (i.e. 1&4, 2&5, and 3&6).Primary current controlWe said primary current is boosted via low resistance ignition coils on an EI system in order to increase secondary output, but the primary circuit current needs to be carefully controlled in order to avoid damaging system components.The primary coil current is controlled by transistors in the ignition control module (ICM). An EI system is equipped with one transistor for each pair of coils that it uses. For example, a V6 engine uses three coils so it would have three transistors. These transistors control the primary current by completing the path to ground. The ICM uses a combination of internal control circuits and external triggering devices such as Hall effect or variable reluctance sensors to control the timing and sequencing of these transistors.The ICM's internal control circuit is responsible for supplying the primary current to the coil's primary windings and also controlling the dwell time. Resistance on an EI coil's primary winding is approximately 1 ohm. If the available system supply voltage (approx. 14V) was applied to the primary windings, a maximum current flow of 14 amps would be possible. Thus, you can see how the high amperage and low resistance enables an EI coil to be fully charged in a minimum amount of time. However, 14 amps is too much current for the system to handle. Therefore, the ICM uses current limiting and dwell control to keep the primary current no more than between 8.5 and 10 amps in order to prevent damage to the system's components.Current limiting is achieved as the ICM monitors the primary circuit current and modifies the base current to the primary control transistor whenever the primary current exceeds the desired range. As a result, the controlling transistor is turned off and the collector emitter current is limited to the desired range. Remember, when a transistor is used as a switch and is turned off, the resistance between the collector and emitter increases. This reduces the amount of amperage in the circuit.The ICM controls the dwell by monitoring the coil's saturation and determines whether maximum current was achieved or not. If maximum current was obtained, the dwell time is reduced limiting the amount of coil saturation time. Likewise if maximum current was not obtained, the ICM lengthens the dwell time to allow a longer saturation time for the ignition coil. In other words, if current limiting occurs prior to coil discharge, dwell will be shortened for the next firing sequence. This process is known as a closed loop dwell system.TriggeringTriggering refers to the type of signal generator used to notify the ICM of the crankshaft and camshaft position. The ICM uses inputs from these sensors to determine when to fire each coil pack and deliver the fuel on sequentially fuel injected engines.The way each GM EI system processes these signals will be discussed later in this article. For now, these signal generators can be separated into two categories: Hall effect and variable reluctance (a.k.a. magnetic sensor).GM vehicles use either a Hall effect or a variable reluctance sensor for the crankshaft sensor. However, there are some exceptions, such as the 1994-present 3.1L/3.4L engines which use both types of sensors for determining crankshaft position. As for the camshaft sensor, all engines, except for the 4.6L Northstar and LS1, use Hall effect type of sensors. The 4.6L Northstar and LS1 engines use a variable reluctance type camshaft position sensor.Though the two types of sensors operate differently, they are easily identifiable. The variable reluctance or magnetic type sensors always produce an analog (AC) signal. On the other hand, Hall effect type of sensors always produce an on/off (square wave) digital (DC) signal. Also, variable reluctance sensors use 2-wire harness connectors and Hall effect sensors utilize 3- or 4-wire harness connectors.Computer Controlled Coil IgnitionI was the first EI system used on GM vehicles and it is the foundation for all of GM's EI systems. C3I first appeared in 1984 on the Buick 3.8L turbocharged engine. The system was later used on the 3.0L, 3300, and the 3.8L (naturally aspirated) engines. The C3I system is comprised of three coil packs, an ICM, and a variety of Hall effect sensors.The coil and ICM can either be a type 1 or a type 2 assembly which can easily be identified. Type 1 coils have three secondary terminals on each side and all three coils are molded into one housing. The coils are not individually replaceable. The type 1 ICM has four wires exiting at the top. One wire is connected to the common power supply for all three coils and the other three wires are individually connected to the ground side of each coil. Whereas type 2 coils have all six secondary terminals on one side. Also, type 2 coils can be individually removed from the ICM and replaced. The type 2 ICM has no wires connecting to the coils; each coil plugs into the ICM via two male terminals.Despite the different types of coils and modules, they function similarly. The C3I ICM fires the coils in the proper sequence based on inputs from a crankshaft position (CKP) and camshaft position (CMP) sensor. The type of sensors used and the way the signals are delivered to the ICM depends on whether the engine is turbocharged and/or whether it uses multi-port (MFI) or sequential fuel injection (SFI). In addition to receiving the CKP and CMP sensor inputs, the ICM buffers the signals and sends them to the PCM to be used for engine speed reference and for spark and fuel control. The ICM also supplies each sensor with its power and ground as well. The ICM generally supplies 10-12V power to the CMP and CKP sensors, but some ICMs contain built-in voltage regulators which supply a reduced voltage, such as 7-9V, to the sensors. Therefore, refer to the vehicle's service manual for exact sensor specifications.Furthermore, C3I systems don't utilize a closed loop dwell system. These systems use a variable dwell control which functions similarly to a DI type system. The dwell on this system is held low at low engine speeds and increases as engine speed increases. The ICM will always supply a little more dwell than is actually needed so the ICM relies on current limiting to prevent the coils from overheating.I on MFI enginesThis type of fuel injection doesn't require a separate CMP sensor. Therefore, a dual Hall effect sensor, a.k.a. dual CKP sensor, is used. The dual CKP sensor, located on the front timing chain cover, combines the CMP and the CKP signals into one sensor. This type of sensor can be easily identified by its 4-wire connector.The dual CKP sensor provides three on/off pulses (3x signal) and one on/off pulse (1x signal), a.k.a. sync pulse, on the reference lines to the ICM per engine revolution. These pulses are created by two interrupter rings as they pass through the dual CKP sensor. The rings are mounted on the inside of the harmonic balancer. The outermost ring contains one window and one vane and it provides the 1x signal for the ICM. The innermost ring contains three windows and vanes which provide the 3x signal for the ICM.The 3x signal pulses are identical in both amplitude and time. Thus, the ICM can't distinguish which pulse to assign to each coil. Therefore, the 1x signal is used during cranking to set up the coil sequencing by being timed with the portion of the 3x signal which represents the second set of companion cylinders in the engine's firing order. All the engines using C3I systems fire the coil pairing cyls. 6 and 3 together first, because the firing order on these engines is 1-6-5-4-3-2. The ICM also has to receive the 1x signal before it starts sending the fuel injector pulse to the PCM. Once the ICM sees the 1x signal which occurs when cylinders 1 and 4 reach 25š ATDC, it knows the crankshaft's position. It will fire the coils upon receipt of the next 3x signal which corresponds with cylinders 6 and 3. Thus, the crankshaftdepending on its position at shutdownmay have to rotate up to two times before the first spark is delivered.Once the setup is complete, the ICM will follow the designated engine firing order and will also remember the coil sequencing for as long as the engine is running. Therefore, if the 1x signal is lost while the engine is running, it will stay running, but it won't restart after it is shut off. However, if the engine speed reference (3x) signal is lost, the engine will immediately stall and fail to restart due to lack of spark.The PCM doesn't monitor the 1x and 3x signal inputs to the ICM, but it does receive a buffered 3x signal from the ICM to determine engine speed. However, there are no diagnostic trouble codes (DTC) which indicate if either signal is lost.I on SFI enginesThe function of the C3I system on these engines is essentially the same as the MFI engine, except for a few differences involving the sensors. The ICM still uses a 3x signal for crankshaft positioning, but the 1x signal is now supplied by a separate CMP sensor. The separate CMP sensor is still supplied with its power and ground via the ICM. Also, since these engines use a separate CMP sensor, a single CKP sensor is used. The single CKP sensor can be easily identified by its 3-wire connector. The single CKP sensor operates in the same manner as the dual CKP sensor, except there is only one interrupter ring on the harmonic balancer.The CMP sensor is a 3-wire Hall effect type, but it can be located in two different areas. On all engines, except the Buick 3.8L turbo, the CMP sensor is located on top of the front timing cover across from the camshaft timing gear.This type of sensor is unique because it doesn't have a magnet built into it. Therefore, it doesn't utilize a vane to turn on and off. The Hall effect switch is on the inside of the timing cover while the harness connector is on the outside. An O-ring is used around the sensor to prevent oil leakage. The Hall magnet is mounted on the camshaft gear which passes the Hall effect switch once per revolution. When the magnet on the cam sprocket passes the switch, it turns on; after the magnet passes the switch, it turns off.The CMP sensor on the Buick 3.8L turbo engine is also a 3-wire Hall effect sensor, except it is encased in a distributor-type housing and mounted on top of the timing cover like a distributor. This type of CMP sensor is adjustable and unless adjusted properly will compromise the fuel injection delivery timing. The camshaft drives a shaft which has a round steel ring with one window in it. The Hall effect switch which is mounted inside turns on and off as the window and vane rotate through it. This type of sensor was last used on the intercooled Buick Regal in the mid-'80s.Regardless of the type of CMP sensor used, it will generate one pulse per two engine revolutions. The CMP sensor pulse indicates when cylinder No. 1 is at TDC on the compression stroke. Cylinder No. 1's position is unimportant to the ICM since it fires the coil every time a cylinder comes to TDC. But, cylinder No. 1's position is important to the PCM because it needs to time the fuel injection with the opening of the intake valves.After the initial pulse to set up the coils, the CMP sensor signal is used by the PCM for only sequencing the fuel injection timing. Thus, the engine will still run if the CMP sensor signal is lost while the engine is running, but it won't start after it is shut down. Also, the PCM is unable to control the sequential injection while the engine is running without a 1x signal. Therefore, the ICM/PCM will revert to pulsing the injectors like a multi-port engine.Additionally, the PCM can't sense if the CMP or CKP sensor signals are reaching the ICM, but a code 41 will set if the CMP signal is not received by the PCM when the engine is running.Fast EnginesThe Fast Start ignition system is an enhanced version of C3I. It was introduced in 1988 and is used exclusively on the 3800 series engine. This system is different from C3I in several ways. Some differences include the use of a separate camshaft position (CMP) sensor in conjunction with a dual CKP sensor which allows improved measurements of crankshaft position and sequentially timed fuel injection. Also, the ICM electronics on this system are smarter than the electronics utilized in the conventional C3I system.The Fast Start ICM performs all the same functions as the conventional C3I ICM, but it includes some additional functions as well. One function in particular is 'rock back detection' (type 2 modules only). Rock back detection prevents the coils from losing their sequence if the starter is engaged at random or disengaged before the engine starts. This is important because the engine may turn backwards from compression when the starter is disengaged prematurely. Rock back could also be caused by low battery voltage. The low current generated by a low battery would be insufficient to overcome the engine's compression, which may allow the crankshaft to spin backwards. If the ICM maintains its original coil sequence and is unable to notice that the crankshaft rotated backwards during engine cranking, the coil may fire a cylinder while the intake valve is open. This would cause a backfire into the intake manifold which could spell instant 'death' for a MAF sensor or PCV valve.Now, let's take a look at the sensors. The dual CKP sensor used on this system is virtually the same as the dual CKP sensor used on the MFI engines equipped with C3I systems. The only difference is the design of the interrupter rings on the harmonic balancer and the type of signals that are generated. The outer interrupter ring on this system has 18 evenly spaced vanes and windows which produce an 18x signal as opposed to the 1x signal found on the conventional C3I systems with dual CKP sensors. Also, the inside interrupter ring on the harmonic balancer still has three windows and vanes which produce a 3x signal, but these windows and vanes are not uniform in size. The windows represent 10, 20, and 30 degrees of crankshaft revolution. Likewise, the vanes between the windows represent 90, 100, and 110 degrees of crankshaft revolution.With this type of setup, the ICM can identify any pair of cylinders within 1/3 (120š) of a crankshaft revolution. Moreover, the ICM is capable of firing any pair of cylinders that reach TDC first and also deliver an injector pulse without waiting for the sync or CMP signalhence the name Fast Start.The trailing edge of one 3x window (10š, 20š, and 30š) to another on the interrupter ring is 120š apart. For example, the crankshaft will have rotated 120š from the end of the 10š window to the end of the 20š window. The ICM is programmed to know that the Nos. 1&4 cylinder pair reach TDC 75š after the 10š window, the No. 3&6 cylinder pair reach TDC 75š after the 20š window, and the No. 2&5 cylinder pair reach TDC 75š after the 30š window.The ICM is able to keep track of each cylinder's position by using the 18x signal as a clock pulse for the 3x signal. For example, the ICM sees one 18x pulse during the 10š window, two 18x pulses during the 20š window, and three 18x pulses during the 30š windowhence each 18x pulse is equal to 10š of crankshaft revolution. By counting the number of clock or 18x pulses, the ICM is able to verify which 3x pulse it is reading and energize the appropriate coil.The ICM uses the 3x input from the CKP sensor strictly as a sync signal for crankshaft position. The 18x signal, in addition to verifying the 3x pulse for the
ICM, is buffered by the ICM and sent to PCM to serve as an engine spark timing reference (a.k.a. 18x high resolution) during cranking and low rpm (below 1200). Furthermore, the ICM takes the same 18x signal and divides it by six to arrive at a 3x signal for the
PCM. The estimated 3x signal (a.k.a. divided signal or 3x output) is then sent from the ICM to the PCM via a different circuit to provide an engine speed reference signal and the pulses for fuel injector control. The 3x signal is also used by the PCM to control spark timing when the engine speed is above 1200 rpm. The ICM also receives and buffers the CMP sensor signal before sending it to the PCM for fuel synchronization control.The vehicle will not start unless the ICM sees both the 18x and the 3x signals during cranking. Both signals must be present or the ICM will not deliver the fuel injector pulse to the
PCM. The 3x signal has the same effect on engine operation as the 1x signal does on conventional C3I systems. If the 3x signal is lost while the engine is running, the engine will continue to run until it is shut off. This occurs because once the ICM is synchronized, it derives the 3x signal from the 18x signal. But, if the 18x signal is lost, so is spark. Additionally, this system has no provisions for codes if the ICM loses the 3x or 18x input signals.However, the PCM is capable of generating codes if the ICM output signal to the PCM is lost. For example, if the 3x or 18x output signals are lost on some 1995 models and all 1996 and newer vehicles equipped with OBD II, the PCM will store code P0336 (18x) and P1374 (3x). OBD I equipped vehicles on the other hand, are only capable of storing codes for a missing spark reference (18x) output to the
PCM. The missing ICM output signals to the PCM on fully compliant OBD II vehicles, unlike the missing ICM inputs from the CKP sensor, will not inhibit the vehicle's ability to start and run. The PCM's logic is capable of operating without a particular input, but the vehicle's performance may be diminished. However, the PCM can't compensate if both the 3x and the 18x signal inputs are lost. If both ICM outputs are lost, the PCM would fail to pulse the injectors.The only other sensor used on the Fast Start system is the CMP sensor and it is identical to the CMP sensor (non-distributor type) used on the C3I systems on engines with
SFI. The CMP sensor signal on the Fast Start system, however, is used by the PCM solely for the purpose of sequencing the fuel injection timing. The engine will still run and start if the CMP sensor signal is lost, but the CMP fail-safe operation of this system is different from the C3I system used on SFI engines. If the CMP sensor signal is lost, the PCM will not revert to pulsing the injectors like a multi-port engine. Instead, the PCM logic will attempt to sequence the injectors based on the 3x signal after two engine revolutions (6 pulses). Therefore, there is a one-in-six chance that the fuel injection pulse will coincide correctly with the valve timing. The estimated fuel injection sequence will allow the vehicle to start and run, but its performance will be reduced (i.e. hesitation).It compensates for a lost CMP sensor differently on this system because of the engine's ability to start within 1/3 of a crankshaft revolution. Since this system can fire the first pair of cylinders to reach
TDC, the ICM sends an injector pulse to the PCM as soon as it receives the 3x and 18x signals. The PCM initially pulses all six injectors for what is known as the priming pulse. The PCM then waits for two engine revolutions (six 3x pulses) before delivering any more fuel. This allows the initial priming pulse to burn off and also allows the PCM to receive the first CMP sensor pulse and begin the sequential fuel injection on the third engine revolution.Furthermore, the PCM can't sense if the CMP sensor signal is reaching the ICM, but a code will set if the CMP output signal is not received by the
PCM. Additionally, vehicles equipped with OBD II will disable the misfire diagnostic routine in the PCM whenever DTC P0341
(CMP sensor circuit) is present.Misfire DiagnosisNow that we have an understanding of how the GM C3I systems work, we can diagnose some problems. If you were paying attention to how a Waste-Spark EI system is wired, you'll realize that one open plug wire could cause a misfire on two cylinders.In order to properly diagnose an engine misfire, a cylinder balance test is needed to determine which
cylinder(s) is causing the problem. However, before performing a cylinder balance test, check the OBD system for any codes. This is important because a malfunctioning component, such as a vehicle speed sensor or CKP sensor in the engine control system can cause a misfire. If any DTCs are present, always repair the affected circuits before proceeding with a misfire diagnosis. Also, most vehicles equipped with fully compliant OBD II systems have provisions for detecting cylinder misfires. They can keep a cylinder misfire history in the PCM's memory.A scan tool can be used to view the cylinder misfire history counts. The counts can help you determine whether the misfire is isolated to a cylinder, cylinder pair, or if it is random. But, regardless of the vehicle's capabilities, it is always best to perform a cylinder balance test to ensure that you have isolated the misfiring
cylinder(s). Remember to clear the PCM's memory after you are finished with the vehicle because your cylinder balance test may have set a code or caused the PCM's historical data to be inaccurate.Once you've isolated a misfiring cylinder or pair of cylinders, check for an injector pulse at the injector harness connectors and verify that there is spark at the plug by using a spark tester. If spark or the fuel injector pulse is missing, continue diagnosis with the appropriate system. But, if both spark and fuel are present, perform an injector balance test to verify that fuel delivery is adequate. Dirty and clogged fuel injectors are common causes of misfires on port-injected vehicles.Good spark and no injector pulseWell, this situation looks worse than it probably is. If the engine has good spark, you know that the CKP and CMP
(SFI only) sensors are working. This indicates that the problem resides in one of three areas: the power circuit to the injectors, the
PCM, or the ICM.Start with the easiest item and verify that there is power to the fuel injectors. If there is no power, it is likely that the injector fuse is blown. If the fuse is blown, don't just install another fuse and send the vehicle on its way; check the circuit for a short. Otherwise, the vehicle may be chauffeured back to you via tow truck in the near future. But if a thorough circuit check reveals no shorts, it's possible that the fuse was fatigued and failed because of repeated thermal cycling.Next, verify that the CKP and CMP signals are being received at the PCM. Remember the ICM controls the coil sequencing based on the inputs from the CKP and CMP sensors, but the PCM controls the injector pulses based on the 1x (C3I) or 3x (Fast Start) inputs from the
ICM. If the PCM doesn't receive an initial 1x signal on a C3I system, it won't pulse the injectors. Likewise if the PCM stops receiving the 3x (and 18x on '95 and newer OBD II systems) signal on a Fast Start system, the fuel injector pulses will cease as well.Finally, check the PCM memory for any codes and repair the affected circuits. Then, recheck for an injector pulse. If the PCM sends a no data message to the scan tool, it is most likely that the PCM isn't receiving power or it has a bad ground. Check all the PCM's power and ground circuits; if the circuits are good, the PCM is probably faulty.Motor Age Diagnostic TipIf there is spark and no injector pulse, you can verify the circuit's integrity by substituting a reference pulse to the PCM over the fuel control circuit (usually a purple wire w/white tracer). Disconnect the injector harness, connect a noid light, and then disconnect the harness connector at the
ICM. Now, connect a test light to the positive battery post. With the ignition switch ON, briefly touch the test light to the fuel control circuit terminal on the ICM harness, then remove it. If the noid light flashes when the test light is connected and removed, the ICM is defective. But if the noid light doesn't flash, the problem is not ignition-related. You will find that the problem is within the wiring between the PCM and the ICM or the fuel injectors, the injectors aren't receiving power, or the PCM is defective.If the PCM is receiving the signal, see if the fuel pump relay is energized. The fuel pump relay should be energized whenever the PCM is receiving reference pulses. If the relay isn't energized and its wiring is OK, the PCM is not processing the signals.No spark from one coilIf your testing reveals that a coil for a pair of companion cylinders isn't firing, testing is simple. On type 1 coils, remove the coil from the ICM and check the power feed to the coil. The type 1 coils use a single power source to all three coils. The power feed can be tested by connecting a test light to the power feed of the coil in question while cranking the engine. If the test light flashes, the coil is defective. If the test light doesn't flash, the ICM is defective.Type 2 coils can also be tested with a circuit test light as mentioned above, except connect the test light across the coil's terminals on the
ICM. An alternative method is to remove and swap the type 2 coils on the ICM since the coils can be removed individually. If the problem follows the coil, the coil is defective. Otherwise, replace the
ICM.But if you are still skeptical and you want more conclusive tests, check the coil's resistance. The primary resistance should be .35 -
1.50W and the secondary resistance should be 10 -
14kW on type 1 coils or
5-7kW on type 2 coils.Well, that wraps up the theory of GM EI and covers two of their four systems. The specifications, codes, and wiring colors cover the majority of the GM vehicle line. However, it is best to refer to the appropriate service material whenever servicing these vehicles. Join us in an upcoming issue for Part II.Motor Age thanks Fred Mackerodt, Inc. in Montvale, NJ; Roberts Auto Mall and
H.G. Motorcar Corp. in Downingtown, PA; and General Motors Corp. for supplying vehicles and parts for this article.
