4-Pin GM HEI Module Notes by Lou Dudzik 8/14/05 update 5/4/09 These notes are specifically for a Wells model #DR100 4-pin GM HEI module. Manufactured by Wells Manufacturing Corp., in Fond du Lac, Wisconsin 54935. It is supplied with thermal transfer gel for the heatsink. The Wells model is no longer available locally in Chicago. A Niehoff (Borg-Warner) model DR400CS appears to be the same unit as the Wells model. In fact, the box and instructions have "DR100" in fine print. As of 2008, it appears the Niehoff DR400CS and the identical BWD model CBE4P are now made in Hong Kong by the same manufacturer. The modules differ from the Wells module in that they have a small metal tab protruding from the side of the body. This tab appears to be an additional ground connection. GM HEI modules use a control chip made by Motorola. It is the MC3334, MCC3334, or MCCF3334 chip. The GM HEI module has four blade terminals and a heatsink. The heatsink acts as the ground connection for the module. It has two mounting holes through it, which are sleeved with metal. These sleeves contact the heatsink when a bolt is used to mount the module. The sleeves are not grounded, but become grounded when a bolt is pressing the heatsink against the sleeve. The mounting hole sleeve may not necessarily provide a good ground. However, some Hong Kong manufactured modules may actually tie the mounting hole to ground. With no input signal, and no current in the spark coil, the HEI module uses 140mA at 13v. That's 1.8 watts. The four terminals are labeled, "B", "C", "G", and "W". B is connected to the supply voltage. Normally this will be from about 10 to 15 volts DC. C is the ignition coil connection. It connects to the negative side of the ignition coil's primary winding and turns the coil "on" and "off" by grounding the negative side of the ignition coil. When "off", C can stop an inductive spike of about 600 volts. When the coil is "on", the voltage drop on C is about 1 volt, but increases slightly as current increases. Current limiting is employed in the coil driver circuit. Terminal C will sink up to a maximum of about 5.5 to 5.8 amps, then will hold at that level (until the signal to turn off the ignition coil is received). If the DC resistance of the ignition-coil primary is so high that a current of 5.5 amps is never reached, the current limit feature does not get used. A 3-ohm ignition coil won't use the current limiter in typical applications. A 2.4-ohm coil probably will use the current limiter a little bit. If the current limiter is in effect, the voltage drop on C may be much higher than 1 volt. The voltage drop on C is constant during current-limit mode. When the current limit mode activates, the voltage on C jumps from 1 volt to the necessary voltage to prevent any further increase in current, and stays there until the dwell period ends. (This may cause dwell meters and electronic tachs to give errant readings because they use the negative side of the coil to get their signal.) G is the connection for the input signal to the module. It is connected to the positive output of the reluctor pickup-coil. The pickup's output voltage controls the GM HEI module. When the voltage is above a certain threshold (turn-on threshold), terminal C is grounded (to about 1 volt) and the ignition coil is then "on". When the input voltage level drops below another threshold (turn-off threshold, which is lower than the turn-on threshold), terminal C is then open and the ignition coil is then "off" and a spark occurs. At this time, C must withstand a voltage spike of several hundred volts. The threshold voltages vary slightly with different supply voltages. To give a rough guideline here are some approximate threshold values. These values may be different from brand to brand or module to module depending on tolerances: Supply voltage: 10v ~ 11v Coil "on" when G > 1.65v Coil "off" when G < 1.46v ~1.48v Supply voltage: 12v ~ 13v Coil "on" when G > 1.64v Coil "off" when G < 1.43v ~1.44v Supply voltage: 13v ~ 14v Coil "on" when G > 1.65v Coil "off" when G < 1.43v Supply voltage: 14v ~ 15v Coil "on" when G > 1.63v Coil "off" when G < 1.37v If the input signal holds somewhere between the two thresholds, (this input will be referred to as "ambiguous"), the output state won't change until the appropriate threshold is crossed. If the module is powered up while the input signal voltage is ambiguous, the output may end up in either one of the two states (ignition coil on, or ignition coil off). Usually, it ends up in the coil-off state. The pickup reluctor is essentially a small AC generator. It provides the voltage signal to turn the coil on then off. The positive portion of the signal turns the coil on, and the negative portion of the signal turns the coil off. The reluctor produces a signal-amplitude, which varies according to the RPM of the motor. At low-RPMs, the signal is small, especially during start-up. This results in a very small amount of time during which the coil is "on". This is referred to as the dwell-time. If the dwell time is too small, a spark won't be generated. In order to increase the dwell time, a DC voltage can be applied to the negative side of the reluctor. This boosts the positive output level of the reluctor, which increases the dwell time. This DC voltage will be referred to as the bias voltage. It can simply be a constant DC voltage or it can be a varying DC voltage. W is normally the terminal providing the bias voltage to the negative side of the pickup-coil. The bias voltage output at W, (when there is no pickup-induced signal), is just slightly below the turn-off threshold. The initial value is about 1.25 volts DC. This is to ensure there is no chance of the input G terminal's voltage being ambiguous when there is no pickup-induced signal present. (A voltage between 1.43 and 1.65 would be ambiguous.) W's bias voltage is not constant. In a normal application, the DC voltage at W will increase with RPM. This is to extend the dwell angle at higher RPMs to provide adequate dwell time for multi-cylinder engines, which use a conventional distributor. W is driven by a buffer connected to a dwell-capacitor. The dwell capacitor gets charged by the signal on the G terminal through a resistor and a diode. This allows the capacitor to get charged by the positive pulses on the G terminal, but does not allow the capacitor to drain through the G terminal. In this manner, the dwell capacitor accumulates a charge. The capacitor slowly drains on its own, but if the signals from the reluctor increase in size and frequency, the capacitor eventually charges faster than it can drain. This causes the voltage on the capacitor to increase when the RPMs go above a certain threshold. The threshold would be determined by the shape and amplitude of the reluctor pulses. Above the threshold, the voltage will increase with RPM. The voltage on the dwell capacitor is detected by a buffer driving the output of the W terminal. The buffer is just a voltage-follower following the voltage on the dwell capacitor. This means the W terminal's output voltage matches the dwell capacitor's voltage. The W terminal's output voltage increases above a certain RPM. As the RPMs increase above the threshold RPM, the W terminal's voltage increases. This, in turn, causes the voltage to the G terminal to increase. This can be considered a slow-acting, positive, DC feedback. It takes about 10 msec for the capacitor to increase from 1.3v to its maximum of 6v if the G terminal is connected to a steady DC voltage (higher than 6v). On its own, the capacitor takes about 150 msec to drain back down to 1.3v. Because of the positive DC feedback, the DC voltage on W can grow higher and higher if left unchecked, as long as there is a significant signal from the reluctor. Without a signal, the feedback loop decays and W ends up back at 1.2 volts. The limit to how high the voltage on W can get seems to be about 6 or 7 volts, but may possibly be higher. If the voltage gets too high (and the reluctor signal can't go negative enough), the dwell may eventually become infinite. In that case, no more sparks will occur, the engine will stumble until the capacitor drains low enough for the input signal to once again cross below the turn-off threshold allowing sparks to resume. The engine may backfire and resume running at this point, or may stall completely. To prevent the feedback loop from increasing unchecked, the HEI module uses the current-limiter to control a circuit, which actively discharges the dwell capacitor at a constant rate (faster than the capacitor drains on its own) while the current limit is activated. The activation of the current limiter implies the dwell was too long on that cycle, so the bias voltage to the reluctor must be lowered. This is done when the current limit circuit drains the dwell capacitor slightly. The capacitor is not necessarily drained all the way on each cycle. It may only get drained slightly if the current limit is only briefly activated. If the capacitor is drained all the way, it is back to its starting level of about 1.25 volts. Normally, the average DC voltage on the W terminal won't drop below 1.2 volts. In a typical car application, ideally, a balance is achieved such that the current limit (approx. 5.5 amps) just starts to activate on each cycle. This ensures the ignition coil has a full charge for each spark, while preventing unnecessarily long dwell times on the coil. This keeps the coil cool, and the ignition system efficient. Normally, W will get high enough such that the ignition coil is almost constantly "on" at high RPMs. It only turns "off" for the duration of a spark. It stays off for about 1ms before it turns back on. It should be noted that the positive DC feedback doesn't affect the amplitude of the signal from the reluctor. The reluctor's output voltage, as measured from W to G stays the same whether the W terminal is allowed to float or is fixed to some steady DC value. However, the signal at the G terminal relative to ground will indeed be affected. The positive DC feedback raises the DC level of the signal at G relative to ground. (The threshold values for coil "on" and "off" are relative to ground.) If a reluctor of different design is used, the dwell may end up longer or shorter than the ideal time. The HEI reluctor rotor is comprised of a wheel with short-duration spikes on it. If a rotor is used with long-duration ramps before the spikes, the bias voltage on W may increase too much, creating the aforementioned stumble at higher RPMs. Also, if a different ignition coil is used with significantly higher DC resistance, the current limiter may not activate enough (if at all) to reduce the dwell. The HEI coil has about .5 ohms in the primary winding. A coil of 3 ohms or higher probably will not activate the current limiter. This will also result in the aforementioned stumble at higher RPMs. It should be noted at this point that the G terminal and W terminal are coupled to their internal circuits through high-resistance resistors (20k ohms). They are also coupled directly to each other through a 10k resistor. (See the sample circuit on the Motorola MC3334 data sheet.) Due to the coupling resistors, strange behavior may be observed if the terminals are viewed on an oscilloscope. For instance, if the W terminal is left unconnected (and an external bias is applied to the reluctor), the 10k resistor coupling will make it appear as though the W terminal has a small positive signal on it mirroring the G terminal. If the W terminal is connected as the reluctor's bias, then it will appear to have a small negative signal mirroring the G terminal. It is important to consider these coupling resistors when analyzing the module's behavior. This apparent signal mirroring is the result of the 10k resistor, and is not some internal function. Also, if a constant, external DC bias (such as 1.4v or .7v) is applied to the negative side of the reluctor while the W terminal is left unconnected, the W terminal of the HEI module will be coupled to that DC level. Therefore the W terminal will no longer behave as a floating DC level. This is important when trying to analyze the behavior of the W terminal. Essentially, it makes it almost impossible to study the behavior of the W terminal while using an external bias. If the reluctor's characteristics are appropriate, the reluctor's bias voltage can be supplied externally, eliminating the need to use the W terminal at all. If the reluctor's bias voltage is supplied by the same power source as the module (which is normally the case), and thus the module and input signal are powered up at the same time, and the input signal is ambiguous, the default state of the output seems to be that of C being open (ignition coil off). This is probably because the dwell-capacitor must be charged before the input comparator receives the full input voltage. The capacitor delays the ambiguous input signal from reaching the comparator when the power is first applied. Thus, the initial input signal is 0v. This means the initial state is that of the ignition coil being off. To be safe, in order to prevent the coil from being "on" at power up (engine not running), any external bias voltage applied to the reluctor should be 1.4v or lower. Another option for creating an appropriate bias would be to limit the amount of DC bias the W terminal would have. This can be done by coupling a constant DC source through a resistor to the W terminal. The resistor allows some fluctuation of W, without allowing full fluctuation. The value of the resistor would determine the amount of fluctuation, or "float", the W terminal would have. This would alter the DC feedback. Some notes on the internal connections and testing: The W terminal and G terminal are connected together by high-resistance resistors. High-resistance resistors are also used to couple the terminal connection to the module's IC. Because of this, some minor coupling occurs between the W and G terminals. This may cause confusion during testing. If one terminal is left open, it will tend to follow the voltage of the other. This may lead the tester to come to a wrong conclusion about the behavior of the open terminal. The voltage on W may appear lower than 1.25 volts, but in fact, it is really just following G. In operation, W does not seem to go much below 1.2v on its own. If a test circuit is built to drive the HEI module, it is very important that the driver circuit is not coupled to the HEI in any other way than through the G and W terminals (unless done so purposely). In other words, an ideal reluctor-simulator will most likely be battery powered by two batteries. (One for positive signals and one for negative signals.) By using batteries, the entire circuit is isolated from the HEI. This will prevent any ground loops etc. from interfering with the operation of the circuit. The driver circuit will be "free-floating" and will behave like a reluctor in terms of accepting a bias voltage. Without this type of driver, the readings at W or G may not be representative of what a real reluctor would produce. Some notes on usage with an early 80's Kawasaki KZ reluctor: With a Kawasaki reluctor connected to W and G and a 3-ohm ignition coil connected to C, the dwell-capacitor (and W terminal) stay at about 1.2 volts until about 3000 or 4000 RPM. Then rapidly increases to over 6 volts. At that point, the ignition coil stays "on" and all sparks stop as if the ignition was shut off. When the RPMs drop back down to about 1500, the sparks resume and a large explosion occurs in the exhaust system. Using a lower-resistance coil can activate the current-limiter, which may solve the problem, but care should be taken. Whenever the current-limiter is activated, there is a potential for a lot of heat to generate inside the HEI module. If the dwell is extended by the reluctor design, as in the Kawasaki, there is the possibility of extended periods of current-limiter activation. The HEI module may destruct. One solution is to not use the W terminal at all and provide a constant 1.4v bias to the pickup externally. The .2v extra bias helps during startup, and by disconnecting the W terminal, the 10k coupling resistor does not bleed off signal, which helps to increase the signal to the G terminal. This results in about 8ms dwell at 1500 RPM which decreases to about 2ms by 10,000 RPM. When a constant DC bias of .7v is used, the dwell remains at approximately 2ms to 2.5ms at 1500 RPM and stays relatively constant to 10,000 RPM where it ends up at about 2.1ms. However, .7v is not enough during start up, and a temporary 1.4v bias will be needed for start up. Some notes on usage with an early 80's Honda CB reluctor: With a Honda reluctor connected to W and G and a 2.5-ohm ignition coil connected to C, the dwell rises to almost 360 degrees just above idle through to 10,000RPM. This is too much dwell and will eventually overheat the coils. Using a lower-resistance coil can activate the current-limiter, which may solve the extended-dwell problem, but causes other problems. The Honda CB pickup has significant "crosstalk" between pickup circuits. This means that each pickup signal contains a small signal from the opposing pickup. This crosstalk will cause unwanted extra sparks. At certain RPMs, the W terminal would produce a bias signal that will cause the crosstalk to generate unwanted sparks somewhere in the range of 100 to 150 degrees (crank) before the spark is supposed to occur. With the Honda CB, using a constant external bias does not work very well. The crosstalk still causes unwanted sparks. Here are some examples: 1) Using ground as the bias (with W unconnected) produces too little dwell time for the stock coil. At 1300 RPM the dwell is 2msec. At 10,000 RPM the dwell is 1.3 msec. The sparks may actually stop entirely (dissipating as a coil "ring") before 10,000 RPM. 2) Using ground as the bias (with W connected to 12 volts) increases the dwell slightly. At 10,000 RPM the dwell is 1.6 msec. The problem is that unwanted sparks become more likely. Even if unwanted sparks don't occur, the crosstalk produces extra coil "rings". That is, a false trigger that is too brief to produce a spark, but interrupts the coil current long enough to make it oscillate or "ring". 3) Using .7v as the bias (with W unconnected) increases the dwell also. At 1300 RPM the dwell is 3.4 msec. At 10,000 RPM the dwell is 1.5 msec. This scheme also produces coil rings (and potential unwanted sparks) near 130 degrees before the desired spark. 4) Using 1.4v as the bias (with W unconnected) increases the dwell significantly. At 1300 RPM the dwell is 10 msec. The dwell jumps from 76 degrees at 1300 RPM to 180 degrees at 1500 RPM. This is because the crosstalk is able to initiate the dwell since the bias is near the threshold for dwell. The dwell remains at 180 degrees through 10,000 RPM. This would appear ideal, at first, but the unwanted sparks (at 135 degrees before the desired spark) become very pronounced in the 1400 to 1500 RPM range during the jump to 180 degrees.