Duralast/Wells DR124 7-Pin GM HEI Module Notes by Lou Dudzik 4/29/09 update 9/21/19 7Pin HEI module Advance is controlled by the ECM, not the ignition module. ------------------------------------------------------------------------------------------------------------------------------------------- 7-Pin HEI module These notes are specifically for the Duralast/Wells model DR124, GM 7-pin HEI ignition module and other manufacturers as noted.The Duralast/Wells is Available at Autozone, distributed by Best Parts, Memphis Tennesee, 38103. It is supplied with thermal transfer gel for the heatsink. It is found on 1986 Chevrolet Caprice 5.7L, 4BL, V-8. The 7-pin GM HEI module has 2 large blade terminals, 5 small blade terminals and a heatsink. The heatsink acts as the ground connection for the module. It has two 4.5mm mounting holes through it, set at about 62mm apart, center-to-center, 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. With no input signal, and no current in the spark coil, the HEI module uses 133mA at 12v. That's 1.6 watts. The two large blade terminals are labeled "C" and "+". "+" is connected to the supply voltage. Normally this will be from about 10 to 15 volts DC. The various examples tested seem to function, at least in Run Mode, with the supply voltage well below normal. The Duralast DR124 (Wells DR124) and the Master Pro seem to function in Run Mode down with the supply voltage down to about 3.9v. The Value Craft Tru-Grade functioned down to about 6.9v. 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 several hundred (600?) volts. When the coil is "on", the voltage drop on C is about 1 volt, but increases slightly as current increases. C has a slight delay on it. The delay on firing the coil (dwell-off) is about 30 to 80 micro-seconds. This delay exists regardless of if the spark is triggered through P or E or B. Current limiting is employed in the coil driver circuit. Terminal C will sink up to a maximum of about 6.3 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 6.3 amps is never reached, the current limit feature does not get used. 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.) The 5 small blade terminals are labeled "B", "R", "E", "N", and "P". N provides a steady DC bias voltage to the reluctor pickup's negative terminal. The voltage on N changes slightly depending on the "mode" of operation discussed below. P is the input for the reluctor's signal. P is connected to the positive terminal of the reluctor pickup. P and N seem to be connected together internally through a 10K-ohm resistor. Therefore, even with nothing connected to the P terminal, it will have a voltage on it close to that of the N terminal. R outputs a squarewave, pulsed DC signal based on the input signal to the P terminal. The signal at P is converted from AC to pulsed DC in the AC-to-DC-converter. A positive signal at P, above a threshold, will produce a positive 5v DC pulse. Below that threshold, the converted signal will be near zero volts. The converted signal then goes through a buffer before it is output to the R terminal. The signal at the R terminal is typically routed to the ECM (electronic control module) of the car. In the ECM, dwell and timing adjustments are made to the signal. B is the terminal which determines the "mode" of operation. When B has 5v on it, a solid-state relay activates inside the HEI module to put the module into "run" mode. With no voltage on B, the relay is deactivated and the module is in "bypass" or "start" mode. While not part of the design spec, it does seem the B terminal can accept a wide range of voltage all the way up to, and possibly above, the supply voltage. Value Craft has about 6k ohms resistance on B to ground. Master Pro has about 10k ohms resistance on B to ground. Duralast/Wells has about 10k ohms resistance on B to ground. The minimum holding voltage on the B terminal to maintain run mode: Duralast/Wells is about 1.3v. E is the input from the ECM. The module signal output at the R terminal is adjusted by the ECM and returned to the HEI module through terminal E as a pulsed 5v DC signal with the dwell and timing altered as necessary.While not part of the design spec, it does seem the E terminal can accept a wide range of voltage all the way up to, and possibly above, the supply voltage. However, some modules cannot accept a negative voltage at the E terminal. Doing so results in a false dwell event when the voltage on E goes adequately negative. The Duralast/Wells DR124 and Master Pro seem to have this behavior. The Value Craft Tru-Grade does not. When the module is in Run Mode, that is when B has 5v (or greater) applied, the E terminal is switched to the coil-driver amplifier. With 13.7v applied to a 2.2k resistor connected to E: Value Craft voltage on E is 5.63v @ 3.7ma. This implies internal resistance to ground is 1.5k ohms. With 13.7v applied to a 1k resistor connected to E: Value Craft voltage on E is 6.34v @ 7.5ma. This implies internal resistance to ground is 850 ohms. Master Pro voltage on E is 12.45v @ 1.3ma. This implies internal resistance to ground is 10k ohms. Duralast/Wells voltage on E is 11.18v @ 1.1ma. This implies internal resistance to ground is 10k ohms. When the module is in Bypass (Start) Mode, that is when B has no voltage or is not connected, the E terminal is switched to ground internally inside the module. This ground connection is not a dead short and different modules have a different resistance to ground in this mode. The ability to sink current for an extended time was tested as described below. The modules were able to withstand this load indefinitely without an external heatsink and 13.7v applied to + and the backplate grounded (module powered up in Start Mode aka Bypass Mode). With 13.7v applied to a 1k resistor connected to E: Value Craft voltage on E is 2.0v This implies ~171 ohms to ground. Master Pro voltage on E is 4.0v to 4.3v. This implies ~434 ohms to ground. Duralast/Wells voltage on E is 4.4v to 4.6v. This implies ~489 ohms to ground. Two main modes of operation: ------------Bypass Mode, Start Mode----------------------------------------------- This mode occurs when there is no voltage on B. With no input signal and supply at 13v: Unconnected N = 2.41v Unconnected P = 2.38v E is grounded. In this mode, there is an internal dwell control circuit which takes effect. It behaves differently for higher resistance coils, which do not use enough current to activate the current limit circuit, versus lower resistance coils which do activate the current limit circuit. With input signal at P, and R connected to E: This should not be done because it shorts out the buffer on the R terminal. Since E is grounded, R is grounded. However, when R would be positive, it doesn't get shorted all the way to zero through E, it actually has about .34v on it. This mode seems to have different behavior among different modules. In some, the thresholds change drastically and suddenly with frequency and signal peak-voltage. Also, on some modules the thresholds have a window in which they must operate. Operating outside this window causes erratic behavior. With input signal at P and peak signal is less than 6.8v: Dwell-on at P > 2.66v Dwell-off at P < 2.52v E is grounded. With input signal at P and peak signal is greater than 6.8v: Dwell-on at -.6v < P < 6.8v Dwell-off at -.6v > P > 6.8v E is grounded. Perhaps this behavior is due to opam phase reversal from input out-of-range. -------------Run Mode--------------------------------------------------------------------- This mode is active when there is 5 to 6v (or possibly much higher voltage) on B: With no input signal and supply at 13v: Unconnected N = 2.01v Unconnected P = 1.99v With input signal at P, and R connected to E: Dwell-on at P > 2.9v Dwell-off at P < 2.2v With dwell-on, R and E are at 4.6v With dwell off, R and E are at 0v. With input signal at P, and R unconnected: R is at 5.8v when P > 2.9v R is at 0v when P < 2.2v Dwell depends on input at E --This method below is the only one that seems reliable with newer manufactured modules-- --Input must only be positive voltages-- Frequency does not seem to alter thresholds much if at all. Peak input signal does not seem to alter thresholds much if at all. Supply voltage does not seem to alter thresholds much if at all except when very low near shutdown. Ignoring N, P, and R and with input signal at E: Dwell-on at E > 1.84v Dwell-off at E < 0.84v The following is not proper operation, but dwell can also be triggered by a negative signal on E: Dwell-on at E < -1.84 Dwell-off at E > -1.84 Duralast/Wells DR124 and Master Pro Dwell-on at E > 2.0v Dwell-off at E < 0.8v Value Craft Tru-Grade Dwell-on at E > 3.2v Dwell-off at E < 1.5v (This module seems to ignore negative input voltage.) -------------Dual Mode----------------------------------------------------------------- This mode is not an actual mode, but is created by alternating from one mode to the other. In this manner, the B terminal can be used as the input. THIS IS NOT RECOMMENDED. When used this way, the module locks up and power must be removed to resume operation. If this method can be made to work reliably, it would eliminate the need for an inverter in order to use this module while being triggered by a set of breaker points. To use this mode, connect P to positive supply voltage. Connect E to ground. Then when B is 5v or 6v, dwell will be off (C is high). When B is 0v, dwell will be on (C is low). A voltage divider can be made from two resistors to feed 6v to B. Then the breaker points can be used to ground the voltage at B to start the dwell. When the points open, B gets 6v and the dwell ends to create a spark.