Hydrogen Hot Rod Ignition Timing Systems

GM 7 / 8 pin Distributor Module

1. The GM 7 /8 pin modules have been used in a wide variety of GM engines from 4 cylinder to V8s (small and big block). The 8 pin distributor was also widely used in marine applications by Indmar, Mercruiser, and others.

GM 7-Pin Module

GM 8-Pin Ignition Module

1. The 7 and 8 pin modules are functionally equivalent and largely share the same wiring. The 7 pin is used in the large coil-in-cap distributors while the 8 pin is used in the small cap distributors with remote mounted coils. The 8 pin has one additional terminal that provides a sensor ground. Both modules provide coil ground via the metal grommets used to secure them to the distributor.

2. These modules provide an simple means for computer controlled timing while retaining the distributor. They were designed to be used with throttle body injection and port injection motors and provide automatic coil current limiting (7.5 amps was the GM specification) and automatic dwell control. The can be adapted to other distributor applications that use either variable reluctor or hall type sensors.

Pin Descriptions and connections

1. • "+" : Battery voltage from a switched ignition source. Provides the power to operate the module.

2. • "C-" : Ignition coil negative connection.

3. • "P & N" : Positive and Negative of the distributor reluctor. Polarity is important. GM distributor connectors can only be connected one way. For use with other distributors, verify polarity of the reluctor leads.

4. • "B" : Ignition bypass. When cranking, grounding this line bypasses computer control of timing. The timing iscontrolled by the module only. This can be done using the Speeduino Cranking Bypass pin function (see below).

5. • "R" : Reference or tach signal. This outputs a 5 volt square wave that serves as the RPM1 input for the Speeduino. To use this, connect it to RPM1 and set JP2 to Hall and JP4 connected (ie, 5 volt pull up).

6. • "E" : Timing control signal. When pin B has 5 volts on it, the module allows Speeduino to control the timing using this pin. The output of Ign1 should be connected to this pin.

7. • "G": (8 pin only) Signal ground. Should be connected to the Speeduino sensor ground. (Module ground is provided through the metal mounting grommets)

8. A timing bypass circuit must be constructed to utilize the Speeduino ignition timing control. The small circuit below should be built in the proto area.

In addition, the cranking bypass should be turned on and the bypass pin should be set to pin 3 in the Cranking Settings dialog (under Starting / Idle in TunerStudio):

Trigger settings (under Starting / Trigger Setup in TunerStudio) should be as shown below. You will need to adjust the trigger angle to get correct timing. Instructions for this are in the wiki.

Timing control is set in the Spark / Spark Settings dialog:

The module works well, however, some applications result in a noisy trigger signal. This shows as an unstable RPM (either at idle, or more frequently, at higher speeds). Trigger filtering may help, but a modification may be necessary to clean up the signal. The circuit below effectively cleans/filters the signal, allowing use with no trigger filtering by the Speeduino. It has been tested with single and 8-pole reluctors and modules from GM, Transpo and a no-name generic.

Coils

1. Current versions of the ECU EMS use low-power output signals, designed to work with external small-signal ignition coil drivers, whether a separate type (module or ICM, igniter, IGBT, etc.), or built into the coil assembly ('smart' coils). This method permits ECU EMSto have great flexibility to control most types of ignition systems. Attaching the ECU EMS outputs directly to a traditional high current passive ("dumb" or 2-pin) ignition coil without an ignition coil driver risks damage on some ecu.

1. How ECU EMS controls ignition circuit power In prior history, the coil driver was a set of mechanical contact points ("points"), simply replaced today by an electronic version. The added coil driver can be anywhere from inside the ECU Ems to inside the coil assembly; though near or in the coil reduces electrical noise.

3. In the example animated image below, the ECU EMS ignition signal is normally low (near ground or 0V) while ECU EMS calculates the time to the next coil firing. At the proper time, ECU EMS switches the ignition output to high (near 5V or 12V selectable) in order to switch the coil driver (example IGBT) on, allowing current to flow through the coil to ground. This is called the 'dwell' period. During dwell an increasing energy field is generated around the ignition coil core and wire windings.

5. At the end of the dwell period and therefore at the proper time for spark; ECU EMSswitches the coil driver off, stopping current flow, which collapses the energy field to create high voltage and the resulting spark:

1. In TunerStudio, the setting for this example would be to fire the spark "going low". The dwell setting is highly variable depending on coil type, voltage levels, etc. Too little dwell will give weak or no spark and excess dwell will rapidly increase heat, possibly damaging any of the ignition components, but usually the ignition coil or coil driver.

1. The wasted-spark version is below to show how it is identical in operation, but with the high-voltage spark returning through the second spark plug to complete the circuit:

1. A good run down of 'smart' coil types can be found at: http://www.megamanual.com/seq/coils.htm. There are many ignition modules available that ECU EMS can use to control standard coils, or for smart coils you can generally use 4 or 5-pin types as these will always be logic level, although some 3-pin coils are also of this variety. GM LS1/2 coils are an example of powerful smart coils that are commonly used and can usually be obtained easily and cheaply.

1. (Note: In the past, some ignition control modules with current limiting or dwell control features (e.g., 1970s GM HEI, Bosch '024' types, and Ford DS1) were referred-to as "smart" modules. While still true, common terminology of individual ignition coils with at least a driver integrated, or newer technology with greater controls (e.g., controlled spark duration or multi-spark) are all considered "smart" coils. You must know the control requirements of the specific drivers, control modules, or coils you intend to use in order to operate them properly with ECU EMS.)