Note: The following info was provided by a third party, and has not been verified. Use at your own risk.

"All distributorless General Motors four-cylinder and V6 models starting in 1987 use a distributorless ignition system called the Direct Ignition System (DIS). This system uses a "waste spark" method of spark distribution. To understand the DIS system better, first read the HEI information above - much of it will apply to the GM DIS system.

Each cylinder is paired by a single coil with its opposing cylinder in the firing order (1&4, 2&3 on a four cylinder, 1&4, 2&5, 3&6 on a V6). This means one cylinder on its compression stroke fires simultaneously with its opposing cylinder on the exhaust stroke.

Since the cylinder on the exhaust stroke requires very little of the available voltage to fire its plug due to the easy ionization of the hot exhaust gases, most of the available voltage is used to fire the cylinder which is on the compression stroke.

A good reference book for this system is available from General Motors STC as book #16020.05-3. It can be ordered through the General Motors Service Technical College catalog at 1-800-393-4831.

The DIS system includes:

• a coil pack (with one coil shared for every two cylinders),
• an ignition module,
• a crankshaft reluctor ring,
• a magnetic crankshaft sensor,
• and the MicroSquirt® controller.

The magnetic crankshaft sensor protrudes through the engine block to within ~0.050" (~1mm) of the reluctor ring. The reluctor ring is a disk cast into the crankshaft, which has notches that a VR sensor locates to acts as a signal generator for the ignition timing.

The system uses Electronic Spark Timing (EST) and control wires from the ECM, just like conventional General Motors distributor systems.

There are example schematics for the DIS modules here:

• V6
• I4

The crank reluctor ring has 7 notches, 6 of which are evenly spaced (i.e. 60° apart) about ³/₁₆" wide (5mm) and ¹/₄" deep (6mm). One additional notch is placed ion the ring to 'sync' the module to the engine's position. This sync notch (the sync pulse) is retarded by 10° from nearest notch.

• In a 6-cylinder application, the evenly spaced notches coincide with TDC. The sync pulse notch coincide with 10° after TDC for the #1 cylinder.
• In a 4-cylinder application, the notches DO NOT coincide with TDC, except for the one at TDC for cylinder#1. The sync pulse comes at 70° after TDC for cylinder #1.

The VR sensor produces a signal that looks like this:

In OEM applications, the ring is about 4.00" (~100mm) to 6.00" (~150mm) in diameter.

There is additional information on how the GM DIS system works in patents #4,750,467 and #4,711,226 (and the patents referenced in these). These are available at www.uspto.gov

There are some DIS module part numbers in this document:

http://www.distester.com/pdf/dis_map.pdf

You can also use the Wells DR145 from a 1994 3.4L 60 degree V6 Camaro (GM PN #10489422).

The DIS coils are AC Delco D555 (~$38) or Standard Motor Products DR39X (~$38), and are used on over 1000 GM vehicles from 1984 to the present.

The connectors are:

• AC Delco PT782 (GM 12167116): 3 position (~$23)
• AC Delco PT787 (GM 12167121): 6 position (~$33)

Wiring and Configuration

The wires to your MicoSquirt® carry the following signals:

• 3X Reference High - The Ignition Control Module (ICM) receives an engine speed signal from the crankshaft sensor and passes it along to the ECM on the purple and white wire.
• 3X Reference Low - Tach signal ground on the red/black wire.
• EST - The ECM calculates ignition timing corrections and sends that information back to the module on the EST line (white wire). This information is used by the ICM to fire the secondary at the correct time.
• Bypass - The tan and black or bypass line will be used when setting the ignition timing or when the engine is cranking. Bypass mode eliminates EST.
• Reference -The black and red line is provides a common ground reference point shared by the ICM and ECM.

On an oscilloscope, you would find:

• RPM sensor = either a square wave or an A/C signal,
• Purple and white = a square wave anytime the engine is cranking or running,
• White = a pulse width modulated square wave when the engine is running,
• Tan and black = zero volts while cranking and 5 volts otherwise,
• Black and red = ground.

Note: it appears some modules use other connections (but use the same wire colors for the same functions). For example, some of the the 2.8 and 3.1 V6 engines have the Tach (white) on C, and the Bypass (tan/black) on A. Most of the remaining wiring (power and VR sensor) appears to be constant.

In your tuning software, set:

To set the dwell to 25.0, you'll have to change the megasquirt-II.ini file. Edit the following line with Notepad.exe (installed on most Windows systems, look under 'Start/All Programs/Accessories') or a similar text editor:

to

The spark duration can already range from 0 to 25.5 milliseconds.

The GM HEI module provides a 'distributor-like' signal to MicroSquirt® controller, so the missing tooth settings should not be used. Set the trigger wheel teeth to 0 (zero).This missing tooth signal goes to the DIS module, and it must be correct, then the DIS module creates a tach signal from that, and this is what is sent to MegaSquirt. So you treat it as if it was a distributor (and let the module do the hard work).