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General Motors DIS Ignition Control with MegaSquirt-II

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:

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:

(They are large: ~950 KB)

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.

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

There are some DIS module part numbers in this document:

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:

Wiring and Configuration

The wires to MegaSquirt-II carry the following signals:

On an oscilloscope, you would find:

To use the GM DIS module, build your MegaSquirt® as shown in the GM HEI section. To connect the module to MegaSquirt:

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:

Ignition Input CaptureRising Edge
Signal inverted by optoisolator (U4)

('Falling Edge' for MicroSquirt only if using the VR input circuit)

Cranking triggerTrigger Rise
Coil Charging SchemeStandard Coil Charge
Spark OutputGoing High (Inverted)

('Going High (Inverted)' for MicroSquirt)

Maximum Spark Duration25.0
Maximum Dwell Duration25.0
(to get a 50% duty cycle)
Trigger Wheel Teeth0 (zero)

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:

max_coil_dur = scalar, U08, 4, "ms", 0.10000, 0.00000, 1.00, 8.00, 1 ; * ( 1 byte)


max_coil_dur = scalar, U08, 4, "ms", 0.10000, 0.00000, 1.00, 25.50, 1 ; * ( 1 byte)

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

The GM HEI module provides a 'distributor-like' signal to MegaSquirt, 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).

MegaSquirt® and MicroSquirt® controllers are experimental devices intended for educational purposes.
MegaSquirt® and MicroSquirt® controllers are not for sale or use on pollution controlled vehicles. Check the laws that apply in your locality to determine if using a MegaSquirt® or MicroSquirt® controller is legal for your application.
©2005, 2010 Bruce Bowling and Al Grippo. All rights reserved. MegaSquirt® and MicroSquirt® are registered trademarks. This document is solely for the support of MegaSquirt® boards from Bowling and Grippo.