MS2-Extra GM HEI Manual

Only for use with the MS2-Extra code (HC9S12C64 based microprocessor - MS2 Daughter Board)

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Please Note:

All of these instructions / diagrams are to be used at your own risk, like most things there is more than one way to do the same thing, what we have tried to do is to offer a method that we have tested or that others have tested for us. No warranty expressed or implied.

Use at your own risk.
If you do have any suggestions or settings that work please let me know Philip.Ringwood(at)

Main Index of MS2-Extra Manuals

Layout of Components -- Suggested Supply points -- List of common components and part numbers

GM 7 pin and 8 pin HEI ignition

Internal Wiring to MS ECU -- Settings

There were a number of different kinds of General Motors HEI modules:

* 4 pin module - electronic ignition, but doesn't do computer timing control, top left
* 7 pin module (large) - electronic module that does timing control, used with coil-in-cap distributors, top right
* 7 pin module (small) - electronic module that does timing control, used with some external coil distributors, bottom right
* 8 pin module - electronic module that does timing control, used with most V8 external coil distributors, bottom left
* 5 pin module - rare and not discussed here. not shown

HEI 7-Pin Module

With a GM HEI 7-pin (and 8-pin modules), the ignition module wires you need to know about are:

* tach signal is a purple wire with a white trace [pin R]
* advance control signal is a white wire [pin E]
* override (cranking) signal is a tan wire with a black trace [pin B]
* ground is a black wire with a red trace [grounded to distributor case]

HEI 8-Pin Module

In addition to the module shown above, you can use the HEI 8-pin module from the "small cap" HEI distributor. This was used on 1987 through 1992 Camaro/Firebird Tuned Port Injection V8 engines, as well as 1987 to 1993 full size cars and 1987 to 1995 pick-up trucks, and V6 MPFI cars (such as the Fiero). It uses a remotely mounted coil (i.e., not in the cap), which is typically is bolted to the intake manifold. The distributors are used primarily on roller cam equipped engines. Note that the GM 8-pin HEI module is basically the same as the 7-pin, except there is one extra pin: the extra pin is 'G' and is used for a ground. The 8-pin module has two 'mushroom' style connectors. One has 2 pins, it is used for the coil. The other has 4 pins, and is used for the electronic timing control. The timing control wires are usually the same colors as described above for the 7-pin module.

Internal Mods for V3.0 PCB's

HEI7/8 mode will use the Opto/Hall input, so the VR components are not required.



Internal Mods for V2.2 PCB's


Wiring the HEI module:



Set Spark mode to Basic Trigger.
GM HEI/DIS to GM Bypass on D16 (This means that D16 will give a 5V signal out on D16 when the engine is in RUN mode, so the module controls cranking spark angle)
Ignition Capture set to Falling Edge and Spark output to Going Low (Normal)
Spark A output pin
set to D14

You can used JS10 for the spark output, but set Spark output to Going High (Inverted)

The Trigger Angle offset value can theoretically be set anywhere physically, however, since it may be used for cranking and 'fault mode' timing (depending on how you have wired the bypass pin), it is best to set it at a reasonable number for cranking and idle, usually 10° BTDC (or whatever is recommended by the module's manufacturer). Check this with a timing light. To get the trigger offset to this value, you may have to physically rotate your distributor.

If you aren't sure where your trigger is physically located, you can set the trigger offset on your vehicle (with the 7 or 8 pin HEI module):

1. Bring the engine to top dead center on cylinder #1 by rotating it in it's normal direction (usually clockwise when viewed from the front, except for some Honda engines). I remove the spark plug from cylinder #1, and feel for compression, then you know the engine is at TDC on the compressions stroke. Rotate the engine a bit more until the timing indicator lines up with the TDC mark.
2. Rotate the distributor to align the VR sensor teeth. In the HEI distributor, there are eight sets of teeth, all of which align with one other tooth simultaneously. Lock the distributor down.
3. Whichever terminal the rotor is pointing at is now cylinder #1. The rotor should be much closer to one terminal than the others (it wil probably have gone by one of the cylinder's terminal by a bit, that's okay - it means it will line up with a bit of advance - you can rotate the engine through very nearly two complete revolutions to check - get it to about 25° to 30° BTDC and it should align with the terminal). If the rotor is pointing between cylinders (i.e., the phasing is off), then it's likely that the distributor is off by one or more teeth at the camshaft (in a V8 HEI install, at least). On a small block Chevrolet, for example, the distributor should be installed so that the rotor is pointing a little in front of the #1 cylinder when the engine is at TDC on the compression stroke for cylinder #1 - all other positions will create phasing problems.
4. Install the remaining plug wires in firing order around the distributor, being very careful about both the direction and the order. On a small block Chevrolet, the rotor spins clockwise when viewed from above, for example.
5. Start the engine with the B pin disconnected, and set the timing as seen on a timing light to 10° BTDC. (With B disconencted the MS ECU will have no control over timing so it defaults to 10deg)
6. Connect the B pin back up to the MS ECU (letting MegaSquirt control the timing), and start TunerStudio, verify that a timing light and the trigger wizard give the same number. Change the trigger offset value in TunerStudio a bit to make them match, if necessary.


Suggested points for Supplies inside the V3.0 ECU



Component Pinouts

Pull up circuit diagram

List of component part numbers used in the ignition section:

DigiKey part numbers:
Farnell part numbers

2N2222A = 497-2598-5-ND
1N4001 = 1N4001/4GICT-ND
1K resistor = 1.0KQBK-ND
470R resistor = 470QBK-ND
2K resistor = 2.0KQBK-NB
330R resistor = 330QBK-ND
680R resistor = 680QBK-ND
200R resistor = 200QBK-ND
270R resistor = 270QBK-ND
1K5 resistor = 1.5KQBK-ND
4K7 resistor = 4.7KQBK-ND
0.01uF Cap = P3103-ND
4N25 (opto) = 4N25ASHORT-ND

2N2222A = 920-7120
1N4001 = 352-5326
1K resistor = 509-164
470R resistor = 543-305
2K resistor = 543-457
330R resistor = 543-263
680R resistor = 543-342
200R resistor = 543-214
270R resistor = 543-240
1K5 resistor = 543-421
4K7 resistor = 543-548
0.01uF Cap = 389-0995
4N25 (opto) = 102-1090

Please note: Above part numbers will need checking, some components will come with a minimum order in multiples of 5 and 10.

If you have a question, comment, or suggestion for this FAQ please post it on the forum.

No part of this manual may be reproduced or changed without written permission from James Murray, Ken Culver and Philip Ringwood.