Megasquirt-3 MS3 GM Optispark


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Introduction - Wiring - Settings

optispark dizzy


The Optispark system was used on GM vehicles from 1993 to 1997 on LT1, LT4 and L99 applications. Internally it uses a Mitsubishi derived optical trigger arrangement. There is a "hi-res" track of 360 slits and a "low-res" track of 8 slots of varying length. The pickup design is sound, but the high-tension side can be problematic with the "correct-a-cap" design - especially if a high energy aftermarket ignition system is used.

The Megasquirt-3 Optispark decoder uses both low and high resolution tracks for improved ignition accuracy. (Most other aftermarket implementation only use the low resolution track.) The system allows for sequential fuel and the single coil as per the original install. However, as an enhancement the single coil can be replaced by a wasted-spark or coil-on-plug setup which would eliminate the troublesome high-tension cap.


opti wiring

The Optispark requires a fused 12V supply. This can be tapped into the same 12V supply as the Megasquirt.
The Ground connection should be run to the sensor ground at the Megasquirt.
The high and low resolution tach signals each require a "pull up" resistor to operate correctly. The unit has been tested with 330R resistors as shown:
opti pullup

Internally, the Megasquirt needs to be set for the VR input, see but unusally needs to use the inverted VR mode.
V3.0 board see
here and adjust per hall, but ensure VROUTinv only is jumpered to TSEL
On V3.57 see here and adjust per hall, but ensure on J1 is jumpered 5-6 only.


opti settings

See the ignition manual for wiring and configuring your coil output(s).
See the fuel manual for setting up your fuel system.

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 and Ken Culver.