I am currently installing an Autronic SM2 aftermarket computer in my ST205. There are already a couple of excellent pages out there for the GT-Four and a wealth of knowledge. One is Dennis Heath's page at http://member.newsguy.com/~gtfour/ and Chris O'Rourke's page at http://www.turbocelica.com/.
I did my install a little differently to those pages. When I rebuilt my car I unwrapped the main engine harness and all the wiring appeared to be in sound condition, expected given the car had only 58000kms on the clock and was only 5 years old when taken off the road. Therefore instead of rewiring the whole engine I simply made a jumper harness to interface between the factory harness and Autronic harness. I made this using a Celica ST182 computer as a source of the vital Toyota plugs. The factory loom was used to run wires where possible but an additional loom was required for the coil-over-spark ignition system. The intention was that the car could be returned to stock ECU in a very short time.
Coil over spark ignitors? I spent a bit of time at engine importers (and used caryards!) measuring these units too see which would fit the ST205 engine. The 1ZZ-FE Corolla and 3S-GTE Caldina units both fit (Celica, 1NZ-FE, and Altezza do not), and as the Corolla units are 1/3 the price of the Caldina units I chose them. The Autronic is capable of running these if fitted with a dwell board which runs them in a wasted spark configuration. A boss needs to be added to the camshaft cover so each can be screwed down. These ignitors are not as expensive as you may think!
My most significant change was to the distributor. The Toyota magnetic distributor is not directly compatible with the Autronic, a number of teeth must be removed and a reluctor interface unit used which changes the signal (since purchasing my Autronic I think a new chip has been released which is compatible). I took the option of converting the distributor to provide an optical square wave (digital) output. A number of late 80's and later Nissans and Mitsubishis use optical distributors. They have an electronics housing with an LED and a light sensor opposite. A slotted rotating disk spins in between cutting the beam of light, providing an electric signal. This signal is directly compatible with the Autronic. The unit I used was from a R32 Skyline with a RB20DE engine. The disk is from a Mitsi.
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