The intention was that the car could be returned to the stock ECU in a very short time. Therefore a spare distributor was modified to optical output while still being able to plug into the factory harness using the factory plugs. Refer to this page for instructions on rebuilding distributors for a basic understanding of the stock 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 . Dennis' page details these changes well. I took the more complex 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 or cam angle sensors. They have an electronic module with an LED and a light sensor mounted opposite. A slotted rotating disk spins in between cutting the beam of light, providing an accurate electrical signal. This signal is directly compatible with the Autronic.
I spent a few hours digging around a large wreckers yard pulling apart distributors. I found all of the stock optical disks have two rows of slots, the inner ring having four slots (for a four cylinder) and the outer having 360. This level of resolution is too much for the Autronic, so custom disks can be bought from Ray Hall to suit a number of Nissan applications which have just one slot instead of 360. That single slot is used for the TDC signal to the ECU.
I searched a large variety of Nissans and Mitsubishis and found there are two disk sizes, 50mm and 54mm. Both brands use both disks dependant on model. Generally the 50mm are Mitsi and later Nissan, the 54mm earlier Nissan. I found a couple of Mitsubishi distributors which use 50mm disks without the 360 slots. One was in a late 80's Galant with a Cyclone engine. It uses four slots (not 360) so is perfect for the Autronic. A 1989 Nissan Bluebird SSS with a CA18DE motor donated its camshaft position optical sensor (50mm disk) for a bargain $10. The Skyline RB20DE unit is essentially the same. This module has a plug which extends out the side of the housing. The distributor based module are identical except the plug exits from the rear. Note that the Nissan distributors are far easier to disassemble than the camshaft angle sensors! An hour with a Dremel was required to disassemble the camshaft position sensor, the distributors cam simply unscrewed.
Now here is something *really* spooky. The Nissans, the Mitsubishis and the Toyota all share the same 55.9mm pitch circle diameter for the screws that hold the electronic modules in place. I'd like to see that level of standardisation among US brands! The modules are almost an exact fit to the ST185 distributor, one hole needs to be plugged then drilled and tapped about 1mm off centre, another existing hole must be tapped. Whichever module is used a slot must be milled for the plug. Three 5mm spacers are needed to position the module at the correct height. Therefore I chose one existing hole as my datum and drilled two more. It is important to get the module central and flat in the Toyota housing.
The Toyota electronic module had the plug cut off and soldered to the Nissan plug to make an interconnection harness. This meant that the Toyota harness could be plugged straight in as normal.
The rotor out of the ST185 distributor can have the sensor pressed off the end. The sleeve from the Mitsi dizzy was machined slightly shorter and bonded to the shaft at the right height using Loctite bearing retainer. The shaft itself was machined to an overall length of 96.45mm and a M6 hole tapped in the end. The distributor was then reassembled and the new disk shimmed to the right height. The correct slot was aligned with TDC (it could be rotated as the 'D' had been machined to a 'O') and it was ready to be installed! Final TDC alignment is made by turning the distributor once installed.
One caveat is that the four cylinder pulses must be at least 5 degrees away from the TDC pulse. The SMC has the ability to select +ve or -ve signal (ie slot or no slot) so this isn't hard to do with a slotted disk.
This could also be done using the 54mm disk and system from a Nissan distributor. I found the slotted 50mm Mitsubishi disks therefore used the Nissan module. Note that the Nissan parts are actually made by Mitsubishi. Confused!?
These optical systems are known to give trouble, most likely due to obstruction of the inaccessible LEDs. The electronic module is easily swapped if required.
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