An easy upgrade for 79 models is to install the 81-85 model electronic ignition distributor to the 79 12A engine. This eliminates the need for periodic points replacement and the inevitable dwell and timing drift associated with points.
The downside is that replacement igniters are quite expensive. A new igniter runs upward of $200 and known good, tested, used igniters can run as much as $125. If you have a local wrecking yard with a good supply of Rx-7s, buy a couple of distributors for spare parts (they’ll come in handy) & snip the factory wiring for the igniters while you’re at it. You should also consider the YAW Direct Fire system before you spend money on igniters. The MSD 6A is only about $130; the price of ONE USED igniter and delivers much hotter spark.
Dropping in the electronic distributor is straightforward. Set the engine to the TDC mark for the leading ignition. Note the position of the rotor and remove the entire assembly. Do not crank the engine again until your done. Disconnect the battery negative terminal.
Position the electronic distributor so the leading igniter faces straightforward and the trailing igniter faces the alternator. Position the rotor to match the position of the old unit removed and drop it in. Install the lock bolt and leave it loose enough to allow for setting timing later.
Now comes the more difficult part. There are several circuits on the ’79 12A that must be modified or eliminated to make this swap work. In these models, the trailing ignition only worked part time. We’ll want full-time operation with the electronic setup. It is vital that you have a wiring diagram for the remainder of this upgrade – doing this wrong could cause major headaches. The Haynes manual (pgs. 177-179) has everything you need.
Install two new coils rated for Mazda electronic ignition. The MSD Blaster II units are excellent and only about $33 each. Your stock coils were designed to operate at 9 volts and will not live long after this upgrade. If you choose to ignore this, expect poor performance and a long walk at some point.
On the front of the left strut tower near the fuse links are four (4) ballast resistors. These must all be removed. Two of the resistors are fed battery power through a black/white (BW) wire. Note these and splice the black/white wire into the wire on the other side of the resistor (solder and insulate with heat shrink tubing – splice connectors will not provide a long-lasting connection). One of the BW wires will be spliced to a yellow (Y) wire that goes to the positive side of the trailing coil. The other BW wire will be spliced to a yellow/black (YB) wire that goes to the positive side of the leading coil. The remaining two resistors can be removed and the connectors taped back to the harness out of the way.
Next, locate the two relays on the radiator support directly behind the left headlamp. Disconnect the relays and tape the connectors out of the way; these controlled the part-time operation of the trailing ignition. You may remove the relays if you wish.
Each igniter has two electrical spade terminals and a metal clip used to retain the factory connector. The terminal closest to the metal clip is the negative terminal. You will connect the negative lead from each coil to one of these. If you salvaged the factory connectors from the doner car, great. This will help protect the connections. If not, don’t worry about it, but be sure to insulate the connections when you’re done.
Connect the leading coil negative lead to the leading igniter (facing forward) negative terminal. If you have the factory igniter connectors, this will be the yellow/blue (YL) wire. Connect the trailing coil negative lead to the negative terminal of the trailing igniter. If you have the factory igniter connector, this is the yellow/green (YG) wire.
Finally, run a new wire from the positive terminal of each coil to the positive terminal of each igniter. Again, if you have the factory connectors, this will be the black/white (BW) wire in each connector. Please use new ring connectors to attach this new wiring to the coil positive terminals and solder all connections and splices. You’ll thank me later.
Next, temporarily disconnect the igniters and reconnect the battery. With the key ON, use a voltmeter to check for battery voltage at the coil positive terminals. If there is more than 0.2 volts difference between the battery reading and the reading at the coils, go back and check your connections. Now check the new positive leads you made for the igniters. It should also be battery voltage. If not, check your connections. A bad ignition switch could also cause low voltage. Try checking other circuits if you show more than 0.2 volts difference from battery voltage.
Turn the key OFF and reconnect the igniters. Install a NEW distributor cap and rotor. The point type cap and rotor will NOT work. New plug wires are recommended also if yours are over two years old.
Start the engine and set timing to the 79 factory specs if your engine is otherwise stock.
Please note that on 79 models, the tach signal came from the leading ignition and still does after this upgrade, since we used the existing negative coil circuits. The 81-85 systems used the trailing ignition for the tach input signal. This will have no affect on performance.
You may see some improvement in performance, but don’t expect miracles. If you used MSD coils you’ll see more. The old point system wasn’t bad, just very old technology. Enjoy you new system and happy rotoring.