I have been avidly watching the alarm/Immobiliser threads for a while, and think it may be time to add my ten pence worth….(sorry it is a bit on the long side)
Most alarm fitters cut the main feed from the ignition switch as this is the easiest and quickest to find, the Black and white wire from your ignition switch on an RX7 is the main supply for the fuel pump, ignition coils and meter supply fuse . This wire is approx 10 AWG rated at 35 Amps continuous , with most alarm immobiliser wires approx 18 AWG rated at best around 10-15Amps continuous. Mazda went to excessive lengths to reduce weight in the car, they would not have over specified a wire unnecessarily!
The three circuits that the black and white wire feed are:
Coil supply, this does not have a fuse after the ignition supply, but is unlikely to be above 8-10 Amps continuous (I have tested a single coil running at 8000 rpm to help back up this estimate)
Fuel pump supply 20 Amp fuse (overload current rating for fuses is 75% of running giving 15 Amps normal running current)
Meter fuse supply 15 Amp fuse ( at 70% running current gives 11.25 Amps)
So total max running current of this circuit can be up to 35-37 Amps!
Effects of voltage drops
Most/some ignition systems compensate for voltage drop by increasing dwell, I am unsure if a power FC operates in this manner, I would have to carry out some testing to prove either way.
If we assume it can compensate for voltage fluctuations, If there is a voltage drop on the coil feed, the ECU will never see this, as the main supply to the ECU is separate from that of the coil supply, with the ECU being supplied by the EGI fuse via the main relay. The EGI fuse has no reference to the coil supply voltage.
The fact the fuel pump wiring is also fed by this wire is especially concerning as any drop in voltage at high demand can/may result in subsequent reduction in fuel pressure, leading to lean running conditions!!
I would recommend avoiding interrupting a feed wire of this level of current draw, instead opting to cut one of the relay activation circuits, which draw a few milliamps at best. If you have no choice but to cut a wire of this size and respective current draw, use a 40Amp+ relay to switch the circuit, this can then be switched using your alarm immobiliser circuit and retaining your alarms integrity, and ensuring the circuit requirements are met
ADDITIONAL COIL INFORMATION (if you are still awake!)
Without going too heavily into ignition coil design, the fundamentals are that an ignition coil is basically a transformer, when current is switched to the primary winding of the ignition coil for a specific time a magnetic field is created, the strength of this field is directly effected by the voltage and time that the coil is switched on for (dwell) When the ignitor switches off the supply, the field collapses, this causes a voltage spike, depending on coil design will be in the region of 350volts, with a turns ration of 100:1 this will give a peak secondary voltage of 35,000 Volts.
The problem of voltage drop at the coil supply is that the current stored in the primary windings is a function of voltage and dwell, therefore reducing or increasing dwell or voltage will effect the spark voltage and spark energy produced by the coil. As a rule of thumb, a 10% drop in supply voltage results in a 10% drop in secondary Spark energy.
Note that spark energy is different to spark peak voltage (secondary voltage)
Hope this makes sense!
Chris
