Morty Mort said:
Well, resistance will change with heat, so it might be simple physics. Alex, you are better than I am at this, so please make a comment from the other side of the world.
M.
LOL at 'better' - hardly, Mr. M...
But yes, well done Aldino for having the patience to spot the problem. I've seen cases where the plug leads arc between each other on the distributor cap.
When you say 'contact breaker', I hope you mean the finned module on the distributor? I think the last of the contact-breaker-Unos were made with the 903cc engine. The FIRE has always had 'breakerless' ignition, to my knowledge.
Incidentally, I thought it was the alternator being overloaded ONLY in connection to the radiator fan being shorted out
I don't think that the smaller current of the ignition would be enough to overload the alternator.
Anyway, assuming you did mean the finned module, I think it's highly likely that its electronics were damaged by the arcing. Why the arcing should only occur at the higher temperature is a bit of a puzzle. It could be that the module itself generates a higher-energy spark when the transistors etc. are warm, when then caused the spark-plug lead insulation to break down.
In an effort to contribute something interesting...
This is my understanding of the conduction process, anyone please feel free to contribute if I have made any mistakes.
Most conductors obey Ohm's Law to a varying extent; V = I x R (Volts = Current x Resistance). Thus for a given voltage, a decrease in resistance gives an increase in current flow.
There are actually two types of behaviour exhibited by conductors (materials that conduct electricity). With metallic materials (e.g. wire, or the filament in a light bulb) the resistance increases with temperature. This is useful because it is effectively self-regulating; the light bulb glows, gets hot from the current flow, resistance rises (due to the random movement of the metal molecules impeding the flow of electrons), and the current flow is restricted. (This is also why light bulbs sometimes blow when first switched on (not when already on...) because the current flow is greatest when the bulb is cold, and the weakened filament can't cope any longer.)
Semiconductors (e.g. doped silicon as used in diodes and transistors) are the opposite story. As the temperature increases, the mobility of the charge carriers (electrons and 'holes') increases. This means the resistance decreases with temperature increase, so the current flow can increase (and this can result in a higher temperature). This is why you get a much bigger BANG when a transistor explodes
and also why most electronic devices include some form of protection (e.g. a thermal fuse).
Meanwhile, some types of transistors (particularly the high-power MOSFET type) were known in the past to be affected by an electric charge in the wrong place ('CMOS latch-up' if anyone's interested) that caused the transistor to effectively become stuck-on as the excess charge would flood the area that was supposed to regulate the current flow through the junction. And similarly, the whole anti-static business is designed to reduce damage to the CMOS devices in memory chips, etc.
From all this, I'm satisfied enough that the current in the transistors would reach a maximum when the electronic module was hot (being bolted to the distributor would mean it will warm up with the engine).
And, the high-tension current sparked across from the spark plug lead to the engine, which is where the electronic module is earthed to. So, this high-voltage 'spike' possibly found its way to the module's transistors, damaging them so that they could no longer handle the full current later, when the module was hot. One would have expected the module to be protected with diodes on the outputs, but nothing is ever perfect and with the electronics crammed into such a small space, the protection is probably compromised in some way.
We know these modules to be a little fickle, but it seems that maintenance of spark plugs and leads may prolong the life of the electronics?
-Alex
