The Fiat Uno 45 FIRE 999 1988 has a breakerless igntion system. In the distributor a four-poled rotor passes a magnetic coil pickup and induces a voltage which switches the ignition coil primary current via some sort of electronic unit.
This car's first troubelsome symptom was a number of years ago when it would not start hot. It seemed to be worse when on a slight downwards slant and I thought it to be the petrol. More recently I was parking it with the air cleaner off if I wished to start it again within the hour.
Then someone suggested to check the spark as they knew of trouble with the magnetic pickup coil going high resistance when hot. I was sure it would be the fuel evapourating too much, perhaps out of the float chamber vent holes, and condensing inside the air cleaner. But I decided to check for spark and there was none at the offending time.
So I measured the magnetic pickup resistance and it was OK when cold at about 800 ohms. But when I heated it a bit with a fam heater, after a while the resistance started to rise to a number of thousands of ohms. When I stopped the heat, after a while the resistance gradually crept back down. What sort of wire is in those pickup coils?
I connected the pickup coil to an oscilloscope and when I rotated the distributor shaft by hand a voltage of a few volts was generated. This continued to work when I heated the pickup coil.
Knowing something aout electronics I thought the pickup coil might be needed to be around its proper 800 ohms to get a transistor of the switching unit to work properly. When the resistance of the pickup coil would go too high the input of the transistor would float up too high and it would stop switching. So I thought to put a resistor across the pickup coil to hold the transistor input steady, hoping that it would not reduce the pickup pulse too much. About 800 ohms has done the trick it seems.
I suppose I shall have to eventually part out quite a lot of money for a new pickup. A few years back I salvaged it when the flexible wire broke inside the distrib. - I replaced them with some de-soldering braid because of its flexibility, and found some flexible insulation to cover it.
But how does that resistance change by a factor of 6 or something?