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
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
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?