Spark from Coil: sometimes YES, sometimes NO

[alfredhamstra]

I thought I solved my mystery:
My 1981 FI Turbo has a problem that the ignition stops working random.
It means (sorry for my bad english...) that sometimes it fires up immediately and sometimes dont start at all.
When I take the sparkcable out from the distributor and put it near earth, and I crank the engine, then there is no spark.
I thought I solved it when I saw that one connection from the pickupwire into the ECM was more in than the other, so not toughing (I thought...)

What i did...
I put a new ECM (though the old one had similar ohm values)
I measured the pickupcoil (750 ohms)
I measured the coil (0.7 and 11000)
I measured the cable (1 k ohm)
There is always a good 12.6 Volts on the positive wire B+ from ignition
pickupcoil to lobe on distributor is 0.55mm

Is there a component that can be sometimes good and sometimes not?
There is not much difference in temperature in my garage...

Any thoughts?

Alfred

[18Fiatsandcounting]

Alfred, I'd try reducing the gap from the distributor lobes to the pickup sensor. I know the spec is 0.55mm (if I recall correctly), but I've seem some pickup sensors that worked better with a gap of around 0.4mm.

Another possibility is that your ignition switch is not making reliable contact, but if you see +12V on the coil but still no spark, then the ignition switch is probably fine.

-Bryan

[alfredhamstra]

Update: i just replaced the lot (distributor, ECM, and white pickupcoil).
Now it is fine, but I dont know which of the 3 were bad... I expect the pickupcoil went bad...
Ill mark them for the next car...

[spider2081]

A few different thoughts: The 2 pin connector of the pick-up coil that mates to the ECM sometimes has issues. sometimes the pins float in the black molding and move back into the molding when the connector is pushed on the the ECM. They then don't make a solid contact. Sometimes they loose their tensile strength and simply make poor connection.
Another possible problem is the magnet cracks and brakes into a few pieces. I think this reduces the strength of the magnetic field creating a weak pulse to the ECM.
Did you move the pick-up wires around in the distributor while checking the coils resistance? sometimes a wire breaks inside its insulation.

[alfredhamstra]

That could be it, the magnet had a little crack in it, not all the way trough, but from one side of the screwhole.
It also was a bit thinner than the new one.
Did not know it makes this much difference...