Got spark by resistor on magnetic pickup

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?
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A poor soldered connection somewhere on the pickup coil can cause the resistance to go high as temperature increases. A coil such as this is usually constructed from enamelled copper wire wound around an iron core.
As the resistance of the coil increases, then the voltage developed by it up to a point will stay the same, until a load is put upon it. As the current drawn from the pickup increases, the excessive resistance will have a voltage drop across it and reduce the voltage at the output. Putting a resistor across the output will reduce the resistance of the coil as seen from the perspective of the ignition amplifier, but it will not make any difference to the amount of voltage output by the coil. In fact, it could make it worse!
An oscilloscope has a very input resistance, so even with a faulty coil, a reasonable output voltage will be attained under test.
HTH
Anthony Remove eight from email to reply.
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Yes but wouldn't that be a sudden change?
A coil such as this is

Yes, but of course I checked the voltage with a resistor across it.
I suppose I had over 6V peak to peak with no resistor, and at least 4 with a resistor, and the heating did not seem to make much difference, though perhaps I was a bit impatient to get on. At first I tried it in the car with a 2700 ohm resistor, the problem continued, so I added another 1200 ohm, and was able to start each time with a hot vehicle. About 1/4 watt resistors fit well inside the transistor module on the side of the distributor.
I am presuming that the transistor switch is fairly good at discriminating the peaks in the coil signal, provided its input is held down well enough so that it is not always switched on.
Is my coil contacting through ferrite or something?
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This might sound a little rude, but it would be a whole lot quicker and a lot less hassle just to replace the damn sensor :)
Hellraiser.........>
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IME the sensor isn't available separately - and I was told by a fairly good garage the distributor isn't carried as a spare anymore.
I got one a few months ago for a Panda (with exactly the same fault) for 20 UKP from a scrapper.
Out of interest, I understand the pick-up device is a Hall-effect switch.
Regards
John H
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If it's anything like the flywheel sensor on the Tipo, it isn't a hall-effect, there aren't enough wires coming out of it for that :)
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D'oh!
Absolutely correct: 3 leads on the HE device..
Doesn't that make the pick-up a coil then??
Must be a lot of thin wire for it to be 800 Ohms, though.
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Don't think that was invented in the days of overhead pairs.

Of course. Makes a nonsense of people worrying about the typical cable impedance on Hi-Fi interconnects.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Possibly, but 30 years ago I was using cabling ( OB to exchange ) which was 600 ohm - with all the bridged "T" balanced equalisers and stuff I can hardly remember - and I'm fairly sure it was star-quad (paper wrapped though!). I think there was still overhead then - but mostly for voice.

Let's not get started on Hi-Fi and the gold-plated connectors, shall we? ;-)
Regards
John H
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I'm trying to remember when star quad arrived for studio use, but it would be about then. And sorted out the problems with the also new thyristor dimmers which caused induced buzz.
IIRC, 600 ohms came about as it was the impedance of a pair of overhead telephone lines stretching to infinity. Or thereabouts. ;-) So would date back to the very early days of telephony.

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Thorn Q_File dates back to the late 60's AFAIK - they were certainly in use in the Concrete Doughnut pre 1972
/nostalgia mode on ferrite core memory /nostalgia mode off

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Yup - think TC1 had the first one.

Now what were those 'valve' thingies in Riverside? They *were* impressive, glowing away. Thyratrons?
Is this the most OT post ever? ;-)
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Thinking about it - star quad is "only" two twisted pairs, it's the balancing which gets rid of the common mode nasties: it's what the Comms folk had been doing for years. Dialing pulse trains in adjacent cables over countless miles of cable, starting with 0.775 volt as "zero level" signal. ISTR -43dB S/N unweighted was the limit ( after equalisation )... -56 weighted, maybe.. it's all too long ago.

Riverside had shut when I got down there - but there were people with fond memories of rowing about on the river at lunch time... Phil Irons ??

Quite possibly :-)
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I was doing the resistance measurement DC.
I was wondering if there were meant to be some temperature effect, a sort of thermistor in the pickup coil to slightly increase the pulse width with a hot engine. But it is revs, not temp which requires different ignition?
Though, as said, there must be a long length of fine wire for 800 ohms, I don't see how it could increase in resistance 6 times or so.
Is there a very thin place which is getting sqeezed by the other wires nearby heating? But I don't think this would be happen to any regular extent.
In reply to the fuss to add a resistor, it only requires taking out the screws which hold the finned electronic unit to the plastic mount on the side of the distributor, and joining in the resistor somehow near the plug connectiton from the pickup coil.
Replacing the pickup coil risks damaging the nylon arm from the vacuum advance. That being said, you might want disconnect that anyway to renew the wires from the pickup coil. They flex and break - intermittent at first. As I said I used desoldering braid.
When I was going to buy my petrol I was still thinking I had to wait till after 1 Sept fow spring, lower vapour pressure petrol. I thought high vapour pressure had been a cause.
To get back off the topic, i) what will 10% ethanol in petrol do to Uno parts? ii) did silicate in antifreeze cause my water pump seal to start to leak after about 4000 K, when it was a good seal before? (Car about 74K Km)
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Or impedance is the resistance at a given frequency.

That's nonsense. Think you're confusing this with the capacitance per meter.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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