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Sometimes it's better to use a decent test light when checking electrical circuits. A sensitive digital voltmeter might well detect near enough battery voltage through a faulty connection, but the same faulty connection might not allow enough current through to light a 21watt bulb.
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Yes but it doesnt explain why voltage disappeared ?
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what happens to the voltage at the + of the coil if the neg is disconnected?
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Residual voltage from the condensor? Where did you measure between?
Kevin...
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to elekie.
voltage disappeared when lead was connected to coil whetehr -ve lead was connected or not. ie only lead on the coil and it still disappeared.
To kevin, voltage was measured from end of lead to earth.
As a help, a lead direct from battery to coil +ve allowed car to start but my problem is understanding the disappearing voltage.
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If the neg lead is not connected then the coil cannot draw any current ,unless it has an internal short to the casing of the coil and it is finding an earth path thro the bracket where it is fixed.Try unbolting the coil so that the case does not touch metal.
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ok, but did this.
coil sat in its bracket or held in my hand and the same result.
I am not electrical specialist but i am son confused with it I had to post it !
The same coil runs, in or out of its bracket, with temporary lead direct from battery, disproving coil failure ????
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This is all true, but, unless the coil has a serious short to earth - expect to see serious sparking during the connection of the +ve wire with the ignition on, then the wire that has the ignition live probably also has a high resistance (duff!) connection upstream.
The point made by none was a very good one. If you test with a voltmeter, you will read 12v until you connect up and try to draw load, because the meter itself draws virtually no current - the meter doesn't load the circuit in a realistic way. If you test points ignition circuits with a 21W light bulb, you will not be misled. I would recommend testing with a bulb, and leaving the voltmeter in the toolbox.
If you are testing an electronic circuit - say a coolant temp sensor circuit, then I would use a voltmeter - the bulb would then draw too much current, and pull down the voltage, because the ECU can only source a limited amount of current.
Number_Cruncher
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As a matter of interest what is the car?
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As a (perhaps perverse!) example, imagine trying to run the ignition using the alternator warning wire as the live feed to the coil.
With the ignition on, and the wire connected only to a voltmeter, you would read 12 volts, because there would be virtually no current flowing through the meter to earth. In other words, all the voltage drop is across the meter, because it is the highest resistance in the circuit.
Connect the wire to the positive connection of the coil, and, if the points are closed (which is usually the case when engines stop naturally), the voltage you would read would drop to almost zero - once the circuit is passing current, most of the voltage is dropped across the dashboard bulb.
OK, this was a silly example, but, it shows that none's point above was correct - it does explain how the voltmeter reading drops to zero upon connection.
If you test with a 21W bulb, which draws a decent amount of current, it would never light, with the wire disconnected, or connected - and so, you would not be misled.
I hope this helps you find the fault. IMO, you need to look for another wire to feed the coil.
What's the car? It wouldn't surprise me if someone on here might know what colour of wire you should be looking for!
Number_Cruncher
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car is vw transporter {added to subject header - DD} with so many extra wires i dont think anybody could help with telling me what colour wire to look for.
Have run a new lead as per the wiring diagram and car starts and turns off as it should; problem solved.
Out of interest I will return to car and test with test light and report back.
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But what confuses me too is that vehicle was running before this. Logic says a wire must have fallen off etc.
Could only find 2 possible wires that would reach/fit and no voltage when done so!
Although I understand the comments I dont understand how its happened if you understand!
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It's good to hear that you have got it running.
I don't know these vans in any detail, but I wouldn't be too surprised to find that this is a ballasted system - you might find a ballast resistor somewhere - the resistor and/or the connections to it might be a bit dodgy.
On the other hand, if the wiring diagram you have shows a direct 12v supply, then your direct wire will be OK.
If the system is ballasted, and you run it at 12volts, then you might find that the points life is a bit short!
Number_Cruncher
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According to info it is non ballast, but if it is then points life is not a major consideration short term, but can always fit 1 in, in future!
Is nice to go back to basics for a change and to test the old grey cells but it was a test!
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what you have done here is to discharge the capacitor thats why you got a o.oov reading ,you need to let the distributor points re energise the condesor with rotation thru 360 to get the spark
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If you measure the coil primary resistance straight across the coil tags with oce disconnects then note the resistance and check in the manual it will reveal whether you have a ballast resistor somewhere as the basllasted ones have the lower primary resistance. I would have thought you would have a ballst resistor on this model. Best check as you can burn out the coil not just the points. Regards Peter
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if it was the original coil I would be fully agreed.
Its not, am doingmore research on it but as far as I can tell on vehicle and wiring info its not ballast sytem.
Short term its not a problem, long term appraoch is ok.
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Is it a 1970s Transporter?, I had a real nasty problem with the engine not starting. It turned out toi be the ignition switch not letting power to the coil when the starter was been turned.
VW have this habit of cutting off lights etc, but iot was a faulty ignition switch that cured it.
Very frustrating, so was the body rot!
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had this problem with a rolls royce once 12 v with wire disconnected but no voltage when connected, simple enough, you can have 12volts but no amps, check with voltmeter 12v but put a test lamp to it (with bulb not l e d) and it wont light up, the fault is further down the line possible ing switch as mentioned earlier
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ty for help but ignition switch dismissed as problem as new wire has cured starting problem.
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all good points
i cant add anything other than multimeters really can have you going in the wrong direction and a bulb is often far better
with regards the ballast resister this does need checking,maybe look on the coil fitted for votage it runs at or change for a 12 volt one if winter starting is not a problem on this vehicle
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its unlikely to be a ballast resisted coil if it was then he would have only 9v to the coil, it only gets 12v during cranking
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