coils??

[sahtuning2000]

on my 64 mk1 i have SW/CB on the coil,i have a new coil to fit,but what is +/- ?????

[Vanh]

I don\'t know if it\'s negative or positive earth but assuming it\'s negative you connect + to the ignition switch (usually white wire) and - to the contact breaker (usually white/black)
Van
 
 
 

[sahtuning2000]

sorry forget to say its positive earth!!!negative is 66 on

[Clifford Pope]

In that case I think the coil should still be connected as if the car had negative earth - ie assuming the coil was marked as if for a more normal negative earth system.
<br>The point is that there is no actual + or - sense to the coil itself, but the wiring of the primary and secondary coils means they share a common earth - ie there are two coils but only 3 terminals.
<br>If connected the wrong way round the HT coil is not earthed properly, but only grounded through the resistance of the primary coil. It will work, but the voltage will be reduced.
<br>The infallible test is to measure the resistance between each LT terminal and the HT. The one that gives the lower resistance should be the earth - ie + in your case.
<br>That\'s my interpretation anyway, after studying my 1956 Odhams Motor Manual.

[Clifford Pope]

PS why not switch to negative earth ? I have (1964) so that I can fit a radio cassette.

[Alec]

Hello SAHT,<br>
Positive earth car, then the + on the coil goes to the distributor, i.e. blackwhite.<br>
<br>
Cliff, the reason for correct polarity of coils is to do with the&nbsp;
spark at the plug, it is easier for the&nbsp; spark to go from the hot
centre electrode than the cold&nbsp; earth electrode.&nbsp; The&nbsp;
old fashioned test was to use a pencil lead, and hold it between the
plug cable and earth, if the spark spread out between the pencil lead
and the engine it was correct and vice versa. Why would you want a radio cassette, isn\'t the exhaust music enough?<br>
<br>
Alec <br>

<edited><editID>Alec</editID><editDate>38430.8950694444</editDate></edited>

[sahtuning2000]

thanks chaps for the advise i will go and fit it in a while,no point in changing to negative,the car has never had a radio fitted from new!!or mirrors,and its got a stainless exhaust,so i think i,am with alec on that one!,cheers

[Clifford Pope]

Surely the direction of the spark at the plug depends on the polarity of the system? If the car is +ve earth then the spark will be -ve. Swapping the coil terminals cannot alter that.
<br>If a spark really does have a \"direction\", then I take the point about the hot and cooler bits, but that would be an argument for the inherent superiority of either + or - earth, whichever produced a spark in the preferred direction.

[sahtuning2000]

all fitted and seems well,must find out why it always shows discharge(even during the day)give it a little rev and it goes into D,any ideas,it did it on the old coil,i think the bulkhead resistor plays up,no ign light on dash(only on start up)

[Vanh]

Alec is right, the spark really does have a direction. That\'s why you should turn the coil round if you convert a car from + to - earth. It\'s also why new coils have + and - on them instead of SW and CB because it\'s less ambiguous.
Cliff is right that the HT coil is connected to one end of the LT coil, and depending which end this is, in theory the HT voltage will be different. But it only makes a difference of 12V in tens of thousands i.e. no difference at all really.
If the car has a dynamo it\'s possible that it\'s discharging on tickover.
Van
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