Triumph Repair: POINTS OVERHEATING AND MELTING THE CAM PROBLEM, resistance values, range rovers

Question
QUESTION: Hello Howard,
I too have a British Car and it has suffered from the followig prpblem. The resistor ballast wire as mentioned in the BL diagram is exposed and melted. I have had to take a wire form a 12v positive rocker switch to the coil. however I am  sure that the coil is getting too much  voltage or current. When running at idle there is little air rush over the distibutor and the points get so hot that they burn you  finger white if you flick them, but they  also get so hot the plastic cam  melts down but my bit, it only takes a few minutes for this to happen - then of course the  engine stops as there is no points gap to dump out the coil.  my question is, can I fit a heft resistor to a)  take down the current  or  b) the voltage.   The coil is an intermotor coil from about 1998  but the car has only done about 5000 miles since then, mainly because it keeps breaking down and I no longer have any confidence in it.  Many thanks for some resistance values.

ANSWER: Hi Edward,
You have a British car? Any particular brand or year model?
Howard

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QUESTION: Well I kept it out of the question on purpose Howard, in case you  told me straight away that you didn't help on RANGE ROVERS.  It is a 2 dr 1978, just about as simple as you can get, except that I cannot solve this problem.  However it is from the same bonkers stable as Triumph so in this regards, I wonder  whether you could make an exception?   I wonder if you know the maximum current the coil should be getting, when  running ( not starting ) and  the sort of resistor I should be fitting in series  to it, ( I have to use a switch on the dash to energise the coil) The switch works well enough, however, bit by bit the plastic cam melts away, until it is so small that the eccentric on the distributor lobe  does not activate the points. I have had 14 sets of points in the last 4 years and  2 of these years  have been non-running off the road. I am sure there is a solution.  Thanks Howard

Answer
Hi Edward,
You are correct that a pointed ignition is probably the same as it is in any of the MGs etc. You may have to dig up a wiring diagram of the vehicle for some of the power for the ignition as I don't have it.

The over heating of the points is a clear indication that too much current is traveling through the points. This can be caused by the wrong coil or the lack of a resistor if the coil required one. Or a problem in the wiring.

I can give you a few things to look for. Most cars that use a low ohm coil (called a 6v coil) use it because in the "Start" position there is less than 12v available to the coil due to the starter motor drawing too much of the current. So they install a 6v coil and when starting the coil may only have about 9v to operate with so the 6v coil works fine. However this creates a problem as soon as the key is released back to the "Run" position where now a full 12v is now available to the 6v coil which conducts far to much current and it overheats the coil and the contact points. So the manufacture then designs the system so that when the key is released back to the "Run" position the current no longer is routed directly to the coil but is routed to a resistor or a resistor wire and then on to the coil thus cutting down on the current and available voltage thus not overheating the coil nor points.

To decide if it is a low ohm coil you need to test the primary windings of the coil. (+) to (-) should read about 1.3 to 1.5 ohm for a 6v coil and probably over 3 ohms on a 12v coil that don't use a resistor system.

A wiring diagram of your year model car will show you if a resistor system is used and thus tell you which coil should be used and allow you to test to see that current is not being directed direct to the contact points because of a wiring problem.

Sorry, that is the best I can do for you as I have no experience nor info on any of the Rovers
Howard