Dale: Do you have a volt-ohm meter? Here is a simple test you can make. See really bad diagram below. The ignition coil (conventional Lucas type not electronic) is a simple auto-transformer. It has a primary winding of perhaps hundreds of turns and a secondaty with many thousands of turns. First, isolate the coil, that is, disconnect the wires that go to the small terminals on top of the coil(one from the wiring harness , the other to the ditributor). Leave the high voltage lead (the one that comes out of the top of the coil) connected to the coil, but disconnect the other end where it attaches to the dist. Take your ohm-meter and set it at the LOWEST setting (maybe 1 - 10 ohms) Now connect the meter probes to the 2 low-voltage coil terminals. What do you read. You should measure between 3 - 5 ohms. Now set the meter for something like 20,000 ohms. Connect to terminals 2 and 3. You should measure maybe between 5000 and 15000 ohms. I have a Lucas Sports coil on the TR250 and I just got a reading of 3.1 ohms for the primary winding and 8.46 k ohms for the high voltage winding.
o(1)---MM(lv winding)MMM---o(2)---MMMMM(HV winding) MMMMMMMMMMMM--->(3)high voltage to top of dist.
In other words:
For my Lucas Sports Coil
from terminal 1 to 2: - <span style="text-decoration: underline">3.1 ohms</span>
from terminal 2 to 3: - <span style="text-decoration: underline">8460 ohms</span>
These reading are from a known good coil that has never acted up. But, these readings only tell you that the windings are not open or shorted, when the coil is not energized and dead cold.
What can happen is that heat will build up in the coil from the current and also because the coil is mounted on the engine block. The heat can cause the high voltage winding (very small wire, many turns)to expand and contract. Sometimes there will be an intermittant open circuit when it gets real hot. When it cools, the circuit will close and everything looks normal. This can be frustrating. To simulate this problem you would have to take the readings while you applied some heat source, like a heat lamp. Your not going to hurt the coil because it gets pretty hot under the hood of a TR6. if you do this experiment and you see no appreciable change, the coil is probably OK. If the readings get very high or if your meter indicates infinite or open circuit , the coil is probably bad. it is also possible for the coil resistance to decrease with temp, but this means that part of the windings are shorting. This also indicates a bad coil. Your resistance reading will increase a little due to the normal change in conductivity with increase temp, but this will be nominal.
You can also test the coil to see if it is producing High Voltage either hot or cold. If you have a 12 volt power supply or even your car battery (please connect a fuse in series if you use your battery), connect 12 volts across terminals 1 and 2. Now, if you disconnect one of these two wires, the magnetic field in the coil will collapse and a high voltage pulse should be produced at terminal 3. This only lasts for a moment. You can test for high voltage at terminal 3 by taking a screwdriver with a plastic handle and bringing the high voltage wie near terminal 1 or 2. Be careful or you 'll knock yourself on your keester! A nice fat blue spark (maybe .25 in. or more) should jump to the terminal. If the spark is yellow and very faint, you either have a bad coil or low battery. If you have a battery charger, connect it for a while and try this test again. Fat blue spark = good. skinny yellow spark = bad. This may only happen when the coil is hot or maybe no spark at all.
Good luck!
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