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TR2/3/3A TR3 Coil question

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CraigLandrum

CraigLandrum

Jedi Hopeful
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The coil we have for our TR3 has two connectors. One is labeled with a "1" and the other with a "15". One of these goes to the distributor and one goes to the white wire from the harness. My problem is that I can't determine which goes where. Of course there is no problem with the central output to the distributor - that is obvious, but those side wires have me stumped.

In the wiring diagram, the way the coil is illustrated sort of hints that if I were to buzz from the central output to each of the side connectors in turn, one of them would have a higher resistance that the other and that one would go to the white wire, but I couldn't get either side connector to indicate anything other than an open circuit when tested with the central output plug. Is my coil defective or what? The resistance between the two side connectors was about 1.5 Ohms.

Any advice welcome on this topic.
 
I don't know about your particular coil, nor how you use your car, but there's a strong possibility it doesn't matter much either way. So try both, and if you can't tell the difference, don't worry about it!
 
That's an odd one as coils usually have + and - markings on the low voltage connectors.

How you connect it will also depend on whether you are running the original positive earth, or have converted/are converting to negative earth.

Perhaps the 1 is an I meaning to connect this side to the ignition switch.

As Roger says, if it doesn't run well one way then it'll be the other, so it's a 50/50 chance. You can't hurt it either way.

If your TR isn't running yet, temporarily swap it into another running car to test it, and set up the wiring.

Merry Xmas,

Viv.
 
CraigLandrum said:
... The resistance between the two side connectors was about 1.5 Ohms.

Any advice welcome on this topic.
Click to expand...

That is the wrong coil for the TR3 unless you're going to run it with a ballast resistor. The original unballasted coil should read about 3.0-3.5 ohms resistance between the two small connectors.
 
martx-5 said:
That is the wrong coil for the TR3 unless you're going to run it with a ballast resistor.
Click to expand...
Or with an electronic ignition (like a MSD or Crane XR3000) that requires a 1.5 ohm coil.

The high tension winding (between one side terminal and the center terminal) has a pretty high resistance (10kohm is typical), so depending on what you are using to buzz with, may well show as an open circuit. Because of the huge resistance ratio, it's pretty much impossible to determine the internal coil wiring that way. Also because of the large ratio, it doesn't much matter which way the primary is connected. The coil will generate a spark either way.

What does matter, some, is the polarity of the spark. At least in theory, you'll get easier starting, fewer misfires, and longer plug life if the center electrode of the spark plug is negative. In practice, I've found that the difference is almost undetectable, but that's the theory.

One way to check spark polarity is to hold a plug wire away from the plug terminal by a small amount, and hold the tip of a 'lead' pencil in the spark. You should be able to see a tiny flare leave the pencil lead inside the spark; and the flare should go towards the plug. If it goes towards the wire, the coil polarity is wrong. The bottom of this page shows the process (just the first page I found with Google). Of course there are other ways, too.
 
Oh, yeah, good point. Make sure you aren't touching the metal of the car or of the pencil at the time.
 
Terminals marked 1 and 15 are common on German coils like those from Bosch. Terminal 15 (on negative ground cars) is the supply from the ignition switch, terminal 1 goes to the distributor/points. The way I remember this is to think of the car's voltage supply... +12V should go to the coil's 15 terminal (higher voltage, higher number). The low connection goes to the terminal with the smaller number (lower volts, lower number). As with Lucas coils, if you swap polarity, you have to switch the coil terminal connections. (i.e. On positive ground cars, terminal 15 would go to the points).
 
TR3driver said:
martx-5 said:
That is the wrong coil for the TR3 unless you're going to run it with a ballast resistor.
Click to expand...
Or with an electronic ignition (like a MSD or Crane XR3000) that requires a 1.5 ohm coil.
Click to expand...

Good catch...thanks Randall.
 
Doug already provided the info I was going to pass on. You have a coil marked for a German wiring system. Could be a replacement for a German car, or could be a German coil...
 
Thanks guys - will double-check the primary resistance this week and will then put the meter on a higher scale to try and get a reading on the high-ohms secondary. Will also give the pencil lead test a go...

Of course I could simply remove a plug, ground the screw-in portion, and see if it is sparking across the gap...

Or even easier - could simply buy a new one that I know works :smile:
 
I should probably mention we have a TR4 engine (and probably TR4 ignition), and that we have a Bosch coil and we are running standard Positive ground.

Knowing that, should we replace the coil?
 
Check the primary resistance of your Bosch coil when deciding about the ballast resistor. The Bosch "blue" coil is typically a 3-Ohm (standard non-ballast) coil. The Bosch "red" coil is often a 1.5 Ohm coil for ballast ignitions. There are also black Bosch coils. I think most of those are standard 3-Ohms but it's best to check with a multimeter. There is no reason to change away from the Bosch coil if it is working and if you don't have an issue with originality.
 
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