Triumph Repair: Ignition failure, triumph spitfire 1500, starter solenoid

Question
QUESTION: 1975 Triumph Spitfire 1500.
Car starts well, mechanically everything is always fine, but sometimes when i turn the ignition key, I hear a little pop sound (sounds like a nestea or milk bottle cap seal being pushed down) and thats it...then i wait a second or two, try again, car turns on fine.

Yesterday, it never turned back on, just that one pop sound everytime I turn the key.  I understand its a relay engaging or something like that.  How can I trouble shoot this to isolate and repair the problem?  I know lucas is not famous for their reliable electrics.   I also have the repair manual if that helps.

Thanks so much!

ANSWER: Hi Daniel,
You will have to catch it in the failed mode to test it. Or you can put a long wire on the trigger wire on the starter solenoid (white w/red tracer) and run it to a test light and place the test light where you can see it when you try to start it. If you hear your sound and the car don't spin over look at the light to see if it lit up. If it didn't you have a failed starter relay and if it did light up you have a failed starter solenoid or a loose connection in either the positive battery cable or the ground cables.
Howard

---------- FOLLOW-UP ----------

QUESTION: Sorry, stupid question.  For the test light, I hook one wire to the trigger wire, and where do I connect the other wire?

Answer
If you are using a two wire test light put one lead on the solenoid trigger wire (white w/red tracer) and the other wire of the test light to ground so the light will light up (IF) power is sent to the solenoid. Thus you will know that the solenoid was sent the power to activate.

If you get a light and nothing happens, move the test light lead to the battery cable connection on the solenoid and it should light the test light and then hit the starter and watch the light. The light should stay bright. If it goes out or dims, You have a bad connection on that battery cable either at the battery or at the solenoid.

If the light stays bright but nothing happens move the test lead to a good ground on the engine block and put the other end of the test lead on a good ground on the body and hit the starter again. If the light lights (even dimly) you have a bad ground strap on the engine or a bad ground cable on the battery or the body.

Howard