Triumph Repair: 1978 Triumph Spitfire, solenoid starter, starter solenoid

Question
QUESTION: I am writing to you because I am having electrical problems with my car. I have changed the solenoid, starter and coil thus far. I fired up my car but noticed that my headlights were dim but thought the alternator would recharge the battery, then the other day I was driving stereo on and then plugged in my car charger. I drove a couple of miles then stopped at the store and when I went to start it up it wouldn't turn over. My buddy came over gave me a jump and the car made it to the house with lights fading. I can jump start the car and it will idle but if I drive too slow it shuts off. Lights go dim etc...
The battery is relatively new, I bought it in July. I didn't drive the car for a month or so so I'm not sure if that could have affected it. The battery was disconnected. I could use any help or ideas you may have.

ANSWER: Hi Dennis,
You need to run some tests. You need a voltmeter to test the system. Do this, first charge the battery and then put the volt meter across the battery posts (not the cable ends) You should see 12.5v or a little more. With the voltmeter still on the posts start the engine and run the RPM up to about 1500 RPM and read the voltage. It should read from 13.8v to 14.5v.
Another test is to remove either battery cable and with everything turned off put a 12v test light between the cable end and the battery post. It should either have no light at all or the test light should burn very dim. Put the cable back on. Now take the volt meter and put the scale on a very low volt scale and put the positive test lead on the (+) post (not the cable end) and the negative test lead on the (+) battery post of the starter solenoid and read the voltage when someone tries to start the engine. The voltmeter should read .5v or less. Now put the negative test lead on the negative post (not the cable end) and the positive test lead on a clean metal part of the engine and have someone try to start the engine. Again you should see .5v or less.
Also check the connection of the large brown wire on the alternator and the belt tension.
If all of this checks out ok, have your battery load tested at a parts store or shop.
Howard

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

QUESTION: Howard,

that was an outstanding response. I decided to run to the local shop and have the battery checked and it was shot, so the replaced it for free. Apparently, by changing the starter, it may have damaged the battery. I brought the new battery home and the car fired up right away. My follow up question is, is it imperative to turn off the radio after you urn off the car? I had a new car stereo installed a couple of years ago and was curios to know if turning the car on with the radio could wear down the battery? I also had a faulty light on my cigarette lighter that would stay on and I just took the bulb out. It seems to have done the trick.Not sure how much a spitfire can endure accessories. Any help would be greatly appreciated.

Answer
Dennis, if the radio is still on when you turn the key off than it will run the battery down but if it is wired so that the radio goes off when you turn the key off, then it does not matter if you leave the radio on. This is not true of some high powered amplifiers that draw current form several sources. So it depends on what kind of radio and how they wired it to the car.

You have something wrong in your wiring if the lighter light stays on with the lights off. You need to use a 12v test light and a wiring diagram to trace that circuit to see what is wrong.

If you don't have a diagram let me know and I will put one on my web site for you to copy.
Howard