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Battery or Alternator

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Cain

Cain

Jedi Trainee
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The last couple of times I jumped in my '71 TR6 to drive it the battery was dead. How do I determine if this is a bad battery or a bad alternator?

Note that neither of the possible culprits have been changed out since I got the car almost three years ago. Also, I have no idea how old or how much use either have had prior to buying the car. But, I've only put about 1,000 miles on the car since I've had it.

I was able to jump start the car both times and it ran great. But, all I did was drive the car around on a nice afternoon for about 30 minutes and then return it to the garage. (i.e., No restarting needed.)

I tried to do a search on the forum, but it seems that all I got was a bunch of alternator conversion info (which may come in handy soon).

Thanks
 
You can check things out with a voltmeter. A fully charged battery should be about 12.6 Volts. With the engine running with no loads voltage should increase to about 13.5-14.5 volts depending on engine rpm and load. As you turn on lights and things, voltage will drop as the amperage goes up. It shouldn't go lower then about 13.2 volts. Make sure the alternator belt is tight enough. It can slip without obvious noise. Also, does the ignition light go on when you first turn the key to the on postion?? And, does it go out when the engine starts??

If the alternator checks out OK, then I'd suspect the battery. You can always have it load tested at your local auto parts store. They will usually do this for no cost.
 
Spend $5 & get a hydrometer from local auto parts, check the specific gravity in all the cells, they should all be at about the same level. Do not add water to the batt before you do this. If water needed add after you measure, charge & check again. If a cell is substantially lower than any others the batt is prob junk.

See this link:

https://www.uuhome.de/william.darden/carfaq4.htm
 
Take the car to the nearest large auto store. Nearly all of them (Autozone, CSK, etc) will test the electrical system for free.
 
Even if your alternator was the problem -- having the battery dead several times will surely mean you will also need a new battery.

If you have no political or social objections to Wally-Mart... they do offer a good selection of very reasonably priced batteries. Like with our tires, many of us will find battery replacement more a function of the number of years rather than actual use.
 
If you know for sure the battery is at least 3 years old then it's suspect based on age alone. Once they pass 2 years old they can go at any time.

Also if the car runs and drives fine after a jump start it would seem that the altenator is doing it's job.

Therefore I'd probably take the battery to a local auto parts store and get them to test it,....cuz most likely it's the problem.
 
For about $12 I purchased a tiny LED device with two alligator clips that gives me a voltage reading on the battery, from 5 - 14 volts. It gives a separate reading when you start the car and engage the alternator.

Sammy is right about the big auto parts stores; they all offer free checks, right in the parking lot, of the car's electrical systems. You may have a good alternator that's trying to charge a worn out battery. The battery then won't hold the charge.

When you start the car, shield the generator light so you can see if it's dimly lit even when the car's running. If so, the regulator portion of the alternator is shot. It can be repaired separately. Your alternator would check out fine but the regulator wouldn't be sending a charge to the battery.

Also, an earlier post asked if the red light went on when you turned the key to "on." That's important on British cars with alternators as the bulb is actually part of the circuit. If the bulb is good and it's not lighting up, then the alternator is bad.

Good luck,

Jeff
 
Thanks for all of the replies. The red light is coming on when I start the car. I'll run to the parts store at lunch. I grabbed the battery out of the car this morning on my way to work, so I'll have them check it out at the store.
 
You may find the auto parts store wants you to bring the whole car with the battery installed if you want them to test the battery. Some like to load test the battery and the charging system together to determine where the problem is. They're always happy to bench test an alternator by itself out of the car.
 
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