Where do I measure the output from my Alternator?

esray

esray

Member
Dear All,

My battery keeps going flat. Fully charged, my multimeter is showing 12.96 volts at my battery terminals, but only 13.96 when the engine is running - Is this sufficient, please? Am I even measuring the output correctly?

Also, could someone please tell me which of the three wires (thick white/yellow, thin white/yellow and white/black go to the positive side of the coil and which to the negative side? Will leaving the thick white/yellow disconnected remove the ballast?

Please help me!

Ray
 
Ray

You measure the alternator's output across the battery terminals. I'd expect around 14.5V but just under 14V should be charging the battery provided you're giving it a long enough run and not heavily loading the electrics. Remember, when the engine is running the alternator is providing power to the car's electrics and trying to charge the battery. So as long as the alternator output is sufficiently higher than the battery voltage it will charge. There's a fine line between the alternator not providing enough to do both and providing too much and cooking the battery. Generally anywhere between 14V and 15V should be okay. Which alternator do you have? Does it have the built in voltage regulator (18ACR type) or a separate regulator in the passenger glovebox (11AC type)? There's more to a battery's performance than just the voltage across the terminals. It may be that there's not enough juice (cold-current) in it to crank the engine over. Do you have anyone who can test the battery for you?

Can't comment on the coil connections other than to say that the coil negative would generally go to the dissy points under normal conditions. In actual fact it doesn't make any difference which way around the coil is connected if points are fitted because the coil is just that....a coil of wire, well actually it's 2 coils of wire. Having said that, the spark plugs tend to prefer the standard polarity of coil negative to points, but the ignition system will still work whichever way around the coil is fitted. See this article here for an explanation. Why does coil polarity matter?. It's also a different story if you have electronic ignition fitted.

Also, as the ballast is generally in the coil supply (+12V) disconnecting it would also remove the supply to the coil.

Hope that helps a bit.

Dave
 
Dave3066 said:
Ray

You measure the alternator's output across the battery terminals. I'd expect around 14.5V but just under 14V should be charging the battery provided you're giving it a long enough run and not heavily loading the electrics. Remember, when the engine is running the alternator is providing power to the car's electrics and trying to charge the battery. So as long as the alternator output is sufficiently higher than the battery voltage it will charge. There's a fine line between the alternator not providing enough to do both and providing too much and cooking the battery. Generally anywhere between 14V and 15V should be okay. Which alternator do you have? Does it have the built in voltage regulator (18ACR type) or a separate regulator in the passenger glovebox (11AC type)? There's more to a battery's performance than just the voltage across the terminals. It may be that there's not enough juice (cold-current) in it to crank the engine over. Do you have anyone who can test the battery for you?

Can't comment on the coil connections other than to say that the coil negative would generally go to the dissy points under normal conditions. In actual fact it doesn't make any difference which way around the coil is connected if points are fitted because the coil is just that....a coil of wire, well actually it's 2 coils of wire. Having said that, the spark plugs tend to prefer the standard polarity of coil negative to points, but the ignition system will still work whichever way around the coil is fitted. See this article here for an explanation. Why does coil polarity matter?. It's also a different story if you have electronic ignition fitted.

Also, as the ballast is generally in the coil supply (+12V) disconnecting it would also remove the supply to the coil.

Hope that helps a bit.

Dave
Click to expand...

Dave, Thanks - Helps a lot!

Ray
 
The problem may not be caused by the charging system,something could be draining the battery. For example if the points in the clock have welded shut it will draw approximately 2 amps from the battery constantly. That's enough to flatten the battery in a day; ask me how I know!

Try disconnecting the battery overnight and see if there is any improvement.
 
pat180269 said:
The problem may not be caused by the charging system,something could be draining the battery. For example if the points in the clock have welded shut it will draw approximately 2 amps from the battery constantly. That's enough to flatten the battery in a day; ask me how I know!

Try disconnecting the battery overnight and see if there is any improvement.
Click to expand...

Also, how old is the battery? If anything over 3 years, it could be draining itself, and therefore due for replacement.
 
12.96 volts from a fully charged battery is quite a healthy reading, anything above 12.5 is good. To measure the voltage when the engine is running, you should do it after a run and run the engine up to about 2000 rpm. If the battery is drawing current to charge it then the voltage will be pulled down. If you measure it immediately after starting the engine then it is putting back the energy used to operate the starter. At about 2000 rpm with the lights on you should see about 14 to 14.4 volts. At the risk of stating the obvious, is the drive belt tight enough? If your meter measures current, then connect it in series with the battery with everything switched off and see if any current is being drawn.
 
Things I would try:-

1. Make sure there is nothing obvious discharging the battery when you leave it - like the boot light staying on when the boot lid is closed. If your multi-meter has a current range you can measure the current being drawn off the battery with engine and ignition off. Anything more than a few mA and I would get suspicious that something is drawing current.

2. Make sure battery is fully charged on a battery charger for 24 hours. Refit and leave battery disconnected when car is standing (if car always starts battery is good).

3. Try is putting in a known good fully charged battery. Although your readings sound healthy, sometimes sick batteries can give good readings and be internally discharging.
 
Could a wrongly fitted radio drain the battery? I fitted one and only afterwards (like 2 days) my battery died (might be a coincidence thiough)
 
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