Alternator testing experience.

[RamFan]

So I'm partly relieved and partly irritated about what I'm about to tell you. The Monte Carlo in my sig is my girlfriend's, over the weekend the battery went flat (just put this one in, in September). I immediately think that it probably has a bad alternator which caused it to run flat so quickly, then I notice that the sticker that displays the date is 5/12. So it sat for 4 months prior to my install. So now I know it is either the alternator, battery, or combination of the two. Could have been a perfectly healthy battery at install and a bad alt caused it to run flat. Could have been a partially flat battery at install and it just took a little bit to finally die. Or, option C, it's a combination of the two.

Took the battery back to Advance last night and they threw it on the trickle charger overnight. My girlfriend told her dad that is what I had done and that I would swap it out today after my class. He, an ex Chevrolet service technician, told me an easy way to test the Alternator is to pull the negative once its running.

I had never heard of this before so I did a little searching and found a bunch of people saying that this can damage modern vehicles. I don't question him though I just keep my thoughtful hesitations to myself and do as he asked. So about 30 minutes ago I replace the battery, start her up, pull the negative and she stays running. Yay moment. I let my girlfriend know she was good to go and I'd pick her up from class when she was done and she tells me that I was supposed to pull the positive. Fine. I start her back up, pull the negative, and she stays running. Yay moment number 2.

Now as I sit on my computer I decide to look into it a little more. I look at pulling the positive cable as a way to diagnose a bad alternator. So I find the same thing as I found when I looked up the negative. It can damage modern cars. At one point in time it was a good thing to do, but with modern technology checking with a multi-meter is the more appropriate measure (what I've always thought).

In conclusion, I'm relieved that the car wasn't damaged. But I'm irritated because I have no idea if this is a valid test. Should I just take it to Advance and have them test the alt like I was going to, or is this test good and the alternator is good?

 

[The_Jeff]

Knock it off. Doesn't matter if it's pos or neg, either cable takes the battery out of the equation. They say do the neg because if you ground your wrench it won't spark.

The technique you listed is rednecky and awful. You're lucky to have gotten away with it. It stresses power supply capacitors in your ECM, stereo, etc as I'm sure you read.

Your car probably has a BCM controlled alt. Not a huge deal, but another failure point. Parts stores can probably test these too.

The easiest test is to point your headlights at something at night while it's idling. Turn off the key and the lights should dim. Get fancy and use a voltmeter to see 14.3-15 volts on GM.

 

[Donald]

Four months is within the guidelines for a battery to be on the shelf. Does not mean I would not have looked back in the shelf for a newer one.

The basics of an alternator test are to measure battery voltage with engine off and running fast idle. Should be 12.5 & 14.3 V respectively. Use a decent DVM (not the free one from HF).

And an alternator should never be used to charge up a dead battery.

A carbon pile load tester can be used to load up an alternator and see how much it can put out before the voltage starts to drop.

 

[JHZR2]

That long for the battery to fail, and it isnt the alternator diode, which is always my first concern with a flat battery. Even if the alternator was weak, Im not convinced that the battery would fail that quick.

Im thinking a short someplace is causing an issue, and it has just gotten bad enough to create a problem...

 

[Dave Sherman]

Agreed, it's a bad idea. The procedure from my Honda's service manual is loading with a carbon pile with the engine at a certain speed and verifying the voltage is within a certain range. Lacking an actual carbon pile I just settled for turning on everything electrical I could think of.

Quite simply, alternators are guaranteed to have SOME output no matter how much the regulator tries to reduce output. If there's nothing to absorb some of this output, the voltage goes much higher than normal. The battery will start sucking lots more current if the voltage gets too high, so it will keep things stable. In older vehicles, this was a big problem, which is why there's a big warning label under the hood of my MG warning you to never disconnect the battery while the engine is running. A buddy of mine fried several light bulbs on his motorcycle that way.

One hopes that charging systems are a little more sophisticated now, and since there's a lot more electrical load that's on all the time then this isn't a problem, but I still wouldn't take that bet.

 

[Thax]

Take the alt of and have it bench tested at a local auto parts shop or service center.

 

[RamFan]

Thanks for all the responses. Anything y'all can suggest for further testing?

 

[mechanicx]

You should never unhook the battery while the engine is running or the ignition is on. For a long time GM cars PCM/ECM monitored battery voltage and would shut off if the battery was disconnected. Unhooking the battery is a poor test of the alternator output anway.

What you could do instead to check the alternator output is to turn on loads, headlights then blower fan and see if the dome light and headlights continue to dim.

 

[wrcsixeight]

A 'trickle charge" overnight on a medium sized dead battery, will perhaps bring that battery to 50% state of charge.

Flooded batteries can self discharge 15% a month at 75f, just sitting on the shelf. Once under 80%, the sulfate on the plates begins to harden. The longer the battery sits under 80% and the deeper under 80% it sits, the more capacity the battery loses due to sulfation, never to return. Like a fuel tank which keeps getting smaller as it ages. You can still fill it. It just has less and less to give.

Vehicular Charging systems are designed to NOT overcharge a battery. They are not designed to take a depleted battery and fully recharge it quickly. They are designed to take a 96% charged battery and perhaps try to get the last 4% back into it, but at a minimum to not let it discharge with every stock electrical option running.

So buying a older battery that was sitting on the shelf, installing it in a car that does short trips, and perhaps some radio playing without the engine running, is a recipe for premature battery death.

Removing a battery cable to check the alternator is unwise. This "test" can damage a lot of things, that the 'test' was designed to check.

A digital voltmeter is your friend. If the vehicle can hold 13.2 plus volts for several minutes with every electrical device turned on at hot idle rpm, then the charging system is working fine. If lots of rpm are required to hold 13.2+ then likely the alternator is running on 2 of 3 cylinders(diodes).

Testing volts on a vehicle with a less than fully charged battery will yield low results. It takes a while before the alternator can get a low battery up into the mid 14's. Also if the vehicle thinks the battery is fully charged, it will drop the volts to the mid 13's to prevent overcharging.

There is no set voltage you are looking for. 12.6 to 12.8 is full charged rested battery voltage. If you read this before cranking the engine, and then read more volts when the engine is running, the charging system is working. Perhaps not at 100%

Everybody blames the charging system for their battery death. Most battery death is due to battery abuse, by chronic undercharging, by expecting that an alternator easily and quickly returns a battery to 100% every time the car is driven, when this is in fact a very rare occurrence. It takes a lot of time to squeeze in that last 5 to 15% into the battery regardless of the charging source. The alternator is likely the least effective method in that state of charge range. lead acid Batteries perform best and last longest when they live their entire life as close to 100% as possible, and a green light on an automatic charger is no guarantee of 100% either.

Also battery quality these days is highly suspect. Corners are cut. An acceptable number of warrantied returns are factored into maximizing executive bonus.

The store which sold the battery will load test it, and find it still good 80% of the time. Surprise!!!!!!!!! Perhaps you should buy this alternator too...... Check out our fuzzy dice collection while you are here.

Get a voltmeter, and don't pull any battery cables with the engine running.

 

[Donald]

Originally Posted By: wrcsixeight
If lots of rpm are required to hold 13.2+ then likely the alternator is running on 2 of 3 cylinders(diodes).


There are 6 diodes in the bridge BTW.

 

[Chris142]

Removing a cable with it running worked fine on grandpa's 63 chevy with a generator. Doing that on a computer controlled car is asking for a smoked ecm.

 

[RamFan]

Originally Posted By: wrcsixeight
A 'trickle charge" overnight on a medium sized dead battery, will perhaps bring that battery to 50% state of charge.

Flooded batteries can self discharge 15% a month at 75f, just sitting on the shelf. Once under 80%, the sulfate on the plates begins to harden. The longer the battery sits under 80% and the deeper under 80% it sits, the more capacity the battery loses due to sulfation, never to return. Like a fuel tank which keeps getting smaller as it ages. You can still fill it. It just has less and less to give.

Vehicular Charging systems are designed to NOT overcharge a battery. They are not designed to take a depleted battery and fully recharge it quickly. They are designed to take a 96% charged battery and perhaps try to get the last 4% back into it, but at a minimum to not let it discharge with every stock electrical option running.

So buying a older battery that was sitting on the shelf, installing it in a car that does short trips, and perhaps some radio playing without the engine running, is a recipe for premature battery death.

Removing a battery cable to check the alternator is unwise. This "test" can damage a lot of things, that the 'test' was designed to check.

A digital voltmeter is your friend. If the vehicle can hold 13.2 plus volts for several minutes with every electrical device turned on at hot idle rpm, then the charging system is working fine. If lots of rpm are required to hold 13.2+ then likely the alternator is running on 2 of 3 cylinders(diodes).

Testing volts on a vehicle with a less than fully charged battery will yield low results. It takes a while before the alternator can get a low battery up into the mid 14's. Also if the vehicle thinks the battery is fully charged, it will drop the volts to the mid 13's to prevent overcharging.

There is no set voltage you are looking for. 12.6 to 12.8 is full charged rested battery voltage. If you read this before cranking the engine, and then read more volts when the engine is running, the charging system is working. Perhaps not at 100%

Everybody blames the charging system for their battery death. Most battery death is due to battery abuse, by chronic undercharging, by expecting that an alternator easily and quickly returns a battery to 100% every time the car is driven, when this is in fact a very rare occurrence. It takes a lot of time to squeeze in that last 5 to 15% into the battery regardless of the charging source. The alternator is likely the least effective method in that state of charge range. lead acid Batteries perform best and last longest when they live their entire life as close to 100% as possible, and a green light on an automatic charger is no guarantee of 100% either.

Also battery quality these days is highly suspect. Corners are cut. An acceptable number of warrantied returns are factored into maximizing executive bonus.

The store which sold the battery will load test it, and find it still good 80% of the time. Surprise!!!!!!!!! Perhaps you should buy this alternator too...... Check out our fuzzy dice collection while you are here.

Get a voltmeter, and don't pull any battery cables with the engine running.





Awesome info! Thank you.