Diagnosing a bad alternator?

I have a 2010 toyota tacoma that i suspected of needing a new battery a few months ago when it was sometimes a bit sluggish to turnover. Took it in for free testing at an autoparts store and they told me the battery was fine but that the alternator was bad and I was driving on borrowed time.... I was told to take these testing howevervwith a grain of salt and ultimately decided not to replace the alternator, as a mechanic told me the alternator tested fine. That was about 3000 miles ago. I have driven the car for 15 hours a day without it dying so I would suspect the alternator is fine. This week I came outside to start my truck after not starting it for about a week and it was dead with weak lights still functioning. I was able to jump it and drive it just fine but the next day it was dead again. A jump worked again. Took the battery in to get tested and was told it was good but again the alternator tested as failing at the autoparts store. I have not had any battery light come on while driving and the battery has not died in the last few days again. Wondering if I should spring for a new battery or alternator or by a multimetro amd just test it all myself? Thanks in advance!

 

Id start with a new battery since that's only $100 and quick.

From there, if that fails over the next few days, then you know 100% sure it's the alternator ( which is a bit more work to replace ) Also, make sure your cables connections are clean.

 

All electrical loads on, disconnect negative cable from battery while running. That will tell you if alt is charging regardless of battery condition.

 

Acid covered battery terminals or oxidized posts can cause the symptoms you have. A $5 battery brush available at any auto parts store could solve your problems.

Do you need an alternator? Possibly. If you don’t have the necessary tools and knowledge, have it tested by someone trustworthy.

 

As with any repair always start where the weather is the roughest. Clean and tighten all connections including the ones on the fender. Also clean the top of the battery. Make sure your connections on your alternator are tight and no wires have rubbed on anything. Does your battery warning light work on start up and then go out with the others ?

 

Not sure I'd do that, good chance to damage sensitive electronics such as the PCM or the ABS module.

 

High risk advice for anything with an ECU.

IE, anything roughly in the last 40 years (early 80s on)

Don't do it.

 

Done it a million times without issue. Your results may vary.

 

People get away with murder all the time, consequences still the same if caught. The juice isn't worth the squeeze. Use a clamp on ammeter and a voltmeter and that's all you need to diagnose the charging system.

 

You gotta crack some eggs to make an omlet sometimes

 

not in today's economy, I can't afford that kinda meal.

 

Or it’ll cause a voltage spike that fries your ECU. Probably fine but not really recommended, especially because simply holding a VM to it is even easier.

 

Look i get it, i understand how it works and the risks involved. I have been personally using this method taught to me. It works 100% percent of the time and I have never had, or seen any ecus get fried. I work on cars for a living and i think that carries a little bit of weight in giving advice like this. If he doesnt want to try it, thats fine. And sometimes by the book isnt always the easiest route for someone who cant read through a flow chart with specific pinouts and a comprehension of the data shown. Im defending myself through your quote but its not directed specifically to you.

 

not being able to read a multimeter should be step 1 of realizing you don't know how to troubleshoot electrical issues and take it to a pro.

 

If going by the book, make sure your ammeter is good for at least 30A. Cheap DMMs are definitely not up to that task. Or use a clamp on, but not everyone has one of those

 

Ok, I’m with you fellas.

Lol, my vote is don’t. As taught to me by a tech with 40 years more experience than I got.

I think if you got an older vehicle, say with points in the distributor, ok yes. In something more modern, there’s too much that COULD go wrong.

Seeing how op is asking how, I’m assuming he doesn’t own a multi meter or amp clamp. High beams puts on a good load but, I have a load tester I’ll pull out occasionally.

Battery testers I don’t really trust. If I use my load tester as a battery tester, it fails everything, new batteries included sometimes.

I’ve seen many no start, or no crank, or intermittent both which turned out to only need battery terminals and posts brushed clean.

Lot of the techs I work with will just stick in a new battery. I prefer to be honest, test, clean, retest, then let them decide after giving my opinion. It’s 4 years 6 months old, testing passed but I think it should still be replaced because it’s old. It’s 2 years old, only needed cleaning, testing good, doesn’t need replacing.

 

Just wondering how long it is between shutting off and restarting - could it be a parasitic draw ?

 

I agree.

The DC clamp ammeters have come down enough to finally be affordable.
I think everyone should probably have one if they are going to DIY.
The cheaper ones aren’t too reliable with anything under .5a or so, but plenty good enough for DIY such as alternators.

 

The battery can show 12 plus volts with a multimeter but not pass a load test. A multimeter will tell you right away if your alternator is putting out 14 plus so you do want to check that out. OP, how old is the battery? My money says the battery needs to be replaced.