Dead battery or bad alternator?

My mother-in-law has a 1998 Isuzu Rodeo. Six months ago the battery died and we replaced it with one from NTB. The car was fine until two weeks ago when it died while she was driving. Her mechanic said it was the alternator (which is what I suspected) and he replaced it.

Last night she was driving and the car started losing power and the interior lights started flashing. Luckily she made it home. I tested the battery and it showed 10v. I jumped the car and got it started. With the car running, I took off the jump and tested the battery and it showed 12v. I let the car run for about 20 minutes, turned it off, but then it wouldn't start again (just a clicking noise when started).

She brought it back to her mechanic this morning and he said the alternator was fine and the battery needed to be replaced.

While the alternator still appeared to be charging the battery somewhat, does that necessarily mean that it is working properly? Or does this sound like the battery is the culprit?

 

when the car is running, You should read ca. 15 Volts on the Batt terminals. 12V is definately too low of a reading with engine running. Sounds like the Alternator is no good, and neither is that mechanic. Have it replaced (for free) with a new one.

 

Thanks--that is what I am suspecting (plus I don't like this mechanic either).

So last night, when the car finally died in her driveway, the battery tested 10v. Would this be because the alternator wasn't charging the battery when she was driving and the battery was being drained (assuming the battery is good)?

 

Yep. Regardless of the battery condition, once you get it started you should be able to get a 13-14 volt reading for alternator output and drive it indefinitely even with a bad battery as long as you don't switch it off. If it's putting out only 12v, alternator likely is bad as mark said.

 

Well the mechanic is either dishonest or very incompetent.

I tested the battery they removed from car (the one they said was bad) and it tested at 12v. The new battery in the car tests at 12v but with the engine running it only tests at 13v. And with a full load running (lights, radio and heater) the battery actually tested at 11.86v.

So my other question is this: We now know the alternator is the problem, but how do we know if it's the alternator, the voltage regulator or a poor installation (loose belt, loose bearings, etc.)?

 

Most modern alternators have an internal voltage regulator. I would assume this Rodeo would too. As for a belt - well, you'll just have to perform a visual inspection.

 

Please take the alternator and battery out of the truck, bring it to AutoZone or Advance Auto and have them PROPERLY tested. Sounds like you are just measuring voltage on them which is not a complete test.

Then if they are both good start looking into the terminals/connections and cables. Make sure the connections at the battery and starter are tight and clean. Maybe measure the resistance of the battery cables to see if they are ok. Check the ground connection from the battery to the frame.

 

Had an old honda civic (1980?). Drivin down the road, bling! ALT light came on. Got home ok, checked it out, aha, bad alternator. Replaced it (in the rain, had to loosen motor mount to release tension on belt, OMG who designed this), ok for 2 wks or more. Drivin down the road, BLING, alt light again. Tested everything, ohmed out the wires, Took battery to be tested, everything ok. Musta been a bad rebuild. Got another, under warranty. Checked voltages, unplugged control voltage connector (can't remember now what its called), alternator volts did what it was supposed to do. Ok, done with this job finally

Nest day, BLING, ALT light on. ARRRGHHHH

Took it to a shop, 30 min later, they called said it was fixed.
Partially broken main lead from alt to battery, when it would open, the alternator would have nowhere to send the juice that was being called for from the battery, so it would overheat and burnout the diodes in the rectifier bridge.

BTW, I took it to a shop that specialized in auto electrical systems, not a general mechanic. $50 and a $5 parts charge if i remember right.
Lesson learned, sometimes the experts are.

 

That's a good suggestion about bringing it to an auto electrical shop. I'm having my mechanic look at it today and I'll bring it to a specialist too.

Either way, the original mechanic is an ass. He charged her $485 to replace the alternator and then another $130 to replace the battery when the battery that was in there was only 6 months old and working fine. They are just trying to take advantage of her because she's a 63 year old single lady who doesn't know much about cars. She should have talked to me first, but I hate seeing mechanics take advantage of people like this.

 

Find a new mechanic and BTW any competent mechanic can diagnosis what your problem is without having to refer to a specialist (but I have nothing against specialists).

 

it sounds like you are moving forward with this, but if you want specific tests that will resolve most all charging system complaints, pm me...i have a great 5 step test that will catch alt, regs and that bad wiring that was referred to in the above post...all in about 2 mins once you've done it a few times.

as for autozone and advance, they are parts sellers, not trained diagnosticians. the last time i saw one of those guys test a battery, they tried to perform a load test (high rate discharge) on a large deep cycle battery out of my motor home with a 50 amp load tester...the battery was bad but the test passed.

 

Thank you everyone for your help (especially carguyinva).

It was a bad alternator. The original mechanic replaced it (free of charge of course) and put her old battery back in (which tested fine under load) and refunded her the money for the battery. Everything tested fine after the replacement alternator was installed.

 

excellent, mark...so glad you persevered and got her situation rectified.

 

Why do you have to be a trained diagnosticians to put an alternator on a test bench plug in the proper cable(That is written in the manual) and push START? Sounds like you have morons working at your local AZ or Advanced. Any general car guy should be able to perform those tasks without too much risk. If the dude testing your battery would have READ THE BATTERY(Label that is) or been half competent then he would have tested it at more like 700MCA(Marine Cranking Amps) at least the tester at the local AZ here has a selection for CCA/CA/or MCA.

So pay attention to what someone is doing to your car but I wouldnt stay completely away from FREE testing personally.

 

yeah...my point exactly...if the dude had read (if he even could) the label or been halfway competent...

in most cases, you get what you pay for so the quality of help in AZ and ADV usually isn't too swift cuz they don't pay much...and they don't train their people to diagnose things correctly, they show them how to stick something on a test stand and that's it. and likewise, you get free diagnostics and the majority of the time, they are incomplete or just plain wrong.

and how exactly would the average person pay attention to what the moron in the store is doing and know that what they are doing or saying is good information or bad. they can't...personally it's ok with me if you take your car there cuz when the parts they sell you don't fix your car because they think any trouble code automatically means that the part associated must be bad, you'll bring it to a pro like me and i'll fix it correctly, so it's job security. to each his own tho...mho

 

I see you are an ASE Master and I respect that, I am a certified MECP specialist so I have a pretty good general idea of automobile electronics. I am not the typical AZ/ADV employee either BUT I work part time at AZ and I can say from experiance that NOT ALL employees of AZ or the Local ADV are incompetent. YES alot of them are and YES THEY DONT PAY ENOUGH TO HAVE GOOD KNOWLEDGEABLE employees that we can agree on. Also YES most of them tell customers that they need the part called out in the code and it DRIVES ME BATTY!

Also customers get mad at me (Saying I dont want to help them) when I wont tell them what exactly is wrong with their car when they get an O2 sensor code or when they come in saying "My car hesitates". I tell them they need things TESTED and there is no way for someone to put a code reader or hear a symptom and be 100% sure of the problem without things being tested! If they want to guess at parts I'll sell it to them but I am NOT going to say this part is why something is happening without testing anything.