Battery or Alternator

stomachmonkey · 07-13-2012, 03:00 PM

06' Mazda3.

My DD that I've only been driving once a week or so lately.

Last week go to take it on an errand and the battery is real weak, not dead but not enough juice to start it. Toss a charger on it and take the truck.

Realize later that the rear dome light is on so assmed one of the kids was messing with it and that's what drained the battery.

Couple of days later drive it a good amount but I notice a drop in rpms that seems to coincide with electrical activity, radio, fan blowers, hitting the brakes.

Yesterday I go out and start it and it turns over slow but does start.

Pop the battery cover and there's a crapload of green buildup on the + terminal.

Disconnect/clean/wire brush everything.

Go to run some errands and pull up at a stop light, car starts to stutter and stall.

Mash the go peddle so the revs stay up and take it home. Sitting in the driveway it's behaving fine but it just occurred to me I was holding it with the e-brake and not the peddle so no load on the electrical system.

Back in the day you could pull a battery from a running car and it would continue to run but modern cars do need the battery in the circuit.

I hate to replace stuff if I don't have to.

Any thoughts short of pulling the alternator and getting a bench test?

Thx

kaisen · 07-13-2012, 04:05 PM

Yank the battery and get it load tested first. It is easier.

peppy · 07-13-2012, 04:29 PM

If you have a volt meter check the battery with the car running, should be about 14 volts.

If it is what Eric said.

stomachmonkey · 07-13-2012, 05:08 PM

Quote:

If you have a volt meter check the battery with the car running, should be about 14 volts.

If it is what Eric said.

13.7

Thx

masraum · 07-13-2012, 05:20 PM

Quote:

Originally Posted by kaisen

Yank the battery and get it load tested first. It is easier.

+1

Quote:

Originally Posted by peppy

If you have a volt meter check the battery with the car running, should be about 14 volts.

If it is what Eric said.

+1

Quote:

Originally Posted by stomachmonkey

13.7

Thx

13.7 is what it should be. Get the battery checked. Make sure the battery is completely charged (12.6V) or the parts place will insist on charging it before they test it.

Nostril Cheese · 07-13-2012, 05:41 PM

Also check the voltage at the battery with everything turned on. It shouldnt go below 12V or so.

john70t · 07-13-2012, 07:13 PM

12.6V and above is just a chemical surface charge, similar to static electricity. No potential.
Also, the alternator doesn't send many amps at idle speeds. Needs more like 2500 rpm.

It sounds like a classic bad battery, but you can load test it using the car:
Remove the fuel pump fuse so it won't start.
Turn on headlights, rear defroster, fan, etc. for an electrical load.
Test battery level while cranking.
Shouldn't fall below 9V.
Many of the electronics need minimum 9V to function even if the engine is spinning.

Alternator may be weak, but get a battery first and go from there.
Make sure the grounds(chassis and engine) have good connections and interior of cable isn't green. Vaseline melts and wicks though the cable protecting it.

GDNF2ET · 06:19 AM

Go to your nearest parts store and get them to check it for free..I go to autozone and if I'm not happy with their findings I go to Advance...Both have a computer that they hook to your battery while its in the car that should pinpoint the problem..
If you find out its the Alt., get them to check the NEW one before you leave the store...