bad battery or bad alternator?

anarky321 · July 5, 2010

1998 Ford Explorer Sport

battery dies just from listening to music for half an hour with the engine off

is the voltage across the battery terminals with the car running dependent on whether the battery is good? im getting 12.5V running, wouldn't a running voltage that low be considered a bad or failing alternator? trying to figure out whether i need to replace the battery of the alternator

car doesn't die while running so the alternator is putting out enough voltage to keep everything going including AC and stereo, although sometimes the car will die when put into reverse, which i attributed to other causes but maybe its related

edit: also i get a whine on acceleration from the front, i havent diagnosed it yet because i dont mind the noise at all, i will check tomorrow whether its the alternator making the noise, i remember reading somewhere that a bad voltage regulator would cause whine

on a related note, can i somehow turn off the timer that keeps my headlights on for 5 min after the engine is shut off? a bad charging system and a 5min headlight timer are a bad combination

Rudy602 · July 5, 2010

Make sure you use the "accessory" position, not the "on" position for the key if you are sitting listening to music or even a good battery will drain down quickly. Autozone will test both alternator and battery for free.

anarky321 · July 5, 2010

my radio only works in the ON position, i thought it was a Ford thing =/

i will go to Advance tomorrow to have them test the alternator just thought maybe there was a 99% sure way i could determine it myself

firepop5 · July 5, 2010

Voltage across terminals of battery should be 12.5 with engine off. With engine running voltage should be 13.5 to14.0. My bet is the alternator, but battery could be bad too. Let them test both for you. Also, do you have a killer audio system? They use lots of power for the amps.

anarky321 · July 5, 2010

audio system is stock

voltage across terminals with headlights on and engine off is 11.7V

double checked the radio - only works with the key in the ON position, not in the ACCESSORY position, this is not normal right?

Rudy602 · July 5, 2010

If key is in "on" position, everything is on as if it were running. My 97 plays the radio on accessory. Even with a new battery, it won't start if you leave it in "on" for very long.

anarky321 · July 5, 2010

how would i start to diagnose that? all the fuses are good

Rudy602 · July 5, 2010

It doesn't sound right to me. Is it a factory radio?

anarky321 · July 6, 2010

i haven't had the car for very long, but it looks 100% stock to me, i also know the speakers are stock

edit: possibly related - the blower doesnt work in acc either and the blower only works on "high" speed in the 'on' and 'start' positions

Rudy602 · July 6, 2010

If you pull the fuse designated for the radio does the radio go off? It could have been rewired to a different circuit, - but why? Maybe '98's only play in 'on'

anarky321 · July 6, 2010

fuse diagram lists radio under 20 - 7.5A, 28 - 7.5A and 29 - 25A - can someone explain why its under 3 different circuits? are these the key positions?

20 7.5A RAP Module, Generic Electronic Module
(GEM), Radio, Cellular Phone

28 7.5A Generic Electronic Module (GEM),
Radio, Memory Seat

29 25A Radio

also what is the GEM module and what exactly does it do

Xplorer82 · July 6, 2010

easy way to tell if the alt is bad is to disconnect the neg battery cable while the truck is running. if it dies you need a new alt.

another thing to think about is, if your battery is over 5-6yrs old i would replace it anyway. there should be a sticker on there w/the date/year on it.

but if you only had the radio on for about 30mins, i would point to the battery. it obviously is not holding a charge like it should (maybe a dead cell?).

anywho, good luck. :salute:

96eb96 · July 6, 2010

Xplorer82 said:
easy way to tell if the alt is bad is to disconnect the neg battery cable while the truck is running. if it dies you need a new alt.

This is not a great idea on computer controlled cars, it could cause a surge and destroy electronic modules. You may get lucky but why take the risk? They may not fail right away but it may take years off their service life.

The suggestions here to check voltage at the battery and verify it is around 14.1V with the engine running and some accessories (like headlights) on is the best way to diagnose an alternator. A multitester is a few bucks at harbor freight, sometimes they are on sale for $2, you can keep it in the glove compartment for that price.

Xplorer82 · July 6, 2010

96eb96 said:
This is not a great idea on computer controlled cars, it could cause a surge and destroy electronic modules. You may get lucky but why take the risk? They may not fail right away but it may take years off their service life.

The suggestions here to check voltage at the battery and verify it is around 14.1V with the engine running and some accessories (like headlights) on is the best way to diagnose an alternator. A multitester is a few bucks at harbor freight, sometimes they are on sale for $2, you can keep it in the glove compartment for that price.

they are free at H.F. right now (with a $9.99 purchase though, just got the add yesterday)

and i see what you mean about disconnecting the neg battery cable, i never thought about it that way, but all my vehicles have been computer controlled and i have Never had a problem like that happen. not saying it can't happen, but i will continue w/my method as it has worked time and time again for me and others. but i will advise others of your info if i ever have to answer this kind of question again :thumbsup:

.

to the OP, try my method at your own risk.

anarky321 · July 6, 2010

at this point the alternator is my chief suspect - voltage running is ~12.5V, voltage drop from positive post on alternator to positive post on battery is .18V so thats okay, voltage drop from alternator body to negative terminal of battery is .5V so that's too high but acceptable, no?

as far as the radio not working in ACC mode, on Fords the iginition switch goes ACC-OFF-ON-START.....lesson learned

too lazy to swap the alternator today since its still putting out enough juice for everyday stuff, i will try to get it replaced within a week

offtopic question - does resistance have to be measured with the circuit power off? im struggling getting any resistance readings - eg. battery to engine, battery to firewall, alternator body to car frame, battery to anything, but voltage drop testing is fine =/ brand new multimeter, shorting the leads gives the correct reading, i think i got 65 Ohms engine to negative battery terminal, i will write down all the readings next time

SoNic67 · July 6, 2010

Running voltage should be 14-14.4V (for a "12V" battery). If it is not that, alternator is bad.

anarky321 · July 6, 2010

does anyone know whether the alternator body to negative post would depend on how clean the surface of the alternator body is? for example if i dont have a good clean negative contact on the alternator would that affect the voltage drop reading i get?

the alternator is failing, i just want to make sure wiring isn't causing a high enough voltage drop to make it look like the alternator is failing? or would the alternator voltage regulator compensate for that?

SoNic67 · July 6, 2010

The alternator reads the voltage internally (voltage regulator reads right from the output), so it doesn't compensate for cable voltage drop.
The ones that I have seen had the rectifier diodes burned out (there are six of them) - you might have initially one or two bad, but that will overload the others and burn them out faster and faster.

anarky321 · July 6, 2010

shouldn't i fix the reason im getting .5V drop on the negative side?

i looked at my haynes wiring diagram and i don't even see alternator wiring on there

14 30A* Generator/Voltage Regulator

does the alternator circuit go BATTERY>GROUND>ALTERNATOR>POSITIVE CABLE>BATTERY? as far as checking the wiring on the negative side is there anything else i can check? where does the negative cable connect onto the frame? it just disappears into a wiring harness underneath the battery tray, i wanted to test cable/frame contact point to negative terminal to see if maybe the cable is causing the voltage drop

96eb96 · July 6, 2010

anarky321 said:
does anyone know whether the alternator body to negative post would depend on how clean the surface of the alternator body is? for example if i dont have a good clean negative contact on the alternator would that affect the voltage drop reading i get?

the alternator is failing, i just want to make sure wiring isn't causing a high enough voltage drop to make it look like the alternator is failing? or would the alternator voltage regulator compensate for that?

That is a separate issue. If you suspect high resistance connections, take a voltage reading at the pos battery terminal, then touch the - to the alternator body. You can also take a + reading at the screw terminal at the back of the alternator.

You can also read voltage across a conductor. Ideally, you should have close to zero volts with the + multimeter lead on the - battery terminal and the - lead on the alternator body (a clean connection acts as a perfect conductor). Any substantial voltage across a conductor is a problem (load, resistance, bad connection, corrosion, whatever you want to call it). This is a troubleshooting technique to check for corroded out wires in a hidden harness too.

bad battery or bad alternator?

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