Is it my battery or my alternator, the quintessential thread.

skyhorse said:
With the motor running disconnect the negative post,if she keeps running it's the battery.
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That might work on cars up until the late 70s past that I'd be careful, as computers became more prevalent in the 80s disconnectin' the battery while the car's runnin' could become expensive as you star fryin' computer modules.

Best way is take it to Autozone and let them hook it up to the charging system analyzer they have or ya can put a multimeter to the battery terminals and read the battery charge at rest after the car's been off for a while, voltage should read 12vdc or better and when the car's runnin' at idle you should get about 14vdc or better, too much is not good either.

Consider also a worn bushing on the starter, more typical on 60s-early 80s cars, this would allow the stator and windings to make contact even if ever so slightly and discharge the battery over the course of the night.
 
What are your symtoms, slow cranking, dead battery, doesn't hold a charge, when ya turn yer headlights on does your stall?

Need to know some basic info first.
 
Take her to Autozone (if they have them where you live), and they'll wheel out a cart to the parking lot and test it for free.

Edit: I see that Brother T.E. beat me to it. Great minds . . .
 
T. Erdelyi said:
That might work on cars up until the late 70s past that I'd be careful, as computers became more prevalent in the 80s disconnectin' the battery while the car's runnin' could become expensive as you star fryin' computer modules.
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Uh, no. Disconnecting the battery won't hurt a thing on newer vehicles with computers (especially if you disconnect the negative side and not the positive). If the alternator is OK, nothing happens. If the alternator is no good, the engine stops, and everything loses power.
 
disconnecting the battery does effect sensors, so dont do it if your about to get an emissions test, other than that it just resets things so you good to go. The car only uses the battery to start, after starting the alternator takes over which converts mechanical energy into electrical energy. So if your car wont start, try a jump/charge, if the cars jumps and stays running (without dimming lights, lower radio volume, etc) its your battery. If the electrical equipment is failing while the car is already on, its your alternator.
 
Clarifying what clich said, removing power to the ECU will reset certain sensors' states in the computer's memory (specifically, emissions sensors). It does not hurt anything, but some of the sensors take some time to reset in the ECU's memory. If the alternator is working properly, removing the negative battery cable won't do a thing.
 
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Disconnect while running, That is the definitive test, yes.

You can get some indication by measuring voltage with the engine not running. And, for a quick test, just turn on the headlights with the engine not running.
 
Click on the followin' link and read about disconnectin' your battery while the engine is runnin'.

http://www.aa1car.com/library/battery_disconnect_problems.htm

The issues it creates by resetting sensors, modules and other computer controlled components is just not worth it, unless you're knowledgeable in the field it's just better to take it to car quest auto zone or any number of auto parts stores that offer diagnostic services, usually for free.
 
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I'm not going to argue. While not completely impossible, it is quite rare that disconnecting the battery will cause any major problems, and in fact, this procedure can be used to diagnose a number of problems. Most issues this may cause are minor annoyances that will take care of themselves in short order (think of it as resetting the car to default like when it was new). Things like long term fuel trim are not as long term as you might think (usually only 50-100 miles), and will usually default to a middle-of-the road safe level without long term data stored. Examples of major problems listed on that website are not typical, and are usually recognized by the manufacturers and covered under recalls, etc.

If this was a real problem across the board, we would all have to take our vehicles into the dealer just to simply change the battery. Most people would not put up with that kind of hassle (or being reamed for it), and "the man" would have stepped in long ago to correct it. The webpage you posted does have some good information, but it also has a number of erroneous points that, imho, are intended to be scare tactics to prevent the DIYer from trying to undertake the SIMPLE task of changing their battery (or the site owners are just covering their behinds). They are certainly taking the most conservative approach possible.

I will, however, concede that if you have no knowledge in the field that you *might* be better off taking your vehicle to an Autozone or the like, but that is still risky business. I hope people realize that many of the employees in those stores have no real understanding of what they're doing short of it being demonstrated to them and then repeating the task. I have personally seen an O'reilly's employee fry what was previously a perfectly good alternator because they had no clue how to test it, and another pop bigtime sparks from two batteries when hooking them together backwards... :eek: You'd be much safer going to a dealer, but that will cost you.
 
So it will fire up and run but it wants to and does stall unless I keep my left foot on the brake pedal and light pressure on the accelerator. Standing voltage is ~ 12.2. It dips to about 11 when I fire it but quickly winds up at ~13.5. The dip is what's causing it to stall. Now the check engine light is on so I need to pull the codes and figure out what TF that's all about also. :grumpy: Replacing the alternator is a nightmare in this model so I am hoping to avoid. :grumpy:
 
clich said:
disconnecting the battery does effect sensors, so dont do it if your about to get an emissions test, other than that it just resets things so you good to go. The car only uses the battery to start, after starting the alternator takes over which converts mechanical energy into electrical energy. So if your car wont start, try a jump/charge, if the cars jumps and stays running (without dimming lights, lower radio volume, etc) its your battery. If the electrical equipment is failing while the car is already on, its your alternator.
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Yes, disconnecting the batteries resets the monitors. I have customers come in after they get tested and they don't understand why. "My friend told me to disconnect the battery to clear the check engine light". ..... but yeah. You can get them tested at big name auto stores. Have you ever replaced the battery?
 
T. Erdelyi said:
Click on the followin' link and read about disconnectin' your battery while the engine is runnin'.

http://www.aa1car.com/library/battery_disconnect_problems.htm

The issues it creates by resetting sensors, modules and other computer controlled components is just not worth it, unless you're knowledgeable in the field it's just better to take it to car quest auto zone or any number of auto parts stores that offer diagnostic services, usually for free.
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Resetting the monitors isn't a big deal, they reset as the system goes back through the checks as you drive. For the evap system though, run your tank down to 1/2 tank and then fill up. Topping off at 3/4 or more won't reset that monitor. :-)
 
I bought the car used about a year and a half ago. It does have an after-market battery in it so the previous owner must have replaced it. I have a feeling it is actually a problem with the regulator which is built in to the alternator but I want to be sure before I take it somewhere.
 
I tried disconnecting the battery while running trick on my 74 TBird, blew out the ignition module.
 
My '09 F-150 has a seven step procedure the reset the computer if the battery is removed -unless you have the gadget that holds the info .Dealers should have one.
The first question should be how old is the battery ? They still won't give you much more than 5-6 years !
 
T. Erdelyi said:
That might work on cars up until the late 70s past that I'd be careful, as computers became more prevalent in the 80s disconnectin' the battery while the car's runnin' could become expensive as you star fryin' computer modules.
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+1 to this. you could also damage the diodes in the alternator IIRC.
 
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