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One should start by measuring the voltage of the battery with the engine off. If you measure a low voltage then it is pretty easy to understand that this will not start the car. At this point, you could charge the battery, remove the battery and have it tested (which is free at a car parts store) and if necessary replace it by a good one.


Your car's volt gauge should read about 13.5 to 16 volts with the engine running. It should read 12 volts with the engine off. You mentioned the old battery had a lot of corrosion on the cable connections. Corrosion can cause a voltage drop to the starter, causing it to turn the engine over slowly or not at all. Auto parts stores have testers for batteries.

These simulate the load of the starter on the battery and measure whether the battery has enough juice in it. The machine can also test the alternator while it's on the car to determine if it is charging when the engine is running.

Also there is a quick and dirty test you can do with a multimeter (Note: these are pretty inexpensive and are very useful in other DIY projects involving any electricity.) . Set it to the "Volts DC" scale. With the engine running, touch the red probe of the meter to the positive "+" post and touch the black probe to the negative "-" post of the battery. It should read 13.5 to 16 volts. If it is less, the alternator may not be charging.


One should never disconnect either battery cable with the vehicle running/on, as it could cause an explosion, leakage, or serious electrical damage to vehicle.


You need to hook it up to a battery charger for several hours at 3 or 5 amp slow charge- then bring it to a battery store or maybe a kragen or pep boys auto store and they load test it for free.


How old the battery is generally your first clue. Most only last about 5 years. Second, are you in hot or cold climate? Cold weather drains a battery so the cold cranking amps on it have to be high enough to allow cranking of the battery in really cold wheather.

Age of the battery can reduce the power of the amps needed. Heat in really hot climate can also kill a battery over the years. If you can take it to a auto parts store they can put it on a machine to check to see if the battery is fully dead or is still good. If it is still good and just has a low charge, they can fully recharge it. If it is still in the car and you want to check it without going to the store, try to crank the car. If it drags a little and you are not in a cold climate it just may just need a boost (If in a cold winter climate it probably needs recharging).

You can boost with another car and cables or buy the instant boost sets they have out now (no other car needed and they work great). If it cranks with a loud turnover noise and the car just doesn't start it could be another problem. You also need to remember that the alternator is the equipment that charges your battery. If your alternator is bad the battery will just drain like a flashlight battery until it is dead. The alternator keeps your battery recharged as your car uses it while the engine is running. Just hope its the battery because an alternator is much more expensive.

For more information see the answers to the Related Questions shown below.

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โˆ™ 2010-05-26 17:20:43
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Q: How do you know if you have a bad alternator or car battery?
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Related questions

How do you know you have a bad alternator?

Turn car on remove positive battery terminal if car turns off alternator is bad

How do you know when alternator is bad?

Typically you know your alternator is out when you have to jump start your car numerous times. A way you can test it is by turning on your car and while it's running discount the + terminal. If the car turns off then your alternator is bad. If the car remains in then alternator is good and possible bad battery. Hope this helps

What can make a car battery die when running?

It sounds like the alternator is bad. If the alternator is bad the battery will not hold charge when running

How do you check if my alternator is bad or my batter is bad in my car?

If the car won't start, jump it off. While it is running, remove the positive cable from the battery. If the car dies, then your alternator is bad. If it keeps running then you may have a bad battery.

How do you know if your altinater as gone in your car?

If your battery doesn't hold a charge while running then your alternator is bad

My alternator is not getting power from the battery The wiring from the battery to the solenoid and from the solenoid to the alternator is good is this a bad solenoid the car will start and run?

Check for a bad fusable link at alternator

Why will a car not start unless it is jumped started?

bad battery, and bad alternator not keeping it charged - also bad connections to the battery or loose wire connections on the alternator.

What are the reasons for the car battery light to go on?

its either the alternator the battery is bad or a loose or bad wire on the starter

What charge the car battery?

The alternator charges the car battery.

Can you start your car with a bad alternator?

Yes it can as long as the battery is charged . The purpose of the alternator is to put the power used back into the battery

Can a car start with a bad alternator?

Yes it can as long as the battery is charged . The purpose of the alternator is to put the power used back into the battery.

How to know if the alternator is gone in a 98 mercury mystique?

let the car run and then pull the negative battery cable off. if the car stalls, the alternator is bad! do you know where the bank 1 o2 sensor 2 is?

How many miles are needed to charge car battery?

Car batteries get re-charged by the alternator. If your battery recently went dead, "miles" is not what is measured to recharge the battery, "time" is used. I'd leave the car running for 15-30 minutes. Driving for this amount of time too is fine. If the battery goes dead again, probably a bad battery or bad alternator, or sometimes a bad alternator makes the battery go bad and they both need repaired. (typically a new alternator is $120 - $200)

What problems does a bad alternator cause?

Dead battery, car will not start

Can a bad alternator drain a good battery?

It doesn't actually drain the battery, but without a working alternator to recharge the battery it will only discharge as the car is operated.

2004 ford explorer how do you know the alternator is bad?

turn the car on and wen is on, disconect the battery if the cars keeps on working is good,but if the car shuts off it means is bad

How can you tell if your alternator is bad on a Subaru Outback?

The quick easy method for determining a bad alternator or at least to find out if it is charging, while the car IS running remove the negative battery cable from the battery, if the car stalls then you need to have the alternator service inspected.

How do you know your alternator is bad?

If your battery is always going dead a day or two after a boost, then it most likely the alternator. Two surefire ways to see if the alternator is bad is to take the positive (red) battery cable off after starting the car. If the car still runs, alternator is good. If the car stops running, then it is likely the alternator. The battery is just to turn the car on and then the alternator regulates the initial charge coming from the battery and that powers the car. Another thing you can do is to run a voltage check at either the battery or on the alternator with a Volt Ohm Meter (VOM). If the voltage comes back as 13.5V or better, than the alternator is good. Less than 13.5V, and it is likely the alternator. Be advised, though that what could look like an alternator problem could also be your ignition switch or starter.

MY car battery light is still on after the alternator and the battery has been changed. The battery cables are clean and well attached. Now what?

Battery light usually means alternator not charging. Most common solutions would be be a bad alternator, bad connection, or loose alternator belt. A replacement alternator can be bad. If it is ok than most likely a poor connection to the alternator.

Why would your car have low amps and shut off on you?

Either the battery is bad or the alternator is bad.

What causes headlights on a car to dim?

Could be a few things, weak alternator, bad battery, bad wiring. If you have a subwoofer in your car with no battery cap that will certainly do it.

What will bad alternator do to a cars battery and lights?

A bad alternator will have the effect of draining the battery, resulting in dim lights, trouble starting the car and the like. It would be best to replace the alternator as soon as possible and possibly obtain a new battery. This may vary depending on how much the bad alternator has drained it. You can get your alternator and battery checked any time at Auto Zone.

My battery will not hold a charge with a new alternator?

Here is a test u can do to see if Ur alternator is good or not . when car is running disconnect the negative battery terminal it's the black wire.If car stalls then the alternator is bad if it doesn't stall then it's Ur battery is bad and u have to replace it

What will the car do if the alternator is bad?

The battery will just run down and the engine will stop.

If you unhook the battery in your 1994 acura legend should it stay running?

Why do you want to unhook your battery??? If the alternator is good the car will continue to run, if it dies then you have a bad alternator this is why you would want to unhook the battery cable to check and see if you have a bad alternator

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