84 CK TRK, 2500, 5.7L 55,000 miles have replaced alternator twice - battery still reads 12 volts. Trickle charge on battery OK and engine starts up OK - terminal 1 on alternator wire (brown) reads 2.4volts and wire is good to firewall - I replaced the ignition switch same condition occurs - 2.4 volts at terminal 1 (brown) wire. If I jumper 12volts to the Terminal 1 wire - the alternator puts out 14.4 volts. Take the 12 volts away from terminal 1 battery reads 12.5 volts and starts dropping pretty fast - did it all again alater - voltage now at battery 11.9 volts and no charging from alternator - but place 12volts from battery to terminal 1- 14.4 volts comes out of the alternator - any suggestions - battery is 10 yeras old but starts the truck and holds a charge with trickle charger for one week.

Sounds like 2 possibilities, A) your alternator is not charging the battery or B) your battery is pooched.

To check your alternator, 1) without the engine running apply a volt meter to your battery, it should read somewhere around 12 volts. 2) boost the car, and with the engine running test your battery again, it should read approx. 14-15 volts.

If it reads 15 volts, your alternator is fine, and your battery is suspect, if it only reads 12 volts when running, then your alternator needs to be replaced.

I have a thought you may try testing your battery voltage with the car turned off .It should read 12 volts or higher if not charge battery than check again if battery is lower than 12 volts after charging replace battery.If 12 volts or higher start engine check voltage on battery again it should be 13 volts to 14.5 volts not higher if voltage readings are lower than 13 volts or higher than 14.5 volts replace alternator.

Disconnect battery while car is running. If car dies altenator is bad. With car not running check battery with volt meter. Should read 12 to 12.46 without engine running. Start car and check battery with volt meter, should read 13 to 13.50. If less than 12 with engine running then altenator is no good.

Sounds like a bad alternator. Drained your battery and when you turned on the lights it took all the power left in your battery. When you drain a battery down that far, they are real hard to jump, and coupled with the alternator, the car is not producing enough current to start and stay running.

I would take the alternator off and take it to oyur local auto parts store to be tested. If bad, replace it and charge the battery fully before starting. Then check voltage at battery with motor running. Should be 13.2 -14 Volts DC. If not, you may have a bad battery cable or blown fusible link. (especially that if the alternator tests good).....Mike

Take a battery that has a good charge on it or 12 volts put it in and start it then take a multimeter and put the red lead on to the power lead on the back of the alternator and ground the black lead.It should read between 13-14.5 volts anything more or less replace alternator.If alternator checks out ok check battery voltage after charging it.Must have at least 12-12.6 volts anything less replace battery.

The battery light staying on indicates that your battery is not charging either because of a faulty battery or a faulty alternator/regulator! Seek a second opinion as the diagnostic seems a bit dodgy when you have a lit battery light and your battery is definitely going flat!
If you have a multimeter, measure the voltage at the battery terminal with engine off: it should read around 13.8Volts and anything below 12 Volts is bad news!
With engine running, measure the voltage at the battery terminal again: it should read around 14.5 Volts - if it 13.8Volts or lower or above 15 Volts, you have a bad regulator/alternator.
Replacing an alternator does not require a lot of labor so it is virtually for the cost of the replacement alternator (consider getting a second hand or recondition unit - under $100?).
In all case, always clean up the battery terminals and clean/retighten all battery and alternator cables and terminals.

Hope it helps.

You're alternator is dead. Once started the alternator should generate enough power by itself to keep the engine running.

I am having the same problem with my 2004 Elantra. I have had the alternator tested twice and did replace the battery. This morning I had it running ad put a volt meter on it ( red to the battery pos black to the engine block) I did get a reading of 14 volts for about 30 seconds then the idle lowered and the voltage dropped to 12 volts. I tried this again later but could not get a voltage reading above 12 volts with the motor running. Any suggestions?

It may not be the starter. The battery could have a weak or dead cell in it and not holding a charge.
The alternator could be bad and charging the battery.
If you have a volt meter, or can get one, with the car running, put the leads from the meter to the battery terminals, red positive, black ground.
Put the meter switch to 12 volts. If the meter reads 14 to 15 volts, the alternator is ok, below that it isn't charging. If it does read 14 to 15 volts, replace the battery.
By the way, the starter is ok, or it wouldn't have started with a jump.
It's more a battery/alternator issue. Clean the battery terminals first and see if that makes a difference.

Ck the charge rate at the battery posts. The proper range is 23.6 to 14.2v , The higher the voltage the llower the amps. Ie; at 13v Alternator is about 10% under capacity. @ 12v it is charging at max capacity. @ 15 v isthe alt is not charging but frying your battery. 9amps is ok if battery is fairly charged but at start up it should sit on 13-13.6 volts. for about 15 seconds the start climbing to less charge towards 14.2.