basic automotive question - Honda Prelude Forum

cifer6425 · 12-27-2003, 01:26 AM

I got a basic automotive question. I need a sure fire way to trouble shoot my alternator, with out driving around for hours on end, waiting for the battery to die. Here is what I do, when the car is running, I disconnect the battery, and the car dies, even when cruising if I disconnect the battery the car dies. Does this mean my alternator is gone? I know this is noob question, but I just wanted to make sure before I spent $100 (that I donâ€™t have) on a new alternator.

Thanks

71dsp · 12-27-2003, 06:44 AM

More than likely that means that your alternator is not putting out enough amperage to power the igntion and other components to keep the car running when the battery is disconnected.

Here's what else I would do to test the system. Measure the battery voltage while the engine is off (with the battery connected to the car, of course). It should be about 12 to 12.5 volts. Now start the car. Measure the voltage again. It should be about 12 to 13 volts (closer to 13V would be better). Now have someone hold the RPMs steady at about 4k RPMs. The voltage should be somewhere around 13.5 to 14 volts. If not, I would think that it's safe to assume that there is a problem with the alternator. First check all of the obvious things like wires that have been disconnected, etc. If it's none of that, I'd have someone like Autozone, etc. test the alternator. IIRC, most parts shops offer that service for free.

racinghonda · 12-28-2003, 10:04 PM

The car will die if you disconnect the battery while the engine is running. The battery provides a load for the alternator, and allows it to regulate correctly. Disconnecting it can cause the alternator to spike and kill voltage sensitive devices.

Also, the alternator wont produce a current when its first spun unless there is power in the stator coils, ie you cant spin it and magically get power.. you need a constant source of magnetisim which is provided by the rotating coils in the alternator.

cifer6425 · 12-28-2003, 10:09 PM

Quote:

Originally posted by racinghonda
...disconnect the battery and you have no electrical power of anything (includes ignition, fuel pump, injectors ECU etc).

would this be true on an older car that is not fuel injected? say like, um... maybe a MGB?

come to think of it, thats not a true statement. cause there have been times when i have taken my car battery out of the car and it not die. i had to do this once when my friends alternator went out, we took went to the car (in a full size truck) took out the battery, put the truck battery in and drove home. the truck had no battery.

racinghonda · 12-28-2003, 10:13 PM

Yes, that's also true for a car with no electronics. The alternator is not a dynamo. I edited my previous post to make proper and correct sense.

cifer6425 · 12-28-2003, 10:25 PM

wow i never knew about the voltage spikes. complete OT from my orginal question, but say if you but the old/dead battery (from my friends car) in to the truck, the alternator would "charge" the battery, correct? and this would prevent the voltage spikes, yes/no?

thanks

racinghonda · 12-29-2003, 02:36 PM

Yes, it would run. But you would need to jump start the truck. The alternator needs the battery as a load and to help regulate the voltage.

tenzola · 12-29-2003, 02:59 PM

dude.. go to sears.. buy a multimeter, check the voltage at the battery terminals while the car is running.. if the meter reads below 12.6, then it's dead.. it really should be reading 13.5v-14.5v for a good charge to occure.... and this is even at idle.. then check with your friend holding the rpm's at 3k.. this should read as close to 14.0-14.5v.. if not.. then you know you got a problem.. and if you don't feel like keeping the meter.. bring it back to sears and return it and call it a day

=)

reyke · 12-29-2003, 09:59 PM

Look I know I work at a shop and you're all going to point at me and call me a demon for recomending this, but a battery and charging test at where I work is only 20 bucks and you know what after you spent your 20 bucks you know for sure what is wrong, you can sit here and trouble shoot. Hell the diode pattern in your alternator could be giving out intermittent problems, or it could be constant, 20 bucks to be certain is cheap aside from an alternator and plus a battery on top of that.

LudemanDan · 12-30-2003, 12:00 AM

DO NOT disconnect the battery while the car is running... or if you already have, don't do it again. The battery acts like a big regulator for voltage spikes, which can happen with or without it in the loop.
Voltage spikes are caused by electrical arcing within the alternator, and can raise the voltage to like 400v for a few milliseconds, which is enough to kill the alternator you are trying to test (the details I may have wrong, but the message is the same).
I've done it. There's only a chance (maybe 1 in 4?) that you'll kill the alternator this way, but it's a chance not worth taking.
Like the others said, go to an auto parts store and they'll test your running charge for you for free.

Dan