[svxcess]

Originally Posted by david_12121

try this:
jump start it, then when it is on, disconnect the battery

DONT DO IT!!!

The battery is used as a large capacitor and thus smooths out the half-wave rectified output from the alternator. Disconnecting the battery while the engine is running will cause all systems in the car to be powered from an 'un-smoothed' power source, and could damage some, if not all, of your car's systems. It would also cause a huge momentary power spike, which could also damage systems.

These power spikes can also be generated when connecting / disconnecting jumper cables. I know of people who have destroyed their car's ECU's doing this. The best way to check your alternator is with a multimeter. With the engine at a fast idle (1500rpm or so) and with the lights switched on and on main beam (to load the alternator) you should get a reading across the battery terminals of 13.5 to 14.5 volts. A reading outside of this indicates an alternator problem. Similarly measure the AC voltage across the terminals. It should be less than 10VAC. Any voltage above this indicates that either the battery is failing to smooth the rectified output of the alternator (shorted battery plates usually), or the rectifying diode pack on the alternator is faulty.

As I said earlier, your battery does more than just yield electricity. It acts as a shock absorber. it also shorts AC, spikes and transients to ground. Removing the battery from the circuit allows those spikes and transients to travel around, endangering every semiconductor circuit in your car. The ECU, the speed sensitive steering, the memory seat adjustments, the cruise control, and even the car's stereo.

Even if your computers and stereo remain intact, in a great many cases removing the battery burns out the diodes in the alternator, necessitating a new alternator ( this has been confirmed many times by Christian at Maniac Electric Motors)If disconnecting the battery interferes with the voltage regulator's control voltage input, it's even possible for the alternator to put out hundreds of volts, frying everything.

Even the initial premise was wrong. If you disconnect the battery and the car conks out, you don't know if it conked out due to insufficient alternator current, or whether the resulting transients caused your ECU (the car's computer, which controls fuel mixture, timing, and much more) to spit out bad data, shutting down the car.

Using a multimeter is a good way to check your alternator. If you don't have one, here is a simple way to check it out, without any tools.

It'll help you to diagnose a BAD Alternator in a pinch. It's a fast test and more importantly an effective test to find out if the alternator is in fact bad. This test doesn't require any tools, such as a multimeter or anything else.

Also, you can avoid the most common mistake made when testing an alternator of disconnecting the negative terminal of the battery while the engine is running. Not to mention that this is a really, really bad way of checking for a bad alternator.

Here are the common symptoms of a BAD Alternator:

The vehicle does not start.

If you jump-start the vehicle and it runs, but after a few minutes or seconds after you disconnected the jumper cables from your vehicle's battery, the vehicle dies.

Or the vehicle may stay running for a while until you start turning on accessories (like headlights, a/c, wipers, etc.).

The battery light comes on and stays on the whole time the vehicle is running.

At night, when you turn on the headlights, both headlights and dashlights are very dim.


Here is how to test without any tools

Start the vehicle. Either jump-start it or charge the battery to get it started.

Open the door and leave it open so that the dome light can come on and stay on.

Disconnect the jumper-cables.

Go back to the inside of the vehicle and eyeball the dome light.

If the vehicle stayed running after disconnecting whatever it was that got it going, then notice the dome light's intensity.

Now turn on the headlights and notice the light's intensity once again.*

The light should dim a little and then resume its brilliance to the way it was before the headlights were turned on. If this happens the alternator is doing good.

If the light dims and DOES NOT return to its previous intensity and CONTINUES to dim, the alternator is faulty.

With the vehicle running and with the headlights still on...

Turn on the blower motor on high and notice the light's intensity once again.*

The light should dim a little and then resume its brilliance to the way it was before the headlights and the blower motor were turned on. If this happens then the alternator is doing good.

If the light dims and DOES NOT return to its previous intensity and CONTINUES to dim, the alternator is faulty and not charging the battery or supplying the vehicle's electrical needs.

If the alternator is BAD, as you turn on load after load (headlights, blower motor, wipers, turn signals, etc.) then eventually the dome light and the headlights will get very, very dim till the vehicle stalls.

If the alternator is not BAD, then the dome light and the headlights will stay on and brightly lit (after a slight dim as the load normalizes) no matter what you turn on.

It doesn't matter how discharged the battery is. After the vehicle is started, either by a jump-start or by charging the battery, this test applies.

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