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Re: Battery voltage drop under braking Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 3 Jan 2003 08:46:15
In Reply to: Battery voltage drop under braking, Robin [Profile/Gallery]
, Wed, 1 Jan 2003 22:33:56
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When the engine is running, the electrical power requirements of the car are being supplied by the alternator, not the battery. The battery is a load, being recharged by the alternator from the task of starting the car. The battery will supply a little current if the car's electrical load becomes great enough, but only if the alternator drops to about 12.6 volts.
A few things to check. First, I hope that this problem can be duplicated with the car sitting in the driveway idling. Take a voltmeter and measure the voltage right at the battery, while someone presses the brake pedal. See if it drops the same as voltage the EDU displays. If the voltage drops at the battery, the alternator needs attention (more below). If the voltage at the battery only changes a few tenths of a volt, but the dash indication drops a lot, then the problem is a bad connection. If the headlights dim, then make sure the connections from the battery cables to the battery posts are clean and tight. If the headlights don't dim, just the dashboard lights, then there is a bad connection in the dashboard.
Of course, make sure ALL the brake lights go on. If one light doesn't, find out why. A short circuit in the back could be pulling a lot of current; not enough to blow the fuse, but enough to pull the voltage down.
If when the engine is running, the voltage at the battery drops under electrical load - brakes on, or headlights on, or switching from fan off to fan HI, then suspect the alternator. Most likely it isn't the alternator, but the brushes. The brushes are built into the voltage regulator, which is bolted to the back of the alternator. Replace the voltage regulator, and you get a new set of brushes automatically. A decent Automotive Electrical shop should be able to do an alternator load test with the alternator in the car pretty easily.
You probably don't need a new alternator. Many places like to replace the whole thing, and why not - it's a bigger bill, and less chance of a call-back. The alternator has basically three parts - the voltage regulator/brushes, the diode pack, and some electrical windings and bearings. The voltage regulator bolts on, and unbolts (relatively) easily. The diode pack can fail, and quite often it's worth replacing/rebuilding the alternator for that, but your symptoms don't sound like a diode pack. As to the rest, it's pretty much purely mechanical; replace if the bearings are shot, and a bad bearing shows up as lots of noise, not low alternator output.
Good luck!
posted by 192.249....
, Wed, 1 Jan 2003 22:33:56, Fri, 3 Jan 2003 08:46:15 <-- Viewing This Message, Sat, 4 Jan 2003 14:46:39, Thu, 2 Jan 2003 19:58:12
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