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Look about right Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 26 Apr 2006 07:07:20
In Reply to: What do these voltage readings mean?, Mike C, Tue, 25 Apr 2006 18:39:20 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The D+ line is also called the Field Coil. An alternator is like a generator, but one difference is that is uses an electromagnet to create an electric field. That's called the field coil. So, for an alternator to work, it needs current flowing into the field coil to create the magnetic field. The D+ line is connected directly to the ALT light in the dashboard, then through the key and then the battery. When you turn the key to ON, current flows through the ALT light and into the coil (D+). This provides just enough of a magnetic field that once the alternator starts spinning, it can start making voltage (and current). The Voltage Regulator then takes over, and uses the voltage the alternator is making to control the field coil.
When the engine is off, the voltage you see on D+ is a function of the ALT light bulb resistance and the resistance of the field coil. The coil resistance is much lower, so you see that divided down battery voltage.
The faster the alternator spins, the more voltage it makes, so the VR adjusts the field coil to compensate and regulate the voltage. At idle, where I assume you're doing the testing, the alternator is spinning the slowest, so the field coil voltage will be highest, which is why you see 13 or so D+
If you're seeing 12.9V under load at idle, that's pretty good. I'd say your alternator is working just fine.
If you are regularly eating batteries, then it could be a sneak discharge problem. These are rare - usually it's a charging problem. But your system looks good.
There is a way to test to see if you have a sneak path. You'll need a meter that reads amps.
(1) Disconnect the ground lead on the battery. Get a piece of wire with alligator clips on both sides (doesn't have to be a very fat wire) and electrically connect the clamp to the negative post. Connect the meter, set to amps, between the battery clamp and the negative post. Then remove the jumper.
Why all this? When you disconnect the battery cable and then re-connect it, there is a current spike as stuff turns on. You don't want to run that spike through your meter - the jumper takes the load.
If there is a hood open switch in the engine compartment - usually right by the radiator - put something heavy on it to keep it closed.
(2) Wait 1 minute after you have re-connected the battery connection. Don't open any doors. This time will allow things to settle out, like the dome light to go out, etc. Read the current meter.
25-35 milliamps (0.025 - 0.035 amps) is normal and fine. That's what the clock, radio keep alive, and alarm (if any) consume. A decent battery can drive that current for weeks.
If you see a larger current than that, you've got a current drain somewhere. The way to troubleshoot that is to remove fuses one at a time. Remove a fuse, and see if the current drops to the normal range. If so, you've found a place to start. If the current doesn't change, re-insert the fuse and go onto the next one.
posted by 192.249....
, Wed, 26 Apr 2006 13:41:54, Wed, 26 Apr 2006 07:07:20 <-- Viewing This Message, Wed, 26 Apr 2006 10:52:49, Thu, 27 Apr 2006 05:53:47
, Tue, 25 Apr 2006 21:46:11
, Tue, 25 Apr 2006 22:23:43
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