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The bane of too much information Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 28 Jul 2003 13:15:26
In Reply to: EDU showing fluctuating/low voltage , sign of what ?, Peter   [Profile/Gallery]
, Mon, 28 Jul 2003 07:47:30
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The EDU voltage display has it's uses, but by constantly monitoring it, you can turn into the automotive version of a hypocondriac.
For a first cut, it's bad if the EDU voltage goes much over 14.5 volts, or spends much time below 13.0 volts. If it's running between 13 and the low 14's, that's fine, and spend your attention watching out for goat-ropers in SUV's trying to kill you.
The EDU voltage is affected by so many things that trying to account for them all will drive you batty. As others have mentioned, the EDU is NOT a precision electronic measuring device. The difference between the battery voltage and EDU reading can be 0.1 to 0.3. And it goes downhill from there. The EDU isn't tied directly to the battery, so depending on what electrical loads you've got on (lights, fans, etc), you can get voltage drops both in the EDU voltage and ground lines.
Next, alternator voltage varies as a function of electrical loads, engine speed, and temperature. Typically you get higher voltages at higher RPM, lower voltage with more electrical loads. So the voltage may drop 0.3 volts because the brake lights are on, or the AC clutch pulled in, or the radiator fan is running.
Next, the voltage regulator is temperature compensated. Batteries get hot when they're charged, so in an attempt to keep the battery from overheating, the alternator puts out less voltage as it gets hotter. The VR has a temperature sensor built in, and the assumption is that since both the battery and alternator are under the hood, they're near the same temp. So on a cold morning/cold engine you may see 14.3 volts, but on a hot day stuck in traffic, the alternator may only put out 12.8 volts. That's not a bad alternator - that's an alternator trying to keep the battery from boiling over.
The EDU is a good way to learn your car. Note what it usually does, and see if you get trends. Really high voltages (over 14.5, especially when hot) is a bad sign. Pretty rare. The typical is low voltages as a sign of a VR or alternator on the way out. If you see voltages below 13 volts when running down the highway, that's a worrysome sign. If it's 98 degrees, not as worrysome. If the BAT light flickers at idle under loads, that's a bad sign.
Otherwise, don't sweat it. It sounds like normal variation.
posted by 192.249....
, Mon, 28 Jul 2003 07:47:30, Mon, 28 Jul 2003 13:15:26 <-- Viewing This Message
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