Voltage issue

DIVIDES

MEMBER
2005 AUDI A4
4 CYL
TURBO
2WD
AUTOMATIC
125,000 MILES

When first bought was acting like crazy I knew it was the battery, had it replaced and all good. Fixed a bunch of vacuum hoses, and so on. Recently she randomly will have the battery light on and off. Took it back to Walmart to check battery they said it was defective, replaced under warranty. Still, when I check volts when off it's 12.11, with lights on and engine off 11.80, with engine running as well as ac and lights 12.64, with engine on and ac and lights off 13.26. They also checked charging system there once and it was okay.

2 Replies

CARADIODOC

MECHANIC
33,871 POSTS

Need to know exactly what they checked. The voltages you listed all say the charging voltage is too low. Minimum should be 13.75 to 14.75 volts. Doing that test with a digital voltmeter only means it is okay to perform the rest of the tests, but that requires a professional load tester. Specifically, we want to see the "full-load output current test". A 150-amp generator is the standard one regardless of engine size and optional equipment. The full-load output test will result in a reading of 0 amps, close to 50 amps, or close to 150 amps.

Obviously 0 amps is not good, but that would have shown up regardless what kind of test was performed. Professional testers will also measure "ripple voltage". If one of the six internal diodes has failed, the most any AC generator will develop is exactly one-third of its maximum rating. Ripple voltage will also be shown as "high". With a failed diode, you will still get some charging current, but not enough under all conditions. The voltage depends on how the voltage regulator is designed. Some will run the generator too hard and system voltage will be a little too high. Some interpret the high spots in the ripple voltage as over-charging, so they reduce output. This is where you'll see charging voltage is higher than battery voltage, which it should be, but not high enough.