Draining electrical problem 740 station wagon

[JimBee]

I think it's a 740. The vehicle runs on a charged battery, but drains the battery while running AND also when the engine is shut off, the draining continues. It seems like a bad alternator, but why does the battery continue to drain when the engine and all switches are off?
JB

[billofdurham]

From your description I would agree that you have a problem with the alternator. However, a car battery can run down for any of several reasons:

1. You accidentally left the lights on or some other accessory that pulls power from the battery even when the ignition key is off. Check lights that can't normally be seen - glove box, engine compartment, luggage compartment.
2. The battery is not being recharged while the vehicle is being driven (you have a charging problem)
3. There is a key off parasitic electrical draw on the battery because a relay is sticking, a module is not shutting down, or there is a shorted diode in the alternator. To check for a parasitic draw see here.
4. Your battery is old and will not hold a charge anymore. The battery needs to be replaced.

The first things you want to check are the battery leads for tightness and lack of corrosion. Pay special attention to where the earth (ground) lead is attached to the body. Take out the bolt and clean off any rust/dirt on the connector and the body.

After fully charging the battery you need to it and the output of the alternator. For this you will need a voltmeter. Select the 12 or 20 volt scale and connect the voltmeter red positive test lead to the battery positive (+) terminal, and the black negative test lead to the battery negative (-) terminal.

A fully charged battery should read about 12.6 volts. If the battery reads 12.4 volts or less, it is low (discharged) and needs to be recharged. If it reads below 12 volts there is probably a dead cell in the battery.

Battery Voltage and State of Charge:

12.68v . . . . . . . . . . 100%
12.45v . . . . . . . . . . 75%
12.24v . . . . . . . . . . 50%
12.06v . . . . . . . . . . 25%
11.89v . . . . . . . . . . 0%

(These readings are at 80 degrees F. Battery voltage readings will drop with temperature roughly 0.01 volts for every 10 degrees F.)
(At 30 degrees F. a fully charged battery will read about 12.588 volts, and at zero degrees F it will read about 12.516 volts.)

With the voltmeter still connected to the battery start the engine and note the charging voltage. A charging system that is operating normally should produce about 13.5 to 14.5 or more volts at idle. If the charging voltage is less than 13.5 volts, the alternator is not putting out enough current to keep the battery charged. You should have the alternator tested (or bench tested at an auto parts store). If the current output is not up to specifications, replace the alternator.

Bill.