The battery on your Toyota Previa is the energy source for a number of components in your vehicle. Everything from the spark plugs to the radio depend on the constant supply of voltage and amperes from your battery. A malfunctioning alternator will not supply the battery with replenishing energy from the rotation of the motor, causing the battery to quickly drain. Troubleshoot your Toyota Previa's alternator to avoid being stranded on the side of the road.
Connect the positive cable from the voltmeter to the positive terminal of your Previa's battery. Connect the negative cable from the voltmeter to the ground on the vehicle's alternator. If the voltmeter gives a reading of 0.2 volt or lower, your alternator is most likely performing within factory standards. A reading of higher than 0.2 volt suggests that your alternator is creating significant resistance in your electrical system, which points to a malfunctioning alternator.
Look at the drive belt around the alternator's pulley. If the belt is excessively loose, it may not turn the pulley on the alternator properly, resulting in diminished performance. Tighten the belt, if needed.
Pull the electrical plug from the back of the alternator out and check for debris that may inhibit a proper connection. Insert the plug back into the alternator.
Wiggle the grounding connection on the alternator. If the connection is loose, tighten the bolt connection of the ground.
Do not drive your vehicle if you suspect that the alternator does not work. If the alternator fails, the car will completely drain the battery in as little as 20 minutes, leaving you stranded.