Volkswagen Mark IV Forum banner
1 - 5 of 5 Posts

·
Registered
Joined
·
25 Posts
Discussion Starter · #1 ·
Hi guys,

Wonder if anyone can shed some light. Im trying to eliminate a battery drain on a car (not VW) for a friend but not having much luck so far.

I have read somewhere that it is possible for an alternator to cause a drain if the rectifier pack becomes faulty. I have searched the proper way to check this but can't find and detailed instructions.

I inderstand i need to switch my meter over to AC volts and measure across the battery. Do i take my measurement with the engine running or not and what reading should i expect to see?

Thanks for any help,

Matt
 

·
Registered
Joined
·
14 Posts
Measuring AC across the battery won't tell you anything about drain. You need to put your meter on DC amps, remove the thick wire from the battery +ve to the alternator and connect your meter in series with this wire and battery +ve. Ignition off, meter set to 10A there shouldn't be any current draw.

A quick and easy test is to charge the battery, connect it back up with the engine cold and ignition off,then leave the car for an hour or so. Then touch the alternator casing, if It's warm the alternator is draining the battery. Not fooloproof but a simple check without a meter.
 

·
Registered
Joined
·
25 Posts
Discussion Starter · #4 ·
Cheers for the advice!

The paragraph i was refering to about switching the meter over to AC was this:

"Sometimes a bad alternator diode can cause a car battery to run down. A good diode should only pass current in one direction. If it leaks current in the opposite direction, it may keep the charging circuit on when the engine is not running, causing the battery to run down. This kind of problem can be diagnosed several ways. If your voltmeter has an AC (alternating current) scale, switch to that scale and observe the charging voltage with the engine running. If the meter shows any AC voltage, one or more diodes are leaking and the alternator needs to be replaced."
 

·
Super Moderator
Joined
·
50,230 Posts
I've never known a faulty altenator to produce enough AC to show on a multimeter.

The only way you can do this test accurately is with an oscilloscope.A good alternator will display all the phases equally with a smooth waveform,if a diode goes down the waveform becomes very obviously jagged.

The tests I detailed above are a method that works for me every single working day.The full load test can take about a minute to do and in most cases will tell you everything you need tto know,and the current draw test will tell you if something is wrong should the full load test not prove anything.

Chris.
 
1 - 5 of 5 Posts
Top