Alternator Diagnostics

[steveh2009]

Hi

I have an alternator fault and have taken a few readings and wanted to check whether my diagnosis was sound.

With the engine off, reading 12.10 VDC across battery
With engine running, VDC drops steadily instead of increasing, indicating alternator not charging.

Now, if I take an AC measurement across the battery, I get 18VAC. Is this an indicator that it is the diode/rectifier that has failed, and if so, is this part servicable ?

TIA

Steve

[signumturbo04]

dunno about the rectifier but i chaanged the diode pack in my 2.0t alternator for less than £2

[steveh2009]

sounds like the kinda thing I'm looking for. Where did you get the replacement diodes from ?

[Marky]

Now, if I take an AC measurement across the battery, I get 18VAC. Is this an indicator that it is the diode/rectifier that has failed, and if so, is this part servicable ?

What should the VAC value be if it was a good battery?

[mxt]

It sounds like Diode/Rectifier to me as it sounds as though the AC from the Stators is going straight through. You should not be getting 18VAC.

Get the make/model number of your alternator and check out ebay for a new pack.

Or see if you can get an exchange alternator from your local motor factors.

[LADmachining]

You will normally get a little AC ripple from a working alternator - usually only a matter of a few mV, no way should you be getting volts AC across a battery.

Looking at this ripple voltage on an oscilloscope is a good way of diagnosing faults with the charging system, once you know what to look for.