(supposed) voltage issue when idling

[CinqueTerreElise]

Electrical issue on my Italian Spec 2.0L '96 S4.  Just after engine start sometimes dashboard lights fade. It takes just a blast on the accelerator to bring them back to standard lighting and problem disappears.

When playing this trick, I hear a clicking sound when lighting becomes 'normal' again. Being a mechanical/automotive engineer I strongly suspect a voltage regulator issue.

My Esprit tends to drain battery and it's kept under mantainer. Once it drained the battery when idling due to an hi-power iPad charger plugged in the lighter socket. I jump started the car and after a drive it was fine again and the morning after it started regularly. So I suppose the Valeo stock alternator is working fine.

Any clues?

 

[CarBuff]

Even when they are "working fine", the Valeo alternators provide famously insufficient current at low engine speeds. Your Esprit will be 'running off the battery' at RPMs less than ~1100 especially with electrical loads (headlights, A/C, interior or cooling fans).

 

The voltage regulators can overheat as a result, melting the potting compound around the components. The black goo actually drips down onto the A/C compressor providing a visual indicator of failure.

 

You can easily replace just the VR if you choose (they are available), but many owners have fitted a Bosch alternator that provides more current at low RPMs.