Any ideas on how I can stop the drop in voltage when the fan, lights and wipers are on?
If it happens at idle, then it's unaviodable.
A quick calculation of the power demand of these:
- Dipped lights + rear lights + number plate lights + dash lights = 160W
- Cooling fan = 85W
- Wiper motor (@low speed) = 45W
- Engine management (driving the injectors and throttle bypass valve) = 60W
- Ignition coil = 40W
- Oxygen sensor heating = 12W
402W in total, that's more than 33 Ampers.
The alternator in the S3 serie is able to deliver 23 Ampers @ idle
without voltage drop (13.7-14.2 V), if the charging cables and
contacts, and the alternator are ok. You would need a Mercedes W126 alternator
(Bosch # 0 120 469 588) to get rid of the voltage drop @ these conditions