What voltage reading should i get when car is running?
 

I want to test my alternator but am unsure what the output should be when the car is running (with all electrics on and off) ??

should remain fairly constant electrics on or off while the cars running, should be around 14.2 volts or there abouts

Normally between 14.2 and 14.4, it should dip when you turn on lights or HRW but raise back to the same level.

My scangauge shows a pretty consistent 14.2 volts, as James says it may vary slightly as electrical loads are switched on.

I tested my voltage and to start with i got a reading of about 14.0 / 14.1 but with everything switched on the voltage dropped to 13.0 volts. This was whilst the car was on tick over. Does this sound right or should it recover to about 14.0 v? (heated screen, fans on full, a/c, lights dipped/beam etc)

hmm sounds like it isnt putting out enough tbh, although a poor battery can also drag it down, personally id like to do some drop tests on the battery and altenator