OBD #9 and actual voltage / fluctuations

[pingu]

I recent compared the reading from OBD test 9 with a "proper" digital multimeter (DMM). I measured the "real" system voiltage using the +Vbat jump starting terminal under the bonnet.

The OBC was consistently 0.2V low - not much difference really.

But with the engine idling at 1,000r.p.m., the OBD indicated that the alternator voltage was fluctuaing from 13.6 to 13.9V, whereas the DMM said that the voltage was rock stead at 14.0V.

So if you use OBD #9 and reckon that alternator is fluctuating (and like me, were worrying about worn brushes), maybe it's just the OBD telling a fib.

[shogun]

My OBC is doing the same. Voltage is 0.2-0.4 v lower on the OBC, I checked it with 3 different DMM's to make sure.

[Mr Project]

It's very logical that there would be quite a bit more voltage drop before the OBC readout compared to the jumper terminals under the hood. Think of all the connections and small-gauge wire that are between the alternator and the OBC! :)

The fluctuation is a bit odd, but thanks for your experience...will definitely file that away!