I am curious as to the actual output of the D+ terminal when the signal is being supplied by a "smart" alternator.
I do not have a "smart" alternator, so am not able to verify the output myself.
Assuming the D+ terminal output is fed via a relay which is controlled by the Instrument Cluster, via the alternator.
My questions are :
Is the output a fixed value voltage, regardless of the actual alternator output ?
Is the output "smoothed" to cater for regenerative braking, and when engine "stop - start" features are engaged etc. ?
Will the D+ output remain constant during the entirety of the journey, even when the full suite of the alternators "smartness" capabilities , are activated / deactivated ?
First, the "D+" has only two states: on (+12v) and off (0v).
I do not know how it changes time-wise when the alternator is being "smart".
The EK1 post is connected to relay contacts, and thence to normal power.
But .... the "smart" alternator merely means that it has a bi-directional communication path (the LINbus) with the rest of the Sprinter. It's the Sprinter that commands the alternator, not the other way 'round. So the Sprinter certainly has the power/knowledge to turn D+ on and off as it's having the alternator do strange things.
From an "old school logic" point of view, (ignoring D+) ... the alternator (at maximum smart mode) will be commanded to be in one of four states:
(a) immediately after starting, charge the starter battery at a fairly high "recovery" rate.
(b) after not-too-long a while, drop the charge to a "medium" rate.
(c) an unknown (to me) time later, once the battery is above 80% state-of-charge, tell the alternator to stop charging.
this maximizes MPG by removing the 4 horsepower alternator load (at full 220 amp output).
(d) at times when additional engine braking force is desired, resume charging the battery to put that 4 hp load on the belt.
(this is why the "80%" of (c) ... it leaves "room" to cram more current into the battery when wished)
As someone above mentioned, the Sprinter monitors the charging current reaching the battery (and voltage levels) so it has some idea of what state-of-charge the battery has reached.
Midwestdrifter's idea of simply watching the voltage level with a plug-in meter is quite good. S**t-simple.
My (totally dumb) T1N would show 14.2 for (a), and 13.6 to 13.8 for (b), loads depending (lights, AC, fans).
I use my ScanGauge as my "plug-in" voltmeter.
If the D+ was purely following (chasing?) the alternator's function, i would expect (a)(b) and (d) to be "on".
If you buy MB's charging-rig option (the isolation relay, etc) they (might) "obviously" would tell the control system to perhaps forgo (c).
In the 2007-2013 NCV3 Sprinters, the 4-cylinder engine made use of "smarts" ... the 6-cylinder didn't seem to.
--dick