Don't want to sound like a noob but what is a "DF" cable?
"DF" is a name left over from Ye Olden Days... (before alternators... when what we had were "generators").
The classical name for a generator is a "Dynamo", (hence the "D") and DF is the power being fed to create the magnetic field (there's the "F") in the non-moving (stator) coils of the beast.
Electricity is generated by physically moving coils of wire through a magnetic field.
The belt-and-pulley provides the motion. The moving coil is called the "rotor".
In order to *control* the voltage and current being produced by the dynamo/generator/alternator, you have the two choices of: control the speed or control the field current.
Since the *speed* is being dictated by the engine doing its "move the car" duties, you're left with the one choice of: control the field current.
The alternator's regulator does the "fine tuning" of the actual field currents to produce proper charging for the battery chemistry it's tuned for. For a flooded lead-acid, that's roughly 14v.
The wild-and-wonderful T1N Sprinter uses the "DF" wire from the instrument cluster (IC) to tell the alternator the motor is actually *running*, and that the alternator is now allowed to put its load on the belt. As the training and service manual says, the IC separately monitors the system voltage, turning on the ALT light if it's below 12.7 v
In the NCV3 (certainly with the 4 cylinder engine), the commands to the alternator/regulator are now transfereed by using a computer-messages LINbus instead of a DF wire.
In all of the Bosch alternators i've serviced since 1967 model years, the manuals all said that they were "not self-exciting" and required a DF signal/power. My *experience* was that there usually *was* sufficient residual magnetism in their steel to self-excite and get the currents flowing...
--dick
p.s. alternators vs generators: generators had "segmented commutators" (the things that the brushes rubbed against) and created DC voltages. Alternators have continuously-connected "slip rings" for the brushes to ride upon, and create AC voltages. Alternators require a "diode bridge" (or network) to convert the AC to DC to make the battery happy.