Forward Resistance test of a bad diode.
When a diode reads this low in ether direction, IE, any reading in the low K Ohm range. It is defective.
Below 400k ohm's ??? maybe is a state we call leaky. Change it.
They need to read near, or in the mega ohm range to be good. IE, .700 meg ohms forward resistance to 1.5 and up meg. ohms reverse resistance.
Should the diode read open with a voltage test in both directions it is defective.
If the diode is open this will cause what looks like a dead alternator, when the only problem maybe the lack of excitation voltage at the alternator when the key is in the start position. Note ! there is voltage on this wire in only the start position not in the run position.
No excitation voltage =s no output, so always check to make sure that there is power on this wire when starting the Engine! There is or should be no or a very low voltage present with the key in the run position.