OK so you got the basic single diode system.
Unplug that white connector , start the engine then check the AC & DC between the thick wire and ground.
If you see wildly changing DC but a fairly constant AC then the diode is probably shot.
Confirm this by getting a pin and piercing the wire about 1" or so higher than the connector.
Using the Ohms readings measure between the pin & the end of the terminal.
Should be 0 one way and infinity (O/L) with the probes reversed.
If not the diode is definately cactus.
It is a PIA to replace particularly if you are not a dab hand with a soldering iron.
I do it with the wires plugged into the mower to prevent the terminal getting too hot & softening the plastic, I am not a dab hand.