2007 Knightrain .. Can any one tell me .. definitively .. what the GREY plug is for ... under the seat on the left hand side ... It's a 4 pin .. deustch type .. plug .. with only 3 pins in it. The wires are Grey, Black/green tracer, Green/ violet (?) tracer. I've been told it's an accessory plug .. and/or a diagnostic hook up .. etc.

 

It is a diagnostic plug, used to retrieve codes from the engine or to up date the mileage on a new speedometer

 

Forgot to mention .. there is a accessory fuse in the fuse block

 

Mate, it's the diagnostic plug as already mentioned . Used by the dealer to read the ECM if ever required. Closes off with a waterproof "butt-plug".

It's also where you mount up your race tuner ( SERT, TTS , SEPST ) to change the ECM calibrations yourself if you choose to do that . I do lots of Harley tuning and re-mapping here in Australia using that Deutsch plug always

On all Softails it's where you describe under the seat. Other models have the same plug but mounted elsewhere. Under the left pannier on a dresser ( "bagger" ), under the left sidepanel on a Dyna , headstock on a V-Rod etc etc..

Now that's a definitive answer from Perth Western Australia ( Down under to you !)