I am starting to wire up my MegaSquirt and I'm a bit confused by the defogger circuit. Looking it over, I cannot see why it feeds into the OEM ECU at all. The circuit turns on and off the indicator lamp and defogger just fine with out this connection. Does the ECU change something if the rear defogger is on? Is there any reason to have pin 2L connect to my MS3? Right now I am planning on leaving it floating. Gracias.

 

it appears it's just so the ECU knows, possibly bumps the idle valve up a bit.

if the connection to 2L was removed, the light would still illuminate and the relay would still activate from the ground path back to G400.

 

Exactly. I guess I'll leave it out of my wiring then. Thanks Brain.

 

or use it to make the MS have fun with the defroster.

Turn it on when the IAT is below freezing and engine temp is below some set value (and then off when the engine have reached temp plus some set time)?

Why not add as many "useful" functions we can