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Some of the dealers here in Alberta hook them up before selling them if the truck has the outfitter switches. Gives them another sales pitch when selling them here for the cold climate I guess...
Can this mod be done on the 5.4 trucks, and if so is there any point, or is this mostly a diesel thing?
Its a diesel thing, to help prevent wet stacking during long duration idleing. Diesel engines (unlike gas engines) run cooler when idleing for long periods causing imcomplete combustion which leads to deposits on the valve stems which can lead to stuck valves. Anytime you idle a diesel for 10-minutes or more you should run with elevated idle.
High idle is also used if you run PTO driven equipemnt.
Is that different than the VSS (Vehicle Speed Sensor)?
Yes it is. The VSS is a sensor used by the PCM to monitor vehicle speed and use the data along with other engine and vehicle sensors to adjust the engine parameters.
The VSO wire is a blunt cut customer access wire underneath the dash used to hook up accessories that need to know vehicle speed to work properly (ie GPS mapping systems that locate your car on the screen and give you current speed on there unit and time and distance to next turn).
Actually most aftermarket suff wants you to tap into the VSS wire. For all 05 and up trucks it is available right in the radio harness, get the scosche adapter and it has the correct output with it so you dont have to tap anything. (the bestkits and metra harness don't include the VSS or steering wheel control wire in them so you still have to splice into stock wiring)