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If it used to work, but now it has quit, then I would take a look at the switches for the conditions required for high idle, which are the E-brake must be on, and the service brake must be off. Also must be in Park (or neutral if a manual tranny). There are switches for each of those conditions.
It is a standard, park brake sewitch working (light comes on, tranny in neutral but the spedo does not work....
Tell more about the speedo not working. You mean it doesn't work when you drive down the street?
One of the conditions for high idle is that the truck is stationary. If the speedo isn't working, because of a bad VSS for example, it is logical to assume that the PCM would not allow high idle. The VSS system is how the PCM knows the truck is or isn't stationary. There should be a DTC for something like that.
I don't think there is any separate high idle module. It is a PCM controlled function.
Did the high idle stop working when the speedo died? Do any of the other dash functions work? Are you sure the 12+ signal is making it through your switch?
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