the high speed relay does exactly this. it will "short" to ground the leg of the transistor that will allow full current to flow through the transistor. this gives you the highest speed.Jos Geuze said:If the blower is running at full speed it indicates a short to earth that bypasses the speed regulation electronics.
rtkraken: the high speed relay is on the circuit board next to the two transistors.
there is a thread around here somewhere that someone posted the pn and method to replace these transistors with parts from an electronics store. a little searching should bring it up.