Sounds like your hot water by-pass valve is stuck open
SOURCE: 2003 GMC Sierra heater Blower wont shut off
My 2003 Sierra's blower came on by itself in the garage while I was sleeping and ran the battery completely down. I had to replace the blower switch located under the glove box next to the blower at the bottom of the right corner of the dash. The switch is in front of the blower and held in by two screws. Disconnect the two wire connections. This is a "dealer only" part and cost me 65.00. Also check the wire harness with seven wires connected to it for melted plastic. It will be marked "ABCDEFG". If it has melted, you'll need to replace it. Pay attention when reinstalling either part, that the wires "ABCDEFG" match the switch in that order.
SOURCE: 2005 GMC Sierra Overheating
If your sure there is no trapped air in system after refill and air bleed off, is fan clutch pulling good air thru radiator? Even if it is, the heater should have coolant as hot as what temp gage is showing! Try backwards flushing heater core (dex cool is notorious for plugging these up). There is a TSB for cracked cylinder heads from GM for unexplained coolant loss.Remove drivers side valve cover, and look at center three head bolts for coolant around them, if coolant has been getting in thru this suspect area, there will be a milky/pudding like goo under valve cover and on head. GM had a company called Castec cast these and the areas near the head bolts were found to be too porous (NO recall YET!) Ya, you guessed it, replace heads! Unfortunately, if its gotten hot to many times, it has probably hurt the heads/gaskets. Hope this helps. PS: The castec logo is under the rearmost pair of rocker arms,may have to remove them to see.
SOURCE: Replace starter on 1992 GMC Sierra pickup truck
First thing unhook positive battery cable, unhook main cable on starter selanoid and small wire on selanoid (there may be 2 small wires) There may be a bracket on front of starter that has to be removed, and then the 2 main bolts that hold it to engine block and it should come out.
SOURCE: 1997 GMC Sierra Ignition Switch Replacement
take off the cover on the top and bottom of the steering column just behind the wheel. pop the tilt lever out simply just by grabbing it and give it a good tug. then once you have the new switch and you get in there your going to need an inverted torx socket to remove the small bolts that hold the switch to the column. do not move the gear on the new switch make sure it lines up exactly as the old one did!!!!!!!!!!!!
SOURCE: 2004 GMC Sierra 4dr truck. the fan does not work
The switch or the wire harness at the blower motor has gone bad. The highest setting 5 has no resistor, so the blower runs on high but the high current flow ends up burning up the switch's contact on that setting or the resistor wire harness at the blower motor has gone bad. Common would be the switch it self, if the switch is good. Then it's going to be the wire leads at the speed resistor by the blower motor under the glove box by the passenger leg compartment. Keep me posted, be glad to help you get your Sierra running 100% again soon.
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