My 98 gmc1500 truck blows heat only on high. If I adjust it lower than high the blower does not work and I have to just put it on high and wait for it to come on by itself. sometimes days later. any help would be appreciated.
You probably need a new blower motor resistor block/pack. This is what gives all the lower blower speeds other than high. But the other thing that can cause this is a high speed blower motor relay. This is what switches power between the lower speeds of the blower motor and the highest speed of the blower motor.
“the other thing that can cause this is a high speed blower motor relay.”
Yes, the solution could be something as simple as cleaning the relay contacts…
I would only add that on a 1998 I would also consider that the bearings of the blower fan may be going out and that in turn blowing the resistor pack. Often the best way to determine that is to replace the resistors and if they blow, look to the fan motor.