GMC Repair: A/C Fan, air plenum, gmc 1500

Question
I have a 1990 GMC 1500 pickup. The fan motor for the A/C changes speeds by itself, seem to notice it only on high

Answer
Hi
On this system, there could be a few things. Either the A/C Controller, blower resistor, High speed relay or wires feeding any of these. Check the Blower motor wire harness see if it is not burned. There is a connector behind the glove box where the blower motor plugs in, check it see if it is not burned.
If wires check out, and motor checks out, the High relay and resistor are located on TOP of the air plenum behind the glove box.
The controller on these systems were/are known for being faulty and causing strange faults.
If in doubt, I would take it to a repair facility YOUR familiar with and get them to check it out.
Just advise from experience.
Hope this helps
Good Luck
Paul