1992 Chevy Van Front blower motor only works on high

Its a 92 but checking part #'s it appears to have 91 ACdelco factory a/c. Has a 2 wire blower motor. Only works on hi. Checked resistor. Looked ok, but tried new one. No luck. Checked voltage to motor on all switch positions. Was 11.88-11.91 on all positions. Off was off. Tried running a seperate ground to battery, but again only worked on hi even thogh other switched positions showed same voltage to motor. Its a conversion van w/rear fan that works fine