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91 Suburban blower motor

My blower motor went out. I replaced it, but it only starts on low even when the switch is in the high position. It will go to high speed if I take the hot lead off and plug it back in while the motor is set to high, or by flicking the relay. I changed out the relay and resistor as well and nothing has changed. Thoughts?

You have a high resistance connection in the circuit somewhere.
The motor works thusly; on HIGH it has a direct 12VDC applied and as you lower the speeds the “resister block” adds more resistance at each setting. The resistance “drops” voltage, leaving only the remaining balance to be seen by the motor. On LOW the motor probably only sees 6VDC.

I’d pick up a can of spray Contact Cleaner, available at electronics houses such as Radio Shack, flush the connectors out, and see if that helps. If not, you could have fractured insulation in the wiring harness creating a “high resistance short to ground”. Can you use a multimeter? If so, you can try physically moving the harness around at points where it goes through the access holes in the body structures looking for changes in applied voltage to the motor. That’s usually where these shorts occur.

The blower switches are a weak spot on those. Remove the control panel and test the switch and inspect for a burned connector.