i have a 91 4.3 4x4 blazer and the heater/ac blower will only work on high if i tap the relay. it's a new relay, the old relay did the same thing. thanks for the help!!!

 

It sounds like a poor connection. Maybe one of the terminals in the connector is loose or corroded. You can use a tiny screwdriver to re-form or close the tabs in the harness for a snug fit on the relay tabs

 

thanks for the info! i cleaned the connector with no luck but, i traced the wiring back from the connector to another connector that's plugged in to switch or sensor in the same box as the blower motor, cleaned it , took it off and on a few times and so far it's working fine. thank you c.l.

 

Glad to hear that it's working. You probably cleaned the contacts at the blower motor resistor.
The resistor is a common failure, if the heater blower ever quits, you'll know where to look.