High speed blower

I have a 1971 Delta 88 convertable with air. The two lower speeds on the blower work fine, but not the high speed. Why is the high speed controlled by a seperate relay? Could something besides a defective high speed relay be the problem? Why is there a seperate control relay for the highest speed anyway?

Why are there two relays controlling the top motor: One under the dash and one at the motor? Thanks

The high speed runs thru a relay to keep excessive amperage from going thru the HVAC control head. The older units where everything ran thru the control head would often put on a fireworks show under the dash- not a pleasant experience!

The 1, 2 and 3 speeds run power from the fuse block thru the control head and blower switch to the resistor block, controlling fan speed. When you put blower switch in high, it triggers a solenoid inside the relay which connects 12v directly from the battery to the blower motor so it can run at full speed. The high speed wire (orange/black) runs from the junction block on the firewall to the relay, and it sometimes has a short 30 amp inline fuse. That fuse is sometimes hard to get out if its rubber clip so I usually replace the whole thing with two inline 30 amp fuses wired in parallel. Sometimes I use glass cartridge fuses, sometimes a blade type fuse, either will work fine. (It's a trick I learned working on Pontiacs.) Other than the relay itself or the aforementioned fuse, I don't know of anything else that will cause high speed blower failure. Thsi is a common problem, BTW.

The top motor relay is at the front of the car and serves same purpose as the hi blower relay- to keep excessive amperage from going thru the top switch. The one you see at the back of the car is to change direction of the top motor for raising or lowering the top.

Great answer. Thanks