The blower resistor provides the low and intermediate speeds rather than the higher speeds. The relay provides (only) max speed.
If you have lost all speeds other than the lowest (always on) speed, AND they all disappeared at the same time, you most likely have connection problems and/or switch failure. Note: The max speed (relay) circuit has a separate inline resistor, which you should check, but it is not likely that it is open.
If you lost max speed first, as is most common, followed later by medium speed, both the max (relay) circuit and the blower resistor and/or connections have issues.
The blower motor pulls over 20 Amps on max speed. The spade terminal connections, though adequate when new, become higher resistance connections as the vehicle ages causing the connections to overheat, often melting the connector housing and fatigueing the female terminals, further increasing the resistance and resulting heat.
Once the max speed fails, most owners use second speed, which now overheats at the connection inside the partially melted terminal housing, and subsequently fails. As a result, it is not uncommon to find such vehicles having only the slowest speed functional.
For this reason, I NEVER use MAX blower speed, unless the vehicle is new and well within warranty.
Look for melted or deformed connector housings at the motor (most likely) and/or switch and relay. A reliable repair will REQUIRE NEW female terminals as well as a new connector housing.
An interesting aside: The always on, minimum speed was introduced by GM (at least) in 1968 when vent windows were removed and marketed as a feature of the "Astro Ventillation" system but the real reason was much different.
After alternators were introduced in the early 1960s, manufacturers learned that the energy in the magnetic field of a rapidly spinning alternator could and did damage damage automotive electronics (radio) and even lamp bulbs during a "load dump" condition, which results from a disconnection or intermittant high resistance connection at the battery during a time when the alternator is providing a heavy charge to the battery.
Neither the vibrating contact voltage regulators nor the early (and probably current) electronic voltage regulators could shut down the alternator and absorb the very high energy resulting from the rotating magnetic field in the alternator quickly enough to protect the electronics or even lamp bulbs.
In a former life as a design engineer for GM, I saw laboratory experiments at Delco Electronics in Kokomo, IN, in which load dump conditions destroyed all of the vehicle electronics as well as all of the lamps that were on at the time.
It was decided in the 1960s, to leave the blower connected to the vehicle bus at all times in order to provide an inductive load capable of absorbing PART of the load dump energy making it esier to control the rest.
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Today's Featured Article - Gatherin of the Orange - by Rick Nikolich. In July of 1998 I was talking to fellow Allis Chalmers collector Mike Schilling about the annual "Gathering of The Orange" AC show coming up in August of 1999. He got this wild idea that we should get a convoy of AC tractors and drive them from Charlotte, Michigan 105 miles to LaGrange, Indiana.
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