Just a quick primer on how the heater motor/swith/resistor works on
MOST cars. There are some that get more complex, using relays aetc -
but this is pretty satandard.
On high the resistor is not in the circuit - so if it does not work on
high and someone tries to blame the resistor, he's blowing smoke.
On low, the current to the motor goes through the entire resistor. On
the medium speed(s) the power goes through PART of the resistor.
On some cars the switch feeds the power to the resistor through
different "taps" for each speed. On some it takes the power from the
resistor from different "taps" for each speed, and on others the
switch shorts out different sections of the resistor, one at a time,
untill the whole resistor is bypassed for high.
Many of the systems using relays have a relay to short out the
resistor for each speed.
So one thing you can be CERTAIN of - if it doesn't work on high - it
is NOT the residtor's fault.
That said - if the blower motor is jammed by ice in the blower wheel,
and the switch is left on low or medium - it WILL blow the resistor -
leaving you with the possibility of needing to change both.
Some cars even have 2 fuses for the blower motor - one for high only,
and one for the low speeds. I cannot remember right off hand WHAT car
uses that system, but I HAVE seen it.
Given the situation where the motor slowed down while on high, either
the resistance of the power feed went up (due to bad switc, burned
wire connector, or bad connection) or something went bad in the motor
- my guess from decades of experience and the age of the car is
warn/sticky motor brushes.
If it was a FORD - particularly an Aerostar or a Contour/Mystique my
bet would be on a burnred connection on the blower switch - and that
would likely be the first thing I would CHECK on the Volvo even though
the most likely problem is the motor. The switch is the simplest to
get to, the simplest to check, and the least expensive to fix
(generally speaking - I haven't had to buy a VOLVO heater switch).
If getting to the switch is difficult, simply back-probe the power
connector to the blower motor if it is easier to get to and check the
voltage with the switch on high. If you have 11.5 volts or higher and
the fan is not running at near full speed you have a motor problem. If
the voltage is markedly lower, it is a switch/connector problem.
If ALL speeds are low and the voltage on high is low it is most likely
a problem in the power feed to the switch. If high only, a problem
from the switch to the motor.