Larry:
I inspected the wiring diagrams up to 1976 and there are
only two wires running to the heater motor. That would
indicate that the motors run at one speed unless the speed
is controled by a resistance being added into the circuit by
the switch. All of the later switches are rocker switches.
It is unlikely a rocker switch has two positions, in which
the first postition adds a resistance into the circuit to
slow the motor down.
Examples:
My TD had a heater in which the blower was infinitly
adjustable. The switch contained a reostat wound on a
ceramic plate. The more the resistance added the slower the
motor ran.
My Datsun pickup has two resistance wires placed inside the
blower housing so the blower cools them. When the switch is
on the 1st position, the current flows through the largest
wire. When in the second position it flows through the
smaller wire, and when in the third postion the current
doesn't flow through any resistance. It's heater motor is
like the later MGB's in that it uses permanent magnets for
the fields.
The early MGB's with the generators used series wound motors
and would have been single speed only.
I could scan and send you the wiring diagram from the truck
if you want to see how it was done. I would think it
possible to convert a single speed motor from 1967 and
later. You would need the resistance wires to place inside
the heater housing/blower plenum, and a three position
rotary switch.
Blake
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