One thing I have not seen mentioned is whether the A/C blows through or is
otherwise associated with the heater core. Is the heater really 'off'?
On my '99 F250 I installed an aftermarket valve (vacuum operated) to shut
off all coolant flow through the heater core when the dash control is moved
to 'max A/C'. Otherwise, coolant always flows through the heater core and
the A/C is ducted immediately adjacent to the heater core.
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