20
901271 REV G 02/04/19, 1969-70 MUSTANG wo AC EVAP INSTR PG 20 OF 26
Symptom Condition Checks Actions
N
ot
es
Blower
sta
ys
on
high
speed
when
ignition
is
on.
1a.
No
other
functions
work.
Check
for
damaged
pins
or
wires
in
control
head
plug.
Verif
y
that
all
pins
are
inserted
into
plug.
Ensure
that
no
pins
are
bent
or
damaged
in
ECU
.
Check
for
damaged
ground
wire
(white)
in
control
head
harness.
Verif
y
continuit
y
to
chassis
ground
with
white
control
head
wire
at
various
points.
Loss
of
ground
on
this
wire
renders
control
head
inoper
able.
All
other
functions
work.
Check
for
damaged
blower
switch
or
potentiometer
and
associated
wiring.
Blower
sta
ys
on
high
speed
when
ignition
is
on
or
off
.
Unplug
3-
wire
BSC
control
connector
from
ECU
.
If
blower
shuts
off
,
ECU
is
either
improperly
wired
or
damaged.
Be
sure
the
small,
20
GA
white
ground
wire
is
connected
to
the
battery
ground
post.
If
it
is,
replace
the
ECU
.
Unplug
3-
wire
BSC
control
connector
from
ECU
.
If
blower
sta
ys
running,
BSC
is
either
improperly
wired
or
damaged.
Check
to
ensure
that
no
BSC
wiring
is
damaged
or
shorted
to
vehicle
ground.
The
BSC
oper
ates
the
blower
by
ground
side
pulse
width
modulation
switching.
The
positiv
e
wire
to
the
blower
will
alw
ays
be
hot.
If
the
“ground”
side
of
the
blower
is
shorted
to
chassis
ground,
the
blower
will
run
on
HI.
Replace
BSC
(This
will
require
remo
val
of
ev
apor
ator
from
vehicle).
No
other
part
replacements
should
be
necessary
.
Compressor
will
not
turn
on
(All
other
functions
work).
2.
System
is
not
charged.
System
must
be
charged
for
compressor
to
engage.
Charge
system
or
bypass
pressure
switch.
Danger: Never bypass safety switch with engine running. Serious injury can result.
System
is
charged.
1b.
Troubleshoot
ing Guide
Check
for
fault
y
A/C
potentiometer
or
associated
wiring
(Not
applicable
to
3-pot
controls).
Check
for
disconnected
or
fault
y
thermistor
.
Check
continuit
y
to
ground
on
white
control
head
wire.
Check
for
5V
on
red
control
head
wire.
Check
2-pin
connector
at
ECU
housing.
To
check
for
proper
pot
function,
check
voltage
at
white/blue
wire.
Voltage
should
be
between
0V
and
5V
,
and
will
vary
with
pot
lev
er
position.
Disconnected
or
fault
y
thermistor
will
cause
compressor
to
be
disabled.
Red
wire
at
A/C
pot
should
ha
ve
appro
ximately
5V
with
ignition
on.
White
wire
will
ha
ve
continuit
y
to
chassis
ground.
White/
Blue
wire
should
vary
between
0V
and
5V
when
lev
er
is
mo
ved
up
or
down.
3.
Compressor
will
not
turn
off
(All
other
functions
work).
Check
for
fault
y
A/C
potentiometer
or
associated
wiring.
Check
for
fault
y
A/C
rela
y.
Repair
or
replace
pot/control
wiring.
Replace
rela
y.
See
blower
switch
check
procedure.
