21
901271 REV G 02/04/19, 1969-70 MUSTANG wo AC EVAP INSTR PG 21 OF 26
Symptom Condition Checks Actions
N
ot
es
System
will
not
turn
on,
or
runs
intermittently
.
4.
W
orks
when
engine
is
not
running;
shuts
off
when
engine
is
started
(T
ypically
early
Gen
IV
,
but
possible
on
all
versions).
Noise
interference
from
either
ignition
or
alternator
.
Install
capacitors
on
ignition
coil
and
alternator
.
Ensure
good
ground
at
all
points.
Relocate
coil
and
associated
wiring
aw
ay
from
ECU
and
ECU
wiring.
Check
for
burned
or
loose
plug
wires.
Verif
y
connections
on
power
lead,
ignition
lead,
and
both
white
ground
wires.
Verif
y
proper
meter
function
by
checking
the
condition
of
a
known
good
battery
.
Ignition
noise
(r
adiated
or
conducted)
will
cause
the
system
to
shut
down
due
to
high
voltage
spik
es.
If
this
is
suspected,
check
with
a
qualit
y
oscilloscope.
Spik
es
greater
than
16V
will
shut
down
the
ECU
.
Install
a
radio
capacitor
at
the
positiv
e
post
of
the
ignition
coil
(See
radio
capacitor
installation
bulletin).
A
fault
y
alternator
or
worn
out
battery
can
also
result
in
this
condition.
Will
not
turn
on
under
an
y
conditions.
Verif
y
battery
voltage
is
greater
than
10
volts
and
less
than
16.
Loss
of
mode
door
function.
No
mode
change
at
all.
Check
for
damaged
mode
switch
or
potentiometer
and
associated
wiring.
Partial
function
of
mode
doors.
Typically
caused
by
ev
apor
ator
housing
installed
in
a
bind
in
the
vehicle.
Be
sure
all
mounting
locations
line
up
and
don’
t
ha
ve
to
be
forced
into
position.
Blower turns on and
off
rapidly
.
6.
Battery
voltage
is
at
least
12V
.
Check
for
at
least
12V
at
circuit
break
er
.
Ensure
all
system
grounds
and
power
connections
are
clean
and
tight.
Battery
voltage
is
less
than
12V
.
5.
Troubleshoot
ing Guide (Con
t.)
Check
for
fault
y
battery
or
alternator
.
Charge
battery
.
System
shuts
off
blower
at
10V
.
Poor
connections
or
weak
battery
can
cause
shutdown
at
up
to
11V
.
7.
When
ignition
is
turned
on,
blower
momentarily comes
on,
then
shuts
off
.
This
occurs
with
the
blower
switch
in
the
OFF
position.
This
is
an
indicator
that
the
system
has
been
reset.
Be
sure
the
red
power
wire
is
on
the
battery
post,
and
not
on
a
switched
source.
Also
,
if
the
system
is
pulled
below
7V
for
ev
en
a
split
second,
the
system
will
reset.
R
un
red
power
wire
directly
to
battery
.
Check
for
positiv
e
power
at
heater
valv
e
green
wire
and
blower
red
wire.
Check
for
ground
on
control
head
white
wire.
Check
for
obstructed
or
binding
mode
doors.
Check
for
damaged
stepper
motor
or
wiring.
Err
atic
functions
of
blower
,
mode,
temp
,
etc.
Check
for
damaged
switch
or
pot
and
associated
wiring.
Repair
or
replace.
8.
