Healer C Ircuit D Ia G N O S Is - GMC 1986 P-Truck Service Manual

BLOWER MOTOR INOPERATIVE
(ANY SPEED)
— T
Check fuse
in fuse panel.
1
.......
X
FUSE BLOWN
With ignition switch in
" RUN" position and blower
speed switch "ON" use
meter to locate short in one
of the following wires:
1. From fuse panel to blower
speed switch.
2. From blower speed switch
to heater resistor.
3. From heater resistor to
blower.
Note. Short in circuit may be
intermittent. If meter does
not indicate a short circuit,
move harness around as
much as possible to re-create
short circuit. Watch artd
listen for arcing.
Use 12 volt test light and check feed
terminal (brown) on blower speed switch.
LAMP DOES NOT LIGHT
I
Repair open in brown wire
from blower speed switch
to fuse panel.
o
o
o
rv)
co
HEATER CIRCUIT DIAGNOSIS*
=
.... -
FUSE OK
The following tests should be made with
the ignition switch in " RUN" position
the blower speed switch " ON" and the
lever on heat position.
Check blower motor ground. |
1 POOR OR NO GROUND! j GROUND OK~]
| Repair ground. Check motor connector
with 12 volt test light.
1 ,
| LAMP LIGHTS | |LAMP DOES NOT LIGHT|
Replace Motor Check blower feed wire in connector
on resistor with 12 volt test light.
I
LAMP DOES NOT LIGHT
j LAMP LIGKTS
Replace switch.
BLOWER MOTOR INOPERATIVE
Disconnect resistor connector, connect one lead
of a self powered test light to any one terminal
and use the other lead to probe each of the other
two terminals.
TEST LIGHT DOES NOT
LIGHT ON ALLTERMINALS
I
Replace resistor.
1
"
LAMP LIGHTS ON
ALL THREE WIRES
LAMP LIGHTS
Replace blower
speed switch.
Repair open in feed wire
from resistor to blower motor.
(CERTAIN SPEEDS)
: i z
I
1
TEST LIGHT LIGHTS ON
ALL TERMINALS
With ignition "OFF" disconnect 3 wire
connector from resistor. Connect a
jumper lead from battery positive
terminal to any wire terminal in
connector. Use 12 volt test light to
check for voltage at the corresponding
wire on blower speed switch. Repeat
same test on other wires.
LAMP LIGHTS ON
ALL THREE WIRES
LAMP DOES NOT
LIGHT ON ALL
THREE WIRES
Repair open in
affected wire.
' See heater circuit diagrams.