21
901149 REV D 7/30/14, INST 67-72 CHEVY PICKUP wo AC EVAP KIT PG 21 OF 24
Symptom Condition Checks Actions Notes
System will not
turn on, or runs
intermittently.
4.
Works when engine is not
running; shuts off when
engine is started
(Typically 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 away from ECU and ECU wiring. Check for burned
or loose plug wires.
Verify connections on power
lead, ignition lead, and both
white ground wires.
Verify proper meter function by checking the condition of
a known good battery.
Ignition noise (radiated or
conducted) will cause the
system to shut down due to
high voltage spikes. If this
is suspected, check with a
quality oscilloscope. Spikes
greater than 16V will shut
down the ECU. Install a
radio capacitor at the
positive post of the ignition
coil (See radio capacitor
installation bulletin). A
faulty alternator or worn
out battery can also result
in this condition.
Will not turn on under
any conditions.
Verify 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
evaporator housing
installed in a bind in the
vehicle. Be sure all
mounting locations line up
and don’t have 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 breaker.
Ensure all system grounds and power connections are
clean and tight.
Battery voltage is less
than 12V.
5.
Troubleshooting Guide (Cont.)
Check for faulty 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
even a split second, the
system will reset.
Run red power wire directly to battery.
Check for positive power at heater valve 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.
Erratic functions of
blower, mode,
temp, etc.
Check for damaged switch or
pot and associated wiring.
Repair or replace.
8.
