Chevrolet 1977 light duty truck Service Manual page 33

TROUBLE
Temperature of heater a ir a t outlets too low to heat up pas­
senger compartment.
Temperature of heater a ir a t outlets adequate but the v e h i c l e
w ill not build up s u ffic ie n t heat.
Inadequate defrosting action.
Inadequate c irc u la tio n of heated a i r through v e h i c l e .
E rra tic heater operation.
Hard operating or broken controls.
DIAGNOSIS
1. See "In s u ffic ie n t Heat Diagnostic Chart".
Check fo r body leaks such as:
1. Floor side kick pad ven tilato rs p a r tia lly open.
2. Leaking grommets in dash.
3. Leaking welded seams along rocker panel and windshield.
4. Leaks through access holes and screw holes.
5. Leaking rubber molding around door and windows.
6. Leaks between sealing edge of blower and a ir in le t assembly
and dash, and between sealing edge of heater d istrib u to r
assembly and dash.
1. Check that DEFROST lever completely opens defroster door
in DEF position - Adjust i f necessary.
2. Insure that temperature and a ir doors open f u l ly - Adjust.
3. Look fo r obstructions in defroster ducts - Remove any
obstructions.
4. Check for a ir leak in ducting between defroster o u tle t on
heater assembly and defroster duct under instrument panel -
Seal area as necessary.
5. Check position of bottom of nozzle to heater locating tab -
Adjust.
6. Check position of defroster nozzle openings re la tiv e to in stru ­
ment panel openings.
i f properly in s ta lle d .
1. Check heater a ir o u tlet for correct in s ta lla tio n - R e in s ta ll.
2. Inspect flo o r carpet to insure that carpet lie s f l a t under
fro nt seat and does not obstruct a ir flow under seat, and
also inspect around o u tlet ducts to insure that carpet is
well fastened to flo o r to prevent cupping of a ir flow - Correct
as necessary.
1. Check coolant level - F i l l to proper le v e l.
2. Check for kinked heater hoses - re lie v e kinks or replace hoses.
3. Check operation of a ll bowden cables and doors - Adjust as
necessary.
4. Sediment in heater lin es and radiator causing engine thermo­
s ta t to stick open - flush system and clean or replace thermo­
sta t as necessary.
5. P a r t ia lly plugged heater core - backflush core as necessary.
1. Check fo r loose bowden cable tab screws or mis-adjusted bowden
cables - Correct as required.
2. Check fo r sticking heater system door(s) - Lubricate as required
using a silico n e spray.
CAUSE AND CORRECTION
Mounting tabs provide positive position