Chevrolet 1977 light duty truck, Service Manual

Page: 79 / 861

Fig. IB -3 0 Checking Evaporator Inlet and
A c u u m u la to r O utlet Tem peratures
ON VEHICLE SERVICE
T H E R M O S T A T IC S W ITC H
C-K Systems and Motor Home Chassis Units make
use o f a therm ostatic switch with either an air or
ev a p o ra to r inlet line sensing capillary. This capillary
controls the switch by sensing the tem perature o f the air
leaving the evaporator core (M otor Home Units) or
tem p eratu re o f the core fins (C-K models).
C hecking fo r Proper Operation
M o to r Hom e Chassis Units
1. Install the gauge set and set up the vehicle as
described u n d e r " P erfo rm a n c e Test."
2. M ovem ent o f the tem perature control knob
should result in a definite change in suction pressure and
cycling o f the compressor clutch.
• If compressor continued to operate regardless
o f the knob adjustm ent, it indicates that the switch
points are fused which will lead to evaporator freeze-up.
Replace the switch.
• If the compressor does not operate, regardless
o f the position o f the knob, a loss o f the power element
charge is indicated (provided th at it has been established
that power is supplied to the switch). This, o f course,
results in no cooling. Replace the switch.
C-K Models
1. Install the gauge set and set up the vehicle as
described under " P erfo rm a n c e Test".
2. Set the control at A /C , HI blower, m ax C O L D
and ru n the engine at 2000 rpm.
• The thermostatic switch should cycle the
compressor off w hen the low limit o f the outlet air
tem p eratu re is reached (see Peform ance Data). If it does
not, the switch points are fused which will lead to
evaporator freeze up. Replace the switch.
• If the compressor does not operate, a loss o f
power element charge is indicated (provided that it has
been established that power is supplied to the switch).
This, o f course, results in no cooling. Replace the switch.
N O TE: Do not attem pt to run a Perform ance
Check with the system disassembled since inaccu­
rate readings would be the result. ALW AYS
reinstall switch and capillary and any duct work
before ru n n in g a p erfo rm an ce check.
A M P LIFIE R BOARD P O T E N T IO M E T E R (Fig.
I B - 3 1 )
To cure a "too cold" or "not cold e n o u g h " capacity
complaint, the sensor should be simulated by a 400
± 1%, 1/2 W att resistor. By connecting the resistor as
shown in Figure IB-31, the control pot can be rotated
until the compressor clutch energizes. T he procedure is
as follows:
1. Remove the headlam p switch knob and then
remove the instru m en t panel bezel.
2. Remove the control to in stru m en t panel screws.
3. R em ove the left foot cooler bracket to
instrum ent panel reinforcem ent screws and remove the
foot cooler and duct.
4. With the system in the vehicle and the engine
running, disconnect the terminals F and J from the
connector at the control w iring harness connector.
5. Connect a 400 ohm ± 1 % , 1/2 W att resistor
across terminals F and J.
6. Set the control tem perature lever at C O L D and