Chevrolet 1977 light duty truck, Service Manual

Page: 80 / 861

then adjust by rotating the pot until the compressor
clutch engages.
7. Reverse the adjustm ent by carefully rotating the
pot until the compressor clutch just disengages.
8. Reverse Steps l -4 above and check system
operation.
E X P A N S IO N VALVE (Fig. IB -3 2 )
An expansion valve is used on C-K Model O verhead
Systems, all G M odel systems an d M otor Home Chassis
Units.
A m alfunction o f the expansion valve will be caused
by one o f the following conditions; valve stuck open,
valve stuck closed, broken power element, a restricted
screen or an im properly located or installed power
e lem ent bulb.
A tta c h m e n t o f the expansion valve bulb to the
e v a p o ra to r outlet pipe is very critical. The bulb must be
attached tightly to the pipe and must make good contact
with the pipe along the entire length o f the bulb. A loose
bulb will result in high " h ig h sid e" pressures and poor
cooling.
Indications o f expansion valve trouble provided by
the P e rfo rm an ce Test are as follows"
VALVE S T U C K O PE N
Noisy Compressor.
N o Cooling - Freeze Up.
VALVE S T U C K C L O S E D , P L U G G E D
S C R E E N O R B R O K E N PO W E R E L E M E N T S
Very Low Suction Pressure.
N o Cooling.
Fig. lB-32--Expansion Valve
PO O R L Y L O C A T E D P O W E R E L E M E N T
BULB
N orm al Pressure.
Poor Cooling.
Check fo r D efective Valve
C A P I L L A R Y BULB
OUTLET
The following procedure must be followed to
determ ine if a m alfunction is due to a defective
expansion valve.
1. Check to d eterm ine if the system will meet the
p erform ance test as outlined previously. If the expansion
valve is defective, the low pressure readings will be
above specification.
2. The loss o f system perform ance is not as evident
when the high side pressure is below 200 PSI. Therefore,
it m ay be necessary to increase the system high side
pressure by partially blocking the condenser. Disconnect
the blower lead wire and repeat the "P erfo rm a n c e
C heck" to d eterm ine if the low side pressure can be
obtained.
3. The system will also indicate a low refrigerant
charge by bubbles occurring in the sight glass.
1. Connect 4 00 ± 1% Vi W att
re sistor across te rm in a ls F and J
2. W ith e n g in e ru n n in g , syste m on
A/C , te m p le v e r a t COLD, rota te
sh a ft un til com p re sso r clutch
engages. Then reverse a d ju s tm e n t
un til the com p re sso r just
Fig. IB -3 1 -A m p lifie r Board Potentiom eter
A d ju s tm e n t
E N G IN E IDLE C O M P E N S A T O R
This additional aid to prevent stalling during
prolonged hot weather periods is included with all air
co n d itio n e d vehicles. T he idle c o m p e n s a to r is a
thermostatically controlled air bleed which supplies
additional air to the idle mixture. On V-8 engines, with