Chevrolet 1977 light duty truck, Service Manual

Page: 679 / 861

THROTTLE RETURN CONTROL (TRC)
GENERAL DESCRIPTION
A throttle retu rn control system (T R C ) is used on
10-30 Series Trucks in C a lifo rn ia w ith heavy duty
em issions system s. Also, 305 C.I.D. heavy duty em issions
(except C a lifo rn ia ) vehicles use the T R C system.
W h en the vehicle is coasting ag ain st the engine, the
T R C valve will open at high m anifold vacuum levels to
allow vacuum to o p erate the th ro ttle lever actuator. The
th ro ttle lever actu ato r pushes the throttle lever slightly
open, thus reducing h y d ro -carb o n em issions du rin g coast
down. W hen m anifold vacuum drops below a p re ­
d e te rm in e d level (T R C valve set p oint) the control valve
closes, the throttle actu ato r retracts, and the throttle lever
retu rn s to idle position.
ON-VEHICLE SERVICE
SYSTEM O P E R A T IO N
T he T R C valve and ac tu a to r system should function
to slightly o p en the throttle at high m an ifo ld vacuum
o v erru n conditions (ab o u t 21-23 in. hg. [71 -78kPa]
d e p e n d in g on the p a rtic u la r engine application) and
re tu rn to the curb idle p o sition at lower m an ifo ld
vacuum s. F ailu re to function in this m an n er indicates the
T R C valve o r actu ato r is m isadjusted, a vacuum leak
exists, the valve vent is plugged or there is b in d in g
som ew here in the system.
C heck hoses fo r cracking, ab rasio n , or d eterio ratio n
and replace as necessary. Check system function for
p ro p e r o p e ra tio n and adjust as as necessary.
C H E C K IN G A N D A D J U S T IN G SYSTEM
C ontrol Valve (Figs. 6 E -43,44, 4 5 )
Checking and Adjusting
1. D isc o n n e c t v a lv e -to -c a rb u re to r hose at the
c a rb u re to r an d connect to an external vacuum source
w ith an accurate vacuum gage inserted n e a r the valve.
2. A pply a m inim um o f 25 in. Hg. (84 kPa)
vacuum to the control valve vacuum supply fitting and
seal off the vacuum supply betw een the gage and the
source. The vacuum gage re a d in g will indicate the valve
set p o in t value.
If the gage rea d in g is n o t w ith in .5 in. Hg. (7kPa) o f
the specified value fo r the p a rtic u la r engine being
checked, then the valve needs adjustm ent. If the
tra p p e d vacuum drops off faster th a n .1" Hg. per
second, then the valve is leaking and m ust be
replaced.
3. To ad ju st the valve set point:
a. G en tly p ry off the conical plastic cover.
b. D isengage the ja m nuts by holding the larger
nu t and loosening the sm aller nut. A djust the valve by
tu rn in g the larg er n u t in (clockwise) to raise the set p oint
or out (counter-clockw ise) to lower the set p o in t value.
c. Recheck the valve set p o in t p e r Step 2.
d. R epeat Steps b and c as necessary to o b tain
the p ro p e r set p o in t value.
292-22.5 in. hg
305-22.5 in. hg
350-21.5 in. hg
400-21.5 in. hg
454-23.0 in. hg
.5 (m aroon)
.5 (orange)
.5 (black)
.5 (black)
.6 (green)