GMC 1982 Light Duty Truck, Service Manual

Page: 972 / 1435

6 E -2 8 ENGINE EMISSION CONTROLS
n ected and if the a c tu a to r does n o t e x ten d w hen
o p e ra tin g a b o v e 1890 R P M , re p la c e th e sp eed
sw itch.
6. If the a c tu a to r rem ains extended at all speeds, proceed
as with following steps.
a. Remove connector from vacuum solenoid.
b. If actu a to r rem ains extended, check the orifice in tbe
solenoid side port for plugging. If plugged, clear and
reconnect system and recheck. If the a c tu ato r again
rem ains extended, remove the solenoid connector. If
the a c tu ato r does not re trac t, replace the vacuum
solenoid.
c. If the actu a to r re tracts with the solenoid connector
off, reconnect and then remove the speed switch
connector. If the actu a to r retracts, replace the speed
switch. If the actu a to r does not retract, the solenoid-
to-sw itch wire is shorted to ground in the wiring
harness. R ep air as required.
Throttle Lever A ctu ato r - Checking Procedure
1. Disconnect valve to a c tu a to r hose a t valve and connect
to an external vacuum source equipped with a vacuum
gage.
2. Apply 20 in. Hg vacuum to the ac tu ato r and seal off the
vacuum source. If the vacuum gage reading drops, then
the a c tu ato r is leaking and m ust be replaced.
3. To check the a c tu a to r for proper operation:
a. C heck the th ro ttle lever, shaft, and linkage to be sure
th a t they operate freely w ithout binding or sticking.
b. S ta rt engine and run until w arm ed up and idle is
stable. N ote idle rpm .
c. Apply 20 in. Hg vacuum to the actu ato r. M anually
open the th ro ttle slightly and allow to close against
the extended a c tu a to r plunger. N ote the engine rpm .
d. R elease and re ap p ly 20 in. H g v acuum to the
a c tu ato r and note the rpm to which the engine speed
increases (do not assist the actu ato r).
e. If the rpm obtained in step D is not w ithin 150 rpm of
th a t obtained in step C, then the a c tu ato r plunger
m ay be binding due to d irt, corrosion, varnish, etc.,
or the ac tu a to r d iaphragm m ay be too weak. If
binding is not indicated or cannot be corrected then
the a c tu ato r m ust be replaced.
f. R elease th e v ac u u m from the a c tu a to r and the
engine speed should retu rn to within 50 rpm of the
idle speed noted in step 2. If it does not, the plunger
m ay be binding due to d irt, corrosion, varnish, etc. If
the problem cannot be corrected, the actu ato r m ust
be replaced.
g. If the engine rpm noted in step 3 is not w ithin the
specified T R C speed range, the T R C ac tu ato r m ust
be adjusted. See T h ro ttle Lever A ctu ato r A djusting
Procedure in section 6C.
V A C U U M A D V A N C E C O N TRO L
Trapped V a cu u m Spark
T rapped vacuum spark is on all engines. A therm al
vacuum sw itch (T V S ) is used to sense engine co olant
tem perature. A check valve is betw een manifold vacuum ,
d istrib u to r and therm al vacuum switch.
W hen engine tem p eratu re is below specified value, the
m anifold vacuum signal is routed through the check valve to
the distributor. Ports on TV S are blocked. The check valve
will keep the vacuum to the d istrib u to r at vacuum levels
h ig h er th a n m anifold d epression d u rin g vehicle
acceleration. A sm all sintered iron bleed orifice is provided
in the check valve to allow for a leak-dow n to enable restarts
in case of engine stalls.
W hen engine tem p eratu re is above specified value, the
ports on TV S will be open to allow m anifold vacuum to the
distributor. T he check valve in this mode of operation acts as
a connector.
SPARK V A C U U M DELAY
G e n e ral Description
On heavy duty emission engines a check valve w ith an
internal beed is installed betw een the T rapped V acuum
S p a rk check valve and th e d is trib u to r. W hen m anifold
vacuum increases it opens the check valve which allows the
distributor vacuum to increase to the sam e level. D ecreasing
m anifold vacuum , as during vehicle acceleration; closes the
check valve and the d istributor vacuum will decrease at a
ra te controlled by the internal bleed.
D E T O N A T IO N SENSOR
R eplacem ent
T he D eto n atio n S ensor is an in te g ra l p a rt of the
E lectronic S p ark Control system . It m ounts on the right
hand side of engine in block drain hole.
Upon replacem ent, avoid the use of im pact type tools
and excessive torque.
• M inim um torque 16.3 N -m (12.02 ft. lbs.)
• M a x im u m to rq u e 2 1 .7 N-m (16.01 ft. lbs.)
VEHICLE E M IS S IO N C O N TRO L IN F O R M A T IO N
LABEL
T he Vehicle Emission C ontrol Inform ation Label (Fig.
6E-50) is located in the engine co m p artm en t (fan shroud,
ra d ia to r su p p o rt, hood u n d ersid e, etc .) of every vehicle
produced by G en eral M otors C o rp o ra tio n . T h e label
co n tain s im p o rta n t em ission specifications and settin g
p ro ced u res, as well as a v a cu u m hose sc h e m a tic w ith
emission com ponents identified.
W hen servicing the engine or emission system s, the
V ehicle Em ission C ontrol In fo rm a tio n L abel should be
checked for up-to-date inform ation.