Jeep Cherokee 1999 Electronic Service Manual page 1221

14 - 30 FUEL SYSTEM
DESCRI PT I ON AN D OPERAT I ON (Cont inue d)
EXTENDED IDLE SWITCH—PCM INPUT
OPTIONAL POLICE PACKAGE WITH 4.0L ENGINE
ONL Y
Th e ext en ded idle swit ch is u sed t o r a ise t h e
en gin e idle speed t o a ppr oxim a t ely 1000 r pm . Th is is
wh en t h e sh ift er is in eit h er t h e P a r k or Neu t r a l
posit ion . A r ocker-t ype swit ch (ext en ded idle swit ch )
is m ou n t ed t o t h e in st r u m en t pa n el. Th is swit ch will
su pply a gr ou n d cir cu it t o t h e power t r a in con t r ol
m odu le (P CM). Th e s w itc h is a v a ila ble o n ly w ith
4.0L e n g in e w h e n s u p p lie d w ith th e o p tio n a l
p o lic e p a c k a g e .
F or t est in g a n d dia gn osis of t h is swit ch a n d it s cir-
cu it , r efer t o t h e Dia gn osis a n d Test in g sect ion of t h is
gr ou p.
GENERA TOR OUTPUT—PCM INPUT
OPERA TION
P r ovides a ch a r gin g syst em volt a ge in pu t t o t h e
P ower t r a in Con t r ol Modu le (P CM). It is sen sed a t t h e
ba t t er y in pu t t o t h e P CM.
OXYGEN SENSOR (HO2S)—PCM INPUT
Two h ea t ed O2S sen sor s a r e u sed. Th e sen sor s pr o-
du ce volt a ges fr om 0 t o 1 volt , depen din g u pon t h e
oxygen con t en t of t h e exh a u st ga s in t h e exh a u st
m a n ifold. Wh en a la r ge a m ou n t of oxygen is pr esen t
(ca u sed by a lea n a ir /fu el m ixt u r e), t h e sen sor s pr o-
du ces a low volt a ge. Wh en t h er e is a lesser a m ou n t
pr esen t (r ich a ir /fu el m ixt u r e) it pr odu ces a h igh er
volt a ge. By m on it or in g t h e oxygen con t en t a n d con -
ver t in g it t o elect r ica l volt a ge, t h e sen sor s a ct a s a
r ich -lea n swit ch .
Th e oxygen sen sor s a r e equ ipped wit h a h ea t in g
elem en t t h a t keeps t h e sen sor s a t pr oper oper a t in g
t em per a t u r e du r in g a ll oper a t in g m odes. Ma in t a in in g
cor r ect sen sor t em per a t u r e a t a ll t im es a llows t h e
syst em t o en t er in t o closed loop oper a t ion soon er.
Also, it a llows t h e syst em t o r em a in in closed loop
oper a t ion du r in g per iods of ext en ded idle.
In Closed Loop oper a t ion , t h e P CM m on it or s t h e
O2S sen sor in pu t (a lon g wit h ot h er in pu t s) a n d
a dju st s t h e in ject or pu lse widt h a ccor din gly. Du r in g
Open Loop oper a t ion , t h e P CM ign or es t h e O2S sen -
sor in pu t . Th e P CM a dju st s in ject or pu lse widt h
ba sed on pr epr ogr a m m ed (fixed) va lu es a n d in pu t s
fr om ot h er sen sor s.
Th e Au t om a t ic Sh u t down (ASD) r ela y su pplies ba t -
t er y volt a ge t o bot h t h e u pst r ea m a n d down st r ea m
h ea t ed oxygen sen sor s. Th e oxygen sen sor s a r e
equ ipped wit h a h ea t in g elem en t . Th e h ea t in g ele-
m en t s r edu ce t h e t im e r equ ir ed for t h e sen sor s t o
r ea ch oper a t in g t em per a t u r e.
UPSTREAM HEA TED OXYGEN SENSOR
Th e u pst r ea m O2S sen sor is loca t ed in t h e exh a u st
down pipe (F ig. 6). It pr ovides a n in pu t volt a ge t o t h e
P CM. Th e in pu t t ells t h e P CM t h e oxygen con t en t of
t h e exh a u st ga s. Th e P CM u ses t h is in for m a t ion t o
fin e t u n e t h e a ir /fu el r a t io by a dju st in g in ject or pu lse
widt h .
Fig. 6 Heated Oxygen Sensors
DOWNSTREAM HEA TED OXYGEN SENSOR
Th e down st r ea m h ea t ed oxygen sen sor is loca t ed
n ea r t h e ou t let en d of t h e ca t a lyt ic con ver t or (F ig. 6).
Th e down st r ea m h ea t ed oxygen sen sor in pu t is u sed
t o det ect ca t a lyt ic con ver t or det er ior a t ion . As t h e con -
ver t or det er ior a t es, t h e in pu t fr om t h e down st r ea m
sen sor begin s t o m a t ch t h e u pst r ea m sen sor in pu t
except for a sligh t t im e dela y. By com pa r in g t h e
down st r ea m h ea t ed oxygen sen sor in pu t t o t h e in pu t
fr om t h e u pst r ea m sen sor, t h e P CM ca lcu la t es ca t a -
lyt ic con ver t or efficien cy.
Wh en t h e ca t a lyt ic con ver t er efficien cy dr ops below
em ission st a n da r ds, t h e P CM st or es a dia gn ost ic
t r ou ble code a n d illu m in a t es t h e Ma lfu n ct ion In dica -
t or La m p (MIL). F or m or e in for m a t ion , r efer t o
Gr ou p 25, E m ission Con t r ol Syst em s.
IGNITION CIRCUIT SENSE—PCM INPUT
OPERA TION
Th e ign it ion cir cu it sen se in pu t t ells t h e P ower-
t r a in Con t r ol Modu le (P CM) t h e ign it ion swit ch h a s
en er gized t h e ign it ion cir cu it .
INTAKE MANIFOLD AIR TEMPERA TURE
SENSOR—PCM INPUT
Th e in t a ke m a n ifold a ir t em per a t u r e sen sor is
in st a lled in t h e in t a ke m a n ifold wit h t h e sen sor ele-
m en t ext en din g in t o t h e a ir st r ea m (F ig. 7) or (F ig.
8). Th e sen sor pr ovides a n in pu t volt a ge t o t h e P ow-
er t r a in Con t r ol Modu le (P CM) in dica t in g in t a ke m a n -
X J