Jeep Cherokee 1999, Electronic Service Manual

Page: 1917 / 1948

O2S fa u lt , ver ify t h a t t h e h ea t er cir cu it is oper a t in g
cor r ect ly.
E ffect ive con t r ol of exh a u st em ission s is a ch ieved
by a n oxygen feedba ck syst em . Th e m ost im por t a n t
elem en t of t h e feedba ck syst em is t h e O2S. Th e O2S
is loca t ed in t h e exh a u st pa t h . On ce it r ea ch es oper-
a t in g t em per a t u r e 300° t o 350°C (572 ° t o 662°F ), t h e
sen sor gen er a t es a volt a ge t h a t is in ver sely pr opor-
t ion a l t o t h e a m ou n t of oxygen in t h e exh a u st . Th e
in for m a t ion obt a in ed by t h e sen sor is u sed t o ca lcu -
la t e t h e fu el in ject or pu lse widt h . Th is m a in t a in s a
14.7 t o 1 Air F u el (A/F ) r a t io. At t h is m ixt u r e r a t io,
t h e ca t a lyst wor ks best t o r em ove h ydr oca r bon s (H C),
ca r bon m on oxide (CO) a n d n it r ogen oxide (NOx) fr om
t h e exh a u st .
Th e volt a ge r ea din gs t a ken fr om t h e O2S sen sor
a r e ver y t em per a t u r e sen sit ive. Th e r ea din gs a r e n ot
a ccu r a t e below 300°C. H ea t in g of t h e O2S sen sor is
don e t o a llow t h e en gin e con t r oller t o sh ift t o closed
loop con t r ol a s soon a s possible. Th e h ea t in g elem en t
u sed t o h ea t t h e O2S sen sor m u st be t est ed t o en su r e
t h a t it is h ea t in g t h e sen sor pr oper ly.
Th e O2S sen sor cir cu it is m on it or ed for a dr op in
volt a ge. Th e sen sor ou t pu t is u sed t o t est t h e h ea t er
by isola t in g t h e effect of t h e h ea t er elem en t on t h e
O2S sen sor ou t pu t volt a ge fr om t h e ot h er effect s.
LEAK DETECTION PUMP MONITOR (IF EQUIPPED)
Th e lea k det ect ion a ssem bly in cor por a t es t wo pr i-
m a r y fu n ct ion s: it m u st det ect a lea k in t h e eva por a -
t ive syst em a n d sea l t h e eva por a t ive syst em so t h e
lea k det ect ion t est ca n be r u n .
Th e pr im a r y com pon en t s wit h in t h e a ssem bly a r e:
A t h r ee por t solen oid t h a t a ct iva t es bot h of t h e fu n c-
t ion s list ed a bove; a pu m p wh ich con t a in s a swit ch ,
t wo ch eck va lves a n d a spr in g/dia ph r a gm , a ca n ist er
ven t va lve (CVV) sea l wh ich con t a in s a spr in g loa ded
ven t sea l va lve.
Im m edia t ely a ft er a cold st a r t , bet ween pr edet er-
m in ed t em per a t u r e t h r esh olds lim it s, t h e t h r ee por t
solen oid is br iefly en er gized. Th is in it ia lizes t h e
pu m p by dr a win g a ir in t o t h e pu m p ca vit y a n d a lso
closes t h e ven t sea l. Du r in g n on t est con dit ion s t h e
ven t sea l is h eld open by t h e pu m p dia ph r a gm
a ssem bly wh ich pu sh es it open a t t h e fu ll t r a vel posi-
t ion . Th e ven t sea l will r em a in closed wh ile t h e
pu m p is cyclin g du e t o t h e r eed swit ch t r igger in g of
t h e t h r ee por t solen oid t h a t pr even t s t h e dia ph r a gm
a ssem bly fr om r ea ch in g fu ll t r a vel. Aft er t h e br ief
in it ia liza t ion per iod, t h e solen oid is de-en er gized
a llowin g a t m osph er ic pr essu r e t o en t er t h e pu m p
ca vit y, t h u s per m it t in g t h e spr in g t o dr ive t h e dia -
ph r a gm wh ich for ces a ir ou t of t h e pu m p ca vit y a n d
in t o t h e ven t syst em . Wh en t h e solen oid is en er gized
a n d de en er gized, t h e cycle is r epea t ed cr ea t in g flow
in t ypica l dia ph r a gm pu m p fa sh ion . Th e pu m p is con -
t r olled in 2 m odes:
P u m p Mode: Th e pu m p is cycled a t a fixed r a t e t o
a ch ieve a r a pid pr essu r e bu ild in or der t o sh or t en t h e
over a ll t est len gt h .
Test Mode: Th e solen oid is en er gized wit h a fixed
du r a t ion pu lse. Su bsequ en t fixed pu lses occu r wh en
t h e dia ph r a gm r ea ch es t h e Swit ch closu r e poin t .
Th e spr in g in t h e pu m p is set so t h a t t h e syst em
will a ch ieve a n equ a lized pr essu r e of a bou t 7.5” H 20.
Th e cycle r a t e of pu m p st r okes is qu it e r a pid a s t h e
syst em begin s t o pu m p u p t o t h is pr essu r e. As t h e
pr essu r e in cr ea ses, t h e cycle r a t e st a r t s t o dr op off. If
t h er e is n o lea k in t h e syst em , t h e pu m p wou ld even -
t u a lly st op pu m pin g a t t h e equ a lized pr essu r e. If
t h er e is a lea k, it will con t in u e t o pu m p a t a r a t e r ep-
r esen t a t ive of t h e flow ch a r a ct er ist ic of t h e size of t h e
lea k. F r om t h is in for m a t ion we ca n det er m in e if t h e
lea k is la r ger t h a n t h e r equ ir ed det ect ion lim it (cu r-
r en t ly set a t .040” or ifice by CARB). If a lea k is
r evea led du r in g t h e lea k t est por t ion of t h e t est , t h e
t est is t er m in a t ed a t t h e en d of t h e t est m ode a n d n o
fu r t h er syst em ch ecks will be per for m ed.
Aft er pa ssin g t h e lea k det ect ion ph a se of t h e t est ,
syst em pr essu r e is m a in t a in ed by t u r n in g on t h e
LDP ’s solen oid u n t il t h e pu r ge syst em is a ct iva t ed.
P u r ge a ct iva t ion in effect cr ea t es a lea k. Th e cycle
r a t e is a ga in in t er r oga t ed a n d wh en it in cr ea ses du e
t o t h e flow t h r ou gh t h e pu r ge syst em , t h e lea k ch eck
por t ion of t h e dia gn ost ic is com plet e.
Th e ca n ist er ven t va lve will u n sea l t h e syst em
a ft er com plet ion of t h e t est sequ en ce a s t h e pu m p
dia ph r a gm a ssem bly m oves t o t h e fu ll t r a vel posit ion .
E va por a t ive syst em fu n ct ion a lit y will be ver ified by
u sin g t h e st r ict er eva p pu r ge flow m on it or. At a n
a ppr opr ia t e wa r m idle t h e LDP will be en er gized t o
sea l t h e ca n ist er ven t . Th e pu r ge flow will be clocked
u p fr om som e sm a ll va lu e in a n a t t em pt t o see a
sh ift in t h e 02 con t r ol syst em . If fu el va por, in dica t ed
by a sh ift in t h e 02 con t r ol, is pr esen t t h e t est is
pa ssed. If n ot , it is a ssu m ed t h a t t h e pu r ge syst em is
n ot fu n ct ion in g in som e r espect . Th e LDP is a ga in
t u r n ed off a n d t h e t est is en ded.
MISFIRE MONITOR
E xcessive en gin e m isfir e r esu lt s in in cr ea sed ca t a -
lyst t em per a t u r e a n d ca u ses a n in cr ea se in H C em is-
sion s. Sever e m isfir es cou ld ca u se ca t a lyst da m a ge.
To pr even t ca t a lyt ic con ver t or da m a ge, t h e P CM
m on it or s en gin e m isfir e.
Th e P ower t r a in Con t r ol Modu le (P CM) m on it or s
for m isfir e du r in g m ost en gin e oper a t in g con dit ion s
(posit ive t or qu e) by lookin g a t ch a n ges in t h e cr a n k-
sh a ft speed. If a m isfir e occu r s t h e speed of t h e
cr a n ksh a ft will va r y m or e t h a n n or m a l.
25 - 16 EMISSION CONTROL SYSTEMS X J
DESCRI PT I ON AN D OPERAT I ON (Cont inue d)