Jeep Cherokee 1999 Electronic Service Manual page 1195

14 - 4 FUEL SYSTEM
DESCRI PT I ON AN D OPERAT I ON (Cont inue d)
OPERA TION
Th e fu el pu m p u sed in t h is syst em h a s a per m a n en t
m a gn et elect r ic m ot or. F u el is dr a wn in t h r ou gh a filt er
a t t h e bot t om of t h e m odu le a n d pu sh ed t h r ou gh t h e
elect r ic m ot or gea r set t o t h e pu m p ou t let .
Ch e c k Va lv e Op e ra tio n : Th e pu m p ou t let con -
t a in s a on e-wa y ch eck va lve t o pr even t fu el flow ba ck
in t o t h e t a n k a n d t o m a in t a in fu el su pply lin e pr es-
su r e (en gin e wa r m ) wh en pu m p is n ot oper a t ion a l. It
is a lso u sed t o keep t h e fu el su pply lin e fu ll of ga so-
lin e wh en pu m p is n ot oper a t ion a l. Aft er t h e veh icle
h a s cooled down , fu el pr essu r e m a y dr op t o 0 psi
(cold flu id con t r a ct s), bu t liqu id ga solin e will r em a in
in fu el su pply lin e bet ween t h e ch eck va lve a n d fu el
in ject or s. F u e l p re s s u re th a t h a s d ro p p e d to 0
p s i o n a c o o le d d o w n v e h ic le (e n g in e o ff) is a
n o rm a l c o n d itio n . Refer t o t h e F u el P r essu r e Lea k
Down Test for m or e in for m a t ion .
Volt a ge t o oper a t e t h e elect r ic pu m p is su pplied
t h r ou gh t h e fu el pu m p r ela y.
FUEL GAUGE SENDING UNIT
DESCRIPTION
Th e fu el ga u ge sen din g u n it (fu el level sen sor ) is
a t t a ch ed t o t h e side of t h e fu el pu m p m odu le. Th e
sen din g u n it con sist s of a floa t , a n a r m , a n d a va r i-
a ble r esist or (t r a ck).
OPERA TION
Th e r esist or t r a ck is u sed t o sen d elect r ica l sign a ls t o
t h e P ower t r a in Con t r ol Modu le (P CM) for fu el ga u ge
oper a t ion a n d for OBD II em ission r equ ir em en t s.
F o r fu e l g a u g e
in cr ea ses, t h e floa t a n d a r m m ove u p. Th is decr ea ses
t h e sen din g u n it r esist a n ce, ca u sin g t h e fu el ga u ge t o
r ea d fu ll. As fu el level decr ea ses, t h e floa t a n d a r m
m ove down . Th is in cr ea ses t h e sen din g u n it r esis-
t a n ce ca u sin g t h e fu el ga u ge t o r ea d em pt y.
Aft er t h is fu el level sign a l is sen t t o t h e P CM, t h e
P CM will t r a n sm it t h e da t a a cr oss t h e CCD bu s cir-
cu it s t o t h e in st r u m en t pa n el. H er e it is t r a n sla t ed
in t o t h e a ppr opr ia t e fu el ga u ge level r ea din g.
F o r OB D II e m is s io n m o n ito r re qu ire m e n ts : A
volt a ge sign a l is sen t fr om t h e r esist or t r a ck on t h e
sen din g u n it t o t h e P CM t o in dica t e fu el level. Th e pu r-
pose of t h is fea t u r e is t o pr even t t h e OBD II syst em
fr om r ecor din g/set t in g fa lse m isfir e a n d fu el syst em
m on it or t r ou ble codes. Th e fea t u r e is a ct iva t ed if t h e
fu el level in t h e t a n k is less t h a n a ppr oxim a t ely 15 per-
cen t of it s r a t ed ca pa cit y. If equ ipped wit h a Lea k
Det ect ion P u m p (E VAP syst em m on it or ), t h is fea t u r e
will a lso be a ct iva t ed if t h e fu el level in t h e t a n k is
m or e t h a n a ppr oxim a t ely 85 per cen t of it s r a t ed ca pa c-
it y.
o p e ra tio n : As fu el
FUEL FILTER/FUEL PRESSURE REGULA TOR
A com bin a t ion fu el filt er a n d fu el pr essu r e r egu la -
t or is u sed on a ll en gin es. It is loca t ed on t h e t op of
fu el pu m p m odu le (F ig. 1). A sepa r a t e fr a m e m ou n t ed
fu el filt er is n ot u sed wit h a n y en gin e.
F u e l P re s s u re Re g u la to r Op e ra tio n : Th e pr es-
su r e r egu la t or is a m ech a n ica l device t h a t is n ot con -
t r olled by en gin e va cu u m or t h e P ower t r a in Con t r ol
Modu le (P CM).
Th e r egu la t or is ca libr a t ed t o m a in t a in fu el syst em
oper a t in g pr essu r e of a ppr oxim a t ely 339 kP a
kP a (49.2 psi
t a in s a dia ph r a gm , ca libr a t ed spr in gs a n d a fu el
r et u r n va lve. Th e in t er n a l fu el filt er is a lso pa r t of
t h e a ssem bly.
level
Fig. 3 Fuel Filter/Fuel Pressure Regulator
F u el is su pplied t o t h e filt er /r egu la t or by t h e elec-
t r ic fu el pu m p t h r ou gh a n open in g t u be a t t h e bot -
t om of filt er /r egu la t or (F ig. 3).
Th e r egu la t or a ct s a s a ch eck va lve t o m a in t a in
som e fu el pr essu r e wh en t h e en gin e is n ot oper a t in g.
Th is will h elp t o st a r t t h e en gin e. A secon d ch eck
va lve is loca t ed a t t h e ou t let en d of t h e elect r ic fu el
pu m p. Re fe r to F u e l P u m p —D e s c rip tio n a n d
Op e ra tio n fo r m o re in fo rm a tio n . Als o re fe r to
th e F u e l P re s s u re Le a k D o w n Te s t a n d th e F u e l
P u m p P re s s u re Te s ts .
If fu el pr essu r e a t t h e pr essu r e r egu la t or exceeds
a ppr oxim a t ely 49 psi, a n in t er n a l dia ph r a gm closes
a n d excess fu el is r ou t ed ba ck in t o t h e t a n k t h r ou gh
t h e pr essu r e r egu la t or. A sepa r a t e fu el r et u r n lin e is
n ot u sed.
5 psi) a t t h e fu el in ject or s. It con -
X J
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