Jeep Cherokee 1999 Electronic Service Manual page 1247

14 - 4 FUEL SYSTEM—2.5L DIESEL ENGINE
DESCRI PT I ON AN D OPERAT I ON (Cont inue d)
FUEL INJECTION PUMP
Th e fu el in ject ion pu m p is a m ech a n ica l dist r ibu -
t or –t ype, Bosch VP 36 ser ies (F ig. 3). A gea r on t h e
en d of t h e in ject ion pu m p sh a ft m esh es wit h t h e
dr ive gea r a t t h e fr on t of en gin e. Th e pu m p is
m ech a n ica lly t im ed t o t h e en gin e. Th e MSA ca n
m a ke a dju st m en t s t o t h e t im in g of t h e in ject ion
pu m p.
Fig. 3 Fuel Injection Pump
Th e in ject ion pu m p con t a in s t h e fu el sh u t down
solen oid, fu el t em per a t u r e sen sor, con t r ol sleeve sen -
sor, fu el qu a n t it y a ct u a t or a n d t h e fu el t im in g sole-
n oid (F ig. 3).
In t h e elect r on ica lly con t r olled in ject ion pu m p, t h e
pu m p plu n ger wor ks t h e sa m e a s t h e pu m p plu n ger
in a m ech a n ica lly con t r olled in ject ion pu m p, bu t t h e
a m ou n t of fu el a n d t h e t im e t h e fu el is in ject ed is
con t r olled by t h e veh icle's MSA, in st ea d of by a
m ech a n ica l gover n or a ssem bly. A solen oid con t r olled
by t h e MSA is u sed in pla ce of t h e m ech a n ica l gover-
n or a ssem bly, a n d it m oves a con t r ol sleeve in side t h e
pu m p t h a t r egu la t es t h e a m ou n t of fu el bein g
in ject ed. Th er e is n o m ech a n ica l con n ect ion bet ween
t h e a cceler a t or peda l a n d t h e elect r on ica lly con t r olled
in ject ion pu m p. In st ea d, a sen sor con n ect ed t o t h e
a cceler a t or peda l sen ds a sign a l t o t h e MSA t h a t r ep-
r esen t s t h e a ct u a l posit ion of t h e a cceler a t or peda l.
Th e MSA u ses t h is in pu t , a lon g wit h in pu t fr om
ot h er sen sor s t o m ove t h e con t r ol sleeve t o deliver
t h e a ppr opr ia t e a m ou n t of fu el. Th is syst em is kn own
a s "Dr ive-By-Wir e".
Th e a ct u a l t im e t h a t t h e fu el is deliver ed is ver y
im por t a n t t o t h e diesel com bu st ion pr ocess. Th e MSA
m on it or s ou t pu t s fr om t h e en gin e speed sen sor (fly-
wh eel posit ion in degr ees), a n d t h e fu el in ject or sen -
sor (m ech a n ica l m ovem en t wit h in t h e #1 cylin der
fu el in ject or ). Ou t pu t s fr om t h e Acceler a t or P eda l
P osit ion sen sor, en gin e speed sen sor (en gin e r pm )
a n d en gin e coola n t t em per a t u r e sen sor a r e a lso u sed.
Th e MSA will t h en com pa r e it s set va lu es t o t h ese
ou t pu t s t o elect r ica lly a dju st t h e a m ou n t of fu el t im -
in g (a m ou n t of a dva n ce) wit h in t h e in ject ion pu m p.
Th is is r efer r ed t o a s "Closed Loop" oper a t ion . Th e
MSA m on it or s fu el t im in g by com pa r in g it s set va lu e
t o wh en t h e in ject or #1 open s. If t h e va lu e is gr ea t er
t h a n a pr eset va lu e a fa u lt will be set .
Act u a l elect r ic fu el t im in g (a m ou n t of a dva n ce) is
a ccom plish ed by t h e fu el t im in g solen oid m ou n t ed t o
t h e bot t om of t h e in ject ion pu m p (F ig. 3). F u el t im in g
will be a dju st ed by t h e MSA, wh ich con t r ols t h e fu el
t im in g solen oid.
An over flow va lve is a t t a ch ed in t o t h e fu el r et u r n
lin e a t t h e r ea r of t h e fu el in ject ion pu m p (F ig. 3).
Th is va lve ser ves t wo pu r poses. On e is t o en su r e t h a t
a cer t a in a m ou n t of r esidu a l pr essu r e is m a in t a in ed
wit h in t h e pu m p wh en t h e en gin e is swit ch ed off.
Th is will pr even t t h e fu el t im in g m ech a n ism wit h in
t h e in ject ion pu m p fr om r et u r n in g t o it s zer o posi-
t ion . Th e ot h er pu r pose is t o a llow excess fu el t o be
r et u r n ed t o t h e fu el t a n k t h r ou gh t h e fu el r et u r n
lin e. Th e pr essu r e va lu es wit h in t h is va lve a r e pr eset
a n d ca n n ot be a dju st ed.
Th e fu el in ject ion pu m p su pplies h igh –pr essu r e
fu el of a ppr oxim a t ely 45,000 kP a (6526 psi) t o ea ch
in ject or in pr ecise m et er ed a m ou n t s a t t h e cor r ect
t im e.
F or m ech a n ica l in ject ion pu m p t im in g, r efer t o
F u el In ject ion P u m p Tim in g in t h e Ser vice P r oce-
du r es sect ion of t h is gr ou p.
FUEL INJECTORS
F u el dr a in t u bes (F ig. 4) a r e u sed t o r ou t e excess
fu el ba ck t o t h e over flow va lve a t t h e r ea r of t h e
in ject ion pu m p. Th is excess fu el is t h en r et u r n ed t o
t h e fu el t a n k t h r ou gh t h e fu el r et u r n lin e.
Th e in ject or s a r e con n ect ed t o t h e fu el in ject ion
pu m p by t h e h igh –pr essu r e fu el lin es. A sepa r a t e
in ject or is u sed for ea ch of t h e fou r cylin der s. An
in ject or con t a in in g a sen sor (F ig. 5) is u sed on t h e
cylin der n u m ber on e in ject or. Th is in ject or is ca lled
in st r u m en t ed in ject or #1 or n eedle m ovem en t sen sor.
It is u sed t o t ell t h e MSA wh en t h e #1 in ject or 's
in t er n a l spr in g-loa ded va lve sea t h a s been for ced
open by pr essu r ized fu el bein g deliver ed t o t h e cylin -
der, wh ich is a t t h e en d of it s com pr ession st r oke.
Wh en t h e in st r u m en t ed in ject or 's va lve sea t is for ce
open , it sen ds a sm a ll volt a ge spike pu lse t o t h e
MSA. Th is t ells t h e MSA t h a t cylin der #1 is fir in g. It
is n ot u sed wit h t h e ot h er t h r ee in ject or s.
F u el en t er s t h e in ject or a t t h e fu el in let (t op of
in ject or ) a n d is r ou t ed t o t h e n eedle va lve bor e. Wh en
fu el pr essu r e r ises t o a ppr oxim a t ely 15,000–15,800
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