Automatic Shutdown (Asd) Relay; Crankshaft Position Sensor; Distributor; Ignition Coil - Jeep Cherokee 1999 Electronic Service Manual

8D - 2

IGNITION SYSTEM

DESCRI PT I ON AN D OPERAT I ON (Cont inue d)
a dva n ce, a n d for ch a n gin g en gin e oper a t in g con di-
t ion s.
Th e a m ou n t of elect r on ic spa r k a dva n ce pr ovided
by t h e P CM is det er m in ed by five in pu t fa ct or s:
en gin e coola n t t em per a t u r e, en gin e r pm , in t a ke m a n -
ifold t em per a t u r e, m a n ifold a bsolu t e pr essu r e a n d
t h r ot t le posit ion .

DISTRIBUTOR

All en gin es a r e equ ipped wit h a ca m sh a ft dr iven
m ech a n ica l dist r ibu t or con t a in in g a sh a ft dr iven dis-
t r ibu t or r ot or. All dist r ibu t or s a r e equ ipped wit h a n
in t er n a l ca m sh a ft posit ion (fu el syn c) sen sor (F ig. 2).
Th is sen sor pr ovides fu el in ject ion syn ch r on iza t ion
a n d cylin der iden t ifica t ion .
Fig. 2 Distributor and Camshaft Position Sensor-
Th e dist r ibu t or s on bot h t h e 2.5L 4-cylin der a n d
t h e 4.0L-6 cylin der en gin es do n ot h a ve bu ilt in cen -
t r ifu ga l or va cu u m a ssist ed a dva n ce. Ba se ign it ion
t im in g a n d a ll t im in g a dva n ce is con t r olled by t h e
power t r a in con t r ol m odu le (P CM). Beca u se ign it ion
t im in g is con t r olled by t h e P CM, ba s e ig n itio n tim -
in g is n o t a d ju s ta ble o n a n y o f th e s e e n g in e s .
Th e dist r ibu t or is locked in pla ce by a for k wit h a
slot loca t ed on t h e dist r ibu t or h ou sin g ba se. Th e dis-
t r ibu t or h olddown cla m p bolt pa sses t h r ou gh t h is slot
wh en in st a lled. Beca u se t h e dist r ibu t or posit ion is
locked wh en in st a lled, it s r ot a t ion a l posit ion ca n n ot
be ch a n ged. D o n o t a tte m p t to m o d ify th e d is -
tribu to r h o u s in g to g e t d is tribu to r ro ta tio n .
D is tribu to r p o s itio n w ill h a v e n o e ffe c t o n ig n i-
tio n tim in g . Th e p o s itio n o f th e d is tribu to r w ill
d e te rm in e fu e l s y n c h ro n iza tio n o n ly.
All dist r ibu t or s con t a in a n in t er n a l oil sea l t h a t
pr even t s oil fr om en t er in g t h e dist r ibu t or h ou sin g.
Th e sea l is n ot ser vicea ble.
Typical

SPARK PLUGS

All en gin es u se r esist or t ype spa r k plu gs. Rem ove
t h e spa r k plu gs a n d exa m in e t h em for bu r n ed elec-
t r odes a n d fou led, cr a cked or br oken por cela in in su -
la t or s. Keep plu gs a r r a n ged in t h e or der in wh ich
t h ey wer e r em oved fr om t h e en gin e. A sin gle plu g
displa yin g a n a bn or m a l con dit ion in dica t es t h a t a
pr oblem exist s in t h e cor r espon din g cylin der. Repla ce
spa r k plu gs a t t h e in t er va ls r ecom m en ded in Gr ou p
O, Lu br ica t ion a n d Ma in t en a n ce.
Spa r k plu gs t h a t h a ve low m ila ge m a y be clea n ed
a n d r eu sed if n ot ot h er wise defect ive, ca r bon or oil
fou led. Refer t o t h e Spa r k P lu g Con dit ion sect ion of
t h is gr ou p.

SPARK PLUG CABLES

Spa r k plu g ca bles a r e som et im es r efer r ed t o a s sec-
on da r y ign it ion wir es. Th ese ca bles t r a n sfer elect r ica l
cu r r en t fr om t h e ign it ion coil(s) a n d/or dist r ibu t or, t o
in dividu a l spa r k plu gs a t ea ch cylin der. Th e r esist ive
spa r k plu g ca bles a r e of n on m et a llic con st r u ct ion .
Th e ca bles pr ovide su ppr ession of r a dio fr equ en cy
em ission s fr om t h e ign it ion syst em .

IGNITION COIL

Ba t t er y volt a ge is su pplied t o t h e ign it ion coil pos-
it ive t er m in a l fr om t h e ASD r ela y.
Th e P ower t r a in Con t r ol Modu le (P CM) open s a n d
closes t h e ign it ion coil gr ou n d cir cu it for ign it ion coil
oper a t ion .
B a s e ig n itio n tim in g is n o t a d ju s ta ble o n a n y
e n g in e . By con t r ollin g t h e coil gr ou n d cir cu it , t h e
P CM is a ble t o set t h e ba se t im in g a n d a dju st t h e
ign it ion t im in g a dva n ce. Th is is don e t o m eet ch a n g-
in g en gin e oper a t in g con dit ion s.
Th e ign it ion coil is n ot oil filled. Th e win din gs a r e
em bedded in a n epoxy com pou n d. Th is pr ovides h ea t
a n d vibr a t ion r esist a n ce t h a t a llows t h e ign it ion coil
t o be m ou n t ed on t h e en gin e.
AUTOMA TIC SHUTDOWN (ASD) RELA Y
As on e of it s fu n ct ion s, t h e ASD r ela y will su pply
ba t t er y volt a ge t o t h e ign it ion coil. Th e gr ou n d cir-
cu it for t h e ASD r ela y is con t r olled by t h e P ower t r a in
Con t r ol Modu le (P CM). Th e P CM r egu la t es ASD
r ela y oper a t ion by swit ch in g t h e gr ou n d cir cu it
on -a n d-off.

CRANKSHAFT POSITION SENSOR

Th e cr a n ksh a ft posit ion sen sor is m ou n t ed t o t h e
t r a n sm ission bellh ou sin g a t t h e left /r ea r side of
en gin e block (F ig. 3).
E n gin e speed a n d cr a n ksh a ft posit ion a r e pr ovided
t h r ou gh t h e cr a n ksh a ft posit ion sen sor. Th e sen sor
gen er a t es pu lses t h a t a r e t h e in pu t sen t t o t h e pow-
er t r a in con t r ol m odu le (P CM). Th e P CM in t er pr et s
X J