Jeep Cherokee 1999 Electronic Service Manual page 1219

14 - 28 FUEL SYSTEM
DESCRI PT I ON AN D OPERAT I ON (Cont inue d)
t h e A/C r ela y. Th is will dea ct iva t e t h e A/C com pr es-
sor clu t ch .
If t h e swit ch open s, (in dica t in g t h a t eva por a t or is
n ot in pr oper pr essu r e r a n ge), t h e P CM will n ot
r eceive t h e A/C r equ est sign a l. Th e P CM will t h en
r em ove t h e gr ou n d fr om t h e A/C r ela y, dea ct iva t in g
t h e A/C com pr essor clu t ch .
AUTOMA TIC SHUTDOWN (ASD) RELA Y
SENSE—PCM INPUT
A 12 volt sign a l a t t h is in pu t in dica t es t o t h e P CM
t h a t t h e ASD h a s been a ct iva t ed. Th e ASD r ela y is
loca t ed in t h e P ower Dist r ibu t ion Cen t er (P DC). Th e
P DC is loca t ed in t h e en gin e com pa r t m en t (F ig. 2).
Refer t o la bel on P DC cover for r ela y loca t ion . Th e
r ela y is u sed t o con n ect t h e oxygen sen sor h ea t er ele-
m en t s, ign it ion coil a n d fu el in ject or s t o 12 volt +
power su pply.
Fig. 2 Power Distribution Center (PDC)
Th is in pu t is u sed on ly t o sen se t h a t t h e ASD r ela y
is en er gized. If t h e power t r a in con t r ol m odu le (P CM)
does n ot see 12 volt s a t t h is in pu t wh en t h e ASD
sh ou ld be a ct iva t ed, it will set a dia gn ost ic t r ou ble
code (DTC).
BA TTERY TEMPERA TURE SENSOR—PCM
INPUT
OPERA TION
P r ovides a sign a l t o t h e P CM cor r espon din g t o t h e
ba t t er y t em per a t u r e.
BA TTERY VOLTAGE—PCM INPUT
OPERA TION
Th e ba t t er y volt a ge in pu t pr ovides power t o t h e
P ower t r a in Con t r ol Modu le (P CM). It a lso in for m s
t h e P CM wh a t volt a ge level is su pplied t o t h e ign i-
t ion coil a n d fu el in ject or s.
If ba t t er y volt a ge is low, t h e P CM will in cr ea se
in ject or pu lse widt h (per iod of t im e t h a t t h e in ject or
is en er gized). Th is is don e t o com pen sa t e for t h e
r edu ced flow t h r ou gh in ject or ca u sed by t h e lower ed
volt a ge.
BRAKE SWITCH—PCM INPUT
OPERA TION
Wh en t h e br a ke ligh t swit ch is a ct iva t ed, t h e P ow-
er t r a in Con t r ol Modu le (P CM) r eceives a n in pu t in di-
ca t in g t h a t t h e br a kes a r e bein g a pplied. Aft er
r eceivin g t h is in pu t , t h e P CM m a in t a in s idle speed t o
a sch edu led r pm t h r ou gh con t r ol of t h e Idle Air Con -
t r ol (IAC) m ot or. Th e br a ke swit ch in pu t is a lso u sed
t o disa ble ven t a n d va cu u m solen oid ou t pu t sign a ls
t o t h e speed con t r ol ser vo.
FIVE VOLT SENSOR SUPPL Y—PRIMARY
OPERA TION
Su pplies t h e r equ ir ed 5 volt power sou r ce t o t h e
cr a n ksh a ft posit ion sen sor, ca m sh a ft posit ion sen sor,
MAP sen sor a n d t h r ot t le posit ion sen sor.
FIVE VOLT SENSOR SUPPL Y—SECONDARY
OPERA TION
Su pplies t h e r equ ir ed 5 volt sou r ce t o cer t a in sen -
sor s.
FUEL LEVEL SENSOR—PCM INPUT
OPERA TION
Th e P ower t r a in Con t r ol Modu le (P CM) su pplies
power t o t h e fu el level sen sor (fu el ga u ge sen din g
u n it ). Th e fu el level sen sor will t h en r et u r n a sign a l
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 a fa lse set t in g of m isfir e a n d
fu el syst em m on it or t r ou ble codes. Th is is if t h e fu el
level is less t h a n a ppr oxim a t ely 15 per cen t , or, if
equ ipped wit h a Lea k Det ect ion P u m p (LDP ), 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 cit y.
Th is in pu t is a lso u sed t o sen d a sign a l t o t h e P CM
for fu el ga u ge oper a t ion via t h e CCD or J 1850 bu s
cir cu it s.
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