Jeep Cherokee 1999 Electronic Service Manual page 1223

14 - 32 FUEL SYSTEM
DESCRI PT I ON AN D OPERAT I ON (Cont inue d)
swit ch will open a n d t h e P CM will in cr ea se t h e
en gin e idle speed. Th is will pr even t t h e en gin e fr om
st a llin g.
Wh en pu m p pr essu r e dr ops t o a ppr oxim a t ely 1379
kP a (200 psi), t h e swit ch cir cu it will r e-close a n d
en gin e idle speed will r et u r n t o it s pr eviou s set t in g.
Fig. 9 Power Steering Pump Pressure Switch—2.5L
SENSOR RETURN—PCM INPUT
OPERA TION
Sen sor Ret u r n pr ovides a low n oise gr ou n d r efer-
en ce for a ll en gin e con t r ol syst em sen sor s.
SPEED CONTROL SWITCHES—PCM INPUT
Two sepa r a t e speed con t r ol swit ch m odu les a r e
m ou n t ed on t h e st eer in g wh eel t o t h e left a n d r igh t
side of t h e dr iver 's a ir ba g m odu le. Wit h in t h e t wo
swit ch m odu les, five m o m e n ta ry con t a ct swit ch es,
su ppor t in g seven differ en t speed con t r ol fu n ct ion s
a r e u sed. Th e ou t pu t s fr om t h ese swit ch es a r e fil-
t er ed in t o on e in pu t . Th e P ower t r a in Con t r ol Modu le
(P CM) det er m in es wh ich ou t pu t h a s been a pplied
t h r ou gh re s is tiv e m u ltip le x in g . Th e in pu t cir cu it
volt a ge is m ea su r ed by t h e P CM t o det er m in e wh ich
swit ch fu n ct ion h a s been select ed.
A speed con t r ol in dica t or la m p, loca t ed on t h e
in st r u m en t pa n el clu st er is en er gized by t h e P CM via
t h e CCD Bu s. Th is occu r s wh en speed con t r ol syst em
power h a s been t u r n ed ON, a n d t h e en gin e is r u n -
n in g.
Th e t wo swit ch m odu les a r e la beled: ON/OF F, SE T,
RE SUME /ACCE L, CANCE L a n d COAST. Refer t o
Gr ou p 8H , Speed Con t r ol Syst em for m or e in for m a -
t ion .
Engine
TRANSMISSION PARK/NEUTRAL SWITCH—
PCM INPUT
Th e pa r k/n eu t r a l swit ch is loca t ed on t h e t r a n sm is-
sion h ou sin g a n d pr ovides a n in pu t t o t h e P ower t r a in
Con t r ol Modu le (P CM). Th is will in dica t e t h a t t h e
a u t om a t ic t r a n sm ission is in P a r k, Neu t r a l or a dr ive
gea r select ion . Th is in pu t is u sed t o det er m in e idle
speed (va r yin g wit h gea r select ion ), fu el in ject or
pu lse widt h a n d ign it ion t im in g a dva n ce. Refer t o
Gr ou p 21, Tr a n sm ission s, for t est in g, r epla cem en t
a n d a dju st m en t in for m a t ion . It is a lso u sed a s a con -
dit ion for speed con t r ol oper a t ion .
THROTTLE POSITION SENSOR (TPS)—PCM
INPUT
Th e TP S is m ou n t ed on t h e t h r ot t le body (F ig. 7).
Th e TP S is a va r ia ble r esist or t h a t pr ovides t h e P ow-
er t r a in Con t r ol Modu le (P CM) wit h a n in pu t sign a l
(volt a ge) t h a t r epr esen t s t h r ot t le bla de posit ion . Th e
sen sor is con n ect ed t o t h e t h r ot t le bla de sh a ft . As t h e
posit ion of t h e t h r ot t le bla de ch a n ges, t h e r esist a n ce
of t h e TP S ch a n ges.
Th e P CM su pplies a ppr oxim a t ely 5 volt s t o t h e
TP S. Th e TP S ou t pu t volt a ge (in pu t sign a l t o t h e
P CM) r epr esen t s t h e t h r ot t le bla de posit ion . Th e
P CM r eceives a n in pu t sign a l volt a ge fr om t h e TP S.
Th is will va r y in a n a ppr oxim a t e r a n ge of fr om .26
volt s a t m in im u m t h r ot t le open in g (idle), t o 4.49 volt s
a t wide open t h r ot t le. Alon g wit h in pu t s fr om ot h er
sen sor s, t h e P CM u ses t h e TP S in pu t t o det er m in e
cu r r en t en gin e oper a t in g con dit ion s. In r espon se t o
en gin e oper a t in g con dit ion s, t h e P CM will a dju st fu el
in ject or pu lse widt h a n d ign it ion t im in g.
VEHICLE SPEED AND DISTANCE SENSOR—
PCM INPUT
Th e veh icle speed sen sor is loca t ed on t h e speed-
om et er pin ion gea r a da pt er (F ig. 10) or (F ig. 11). Th e
pin ion gea r a da pt er is loca t ed on t h e ext en sion h ou s-
in g of t h e t r a n sm ission (dr iver s side—2WD), or on
t h e t r a n sfer ca se (4WD). Th e sen sor in pu t is u sed by
t h e power t r a in con t r ol m odu le (P CM) t o det er m in e
veh icle speed a n d dist a n ce t r a veled.
Th e speed sen sor gen er a t es 8 pu lses per sen sor
r evolu t ion . Th ese sign a ls, in con ju n ct ion wit h a
closed t h r ot t le sign a l fr om t h e t h r ot t le posit ion sen -
sor, in dica t e a closed t h r ot t le deceler a t ion t o t h e
P CM. Wh en t h e veh icle is st opped a t idle, a closed
t h r ot t le sign a l is r eceived by t h e P CM (bu t a speed
sen sor sign a l is n ot r eceived).
Un der deceler a t ion con dit ion s, t h e P CM a dju st s
t h e idle a ir con t r ol (IAC) m ot or t o m a in t a in a desir ed
MAP va lu e. Un der idle con dit ion s, t h e P CM a dju st s
t h e IAC m ot or t o m a in t a in a desir ed en gin e speed.
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