Jeep Cherokee 1999, Electronic Service Manual

Page: 319 / 1948

If a n oscilloscope is n ot a va ila ble, spa r k plu g ca bles
m a y be t est ed a s follows:
CAUTION: Do not leave any one spark plug cable
disconnected for longer than necessary during test-
ing. This may cause possible heat damage to the
catalytic converter. Total test time must not exceed
ten minutes.
Wit h t h e en gin e r u n n in g, r em ove spa r k plu g ca ble
fr om spa r k plu g (on e a t a t im e) a n d h old n ext t o a
good en gin e gr ou n d. If t h e ca ble a n d spa r k plu g a r e
in good con dit ion , t h e en gin e r pm sh ou ld dr op a n d
t h e en gin e will r u n poor ly. If en gin e r pm does n ot
dr op, t h e ca ble a n d/or spa r k plu g m a y n ot be oper a t -
in g pr oper ly a n d sh ou ld be r epla ced. Also ch eck
en gin e cylin der com pr ession .
Wit h t h e en gin e n ot r u n n in g, con n ect on e en d of a
t est pr obe t o a good gr ou n d. St a r t t h e en gin e a n d r u n
t h e ot h er en d of t h e t est pr obe a lon g t h e en t ir e
len gt h of a ll spa r k plu g ca bles. If ca bles a r e cr a cked
or pu n ct u r ed, t h er e will be a n ot icea ble spa r k ju m p
fr om t h e da m a ged a r ea t o t h e t est pr obe. Th e ca ble
r u n n in g fr om t h e ign it ion coil t o t h e dist r ibu t or ca p
ca n be ch ecked in t h e sa m e m a n n er. Cr a cked, da m -
a ged or fa u lt y ca bles sh ou ld be r epla ced wit h r esis-
t a n ce t ype ca ble. Th is ca n be iden t ified by t h e wor ds
E LE CTRONIC SUP P RE SSION pr in t ed on t h e ca ble
ja cket .
Use a n oh m m et er t o t est for open cir cu it s, exces-
sive r esist a n ce or loose t er m in a ls. Rem ove t h e dis-
t r ibu t or ca p fr om t h e dist r ibu t or.
D o n o t re m o v e
c a ble s fro m c a p . Rem ove ca ble fr om spa r k plu g.
Con n ect oh m m et er t o spa r k plu g t er m in a l en d of
ca ble a n d t o cor r espon din g elect r ode in dist r ibu t or
ca p. Resist a n ce sh ou ld be 250 t o 1000 Oh m s per in ch
of ca ble. If n ot , r em ove ca ble fr om dist r ibu t or ca p
t ower a n d con n ect oh m m et er t o t h e t er m in a l en ds of
ca ble. If r esist a n ce is n ot wit h in specifica t ion s a s
fou n d in t h e SPARK P LUG CABLE RE SISTANCE
ch a r t , r epla ce t h e ca ble. Test a ll spa r k plu g ca bles in
t h is m a n n er.
To t est ign it ion coil-t o-dist r ibu t or ca p ca ble, do n ot
r em ove t h e ca ble fr om t h e ca p. Con n ect oh m m et er t o
r ot or bu t t on (cen t er con t a ct ) of dist r ibu t or ca p a n d
t er m in a l a t ign it ion coil en d of ca ble. If r esist a n ce is
n ot wit h in specifica t ion s a s fou n d in t h e Spa r k P lu g
Ca ble Resist a n ce ch a r t , r em ove t h e ca ble fr om t h e
dist r ibu t or ca p. Con n ect t h e oh m m et er t o t h e t er m i-
n a l en ds of t h e ca ble. If r esist a n ce is n ot wit h in spec-
ifica t ion s
a s fou n d in t h e Spa r k P lu g Ca ble
Resist a n ce ch a r t , r epla ce t h e ca ble. In spect t h e ign i-
t ion coil t ower for cr a cks, bu r n s or cor r osion .
SPARK PLUG CONDITIONS
NORMAL OPERATING
Th e few deposit s pr esen t on t h e spa r k plu g will
pr oba bly be ligh t t a n or sligh t ly gr a y in color. Th is is
eviden t wit h m ost gr a des of com m er cia l ga solin e
(F ig. 11). Th er e will n ot be eviden ce of elect r ode
bu r n in g. Ga p gr owt h will n ot a ver a ge m or e t h a n
a ppr oxim a t ely 0.025 m m (.001 in ) per 3200 km (2000
m iles) of oper a t ion . Spa r k plu gs t h a t h a ve n or m a l
wea r ca n u su a lly be clea n ed, h a ve t h e elect r odes
filed, h a ve t h e ga p set a n d t h en be in st a lled.
Som e fu el r efin er s in sever a l a r ea s of t h e Un it ed
St a t es h a ve in t r odu ced a m a n ga n ese a ddit ive (MMT)
for u n lea ded fu el. Du r in g com bu st ion , fu el wit h MMT
ca u ses t h e en t ir e t ip of t h e spa r k plu g t o be coa t ed
wit h a r u st color ed deposit . Th is r u st color ca n be
m isdia gn osed a s bein g ca u sed by coola n t in t h e com -
bu st ion ch a m ber. Spa r k plu g per for m a n ce m a y be
a ffect ed by MMT deposit s.
COLD FOULING/CARBON FOULING
Cold fou lin g is som et im es r efer r ed t o a s ca r bon
fou lin g. Th e deposit s t h a t ca u se cold fou lin g a r e ba si-
ca lly ca r bon (F ig. 11). A dr y, bla ck deposit on on e or
t wo plu gs in a set m a y be ca u sed by st ickin g va lves
or defect ive spa r k plu g ca bles. Cold (ca r bon ) fou lin g
of t h e en t ir e set of spa r k plu gs m a y be ca u sed by a
clogged a ir clea n er elem en t or r epea t ed sh or t oper a t -
in g t im es (sh or t t r ips).
SPARK PLU G CABLE RESI STAN CE
MINIMUM MAXIMUM
250 Ohms Per Inch 1000 Ohms Per Inch
3000 Ohms Per Foot 12,000 Ohms Per Foot
Fig. 11 Normal Operation and Cold (Carbon) Fouling
8D - 6 IGNITION SYSTEM X J
DI AGN OSI S AN D T EST I N G (Cont inue d)