īī
Electrical System TroubleshootingElectrical System Troubleshooting
Some of the components described in this sectionSome of the components described in this section
are optional equipment.are optional equipment.
Eng Oil lightEng Oil lightī(DS1, ī(DS1, IllustIllustration 6-16)ration 6-16).. īThe engineīThe engine
oil light will illuminate when engine oil pressureoil light will illuminate when engine oil pressure
switswitch (S17, Illustrch (S17, Illustration 1-6) has closed. ation 1-6) has closed. TheThe
engine oil pressure switch will close between 7 -engine oil pressure switch will close between 7 -
10 psi on falling engine oil pressure.10 psi on falling engine oil pressure.
Battery lightBattery lightī(DS3, Illustration 6-16)ī(DS3, Illustration 6-16).. īWhen theīWhen the
battery light illuminates, it lets the operator knowbattery light illuminates, it lets the operator know
that there is a problem witthat there is a problem with the alternatorh the alternator. . ReferRefer
to theto theīComponent TroubleshootingīComponent Troubleshootingīof theīof theīAlter-īAlter-
natornatorīfound earlier in this section.īfound earlier in this section.
Seat Belt lightSeat Belt lightī(DS4, Illustration 6-16)ī(DS4, Illustration 6-16).. īThe seatīThe seat
belt light (DS4) will only illuminate when the key isbelt light (DS4) will only illuminate when the key is
in the ignition position and the seat belt is unfas-in the ignition position and the seat belt is unfas-
tened. tened. With the sWith the seat belt faseat belt fastened, the seat beltened, the seat beltt
switch (S18) will send a ground through wire #38,switch (S18) will send a ground through wire #38,
energizing the seat belt relay (K2) and breakingenergizing the seat belt relay (K2) and breaking
the contact between pins #30 and #87A of relaythe contact between pins #30 and #87A of relay
(K2), removing the ground from the seat belt light(K2), removing the ground from the seat belt light
(DS4).(DS4).
Shift Fail lightShift Fail lightī(DS6, Illustration 6-16, if equippedī(DS6, Illustration 6-16, if equipped
with APC 70)with APC 70).. īThe shift fail light (DS6) is con-īThe shift fail light (DS6) is con-
trolled by the APC 70 and will illuminate only whentrolled by the APC 70 and will illuminate only when
the operator has attempted an improper shiftingthe operator has attempted an improper shifting
procedure. procedure. The only purpose of thiThe only purpose of this light is ts light is to leto let
the operator know that he or she is outside of thethe operator know that he or she is outside of the
operating operating parameterparameters.s.
Low Air lightLow Air lightī(DS7, ī(DS7, IllustIllustration 6-16)ration 6-16).. īThe low airīThe low air
light (DS7) will illuminate when the air pressurelight (DS7) will illuminate when the air pressure
falls below 75 psi. falls below 75 psi. The low air pressurThe low air pressure switche switch
(S13), a normally closed switch, will close when(S13), a normally closed switch, will close when
the air pressure drops below 75 psi, sendingthe air pressure drops below 75 psi, sending
ground through wire #31 (Illustration 6-16) com-ground through wire #31 (Illustration 6-16) com-
pleting the cipleting the circuit to the low rcuit to the low air light (air light (DS7). DS7). InIn
addition to the low air light being illuminated, anaddition to the low air light being illuminated, an
audible alarm will be heard from the low air / shutaudible alarm will be heard from the low air / shut
down buzzer down buzzer (LS5). (LS5). When the air When the air pressurpressure dropse drops
to 60 psi, the parking brake valve will pop out,to 60 psi, the parking brake valve will pop out,
applying the parking brake, closing the parkingapplying the parking brake, closing the parking
brake switcbrake switch (S11). h (S11). As a result, this will energizAs a result, this will energizee
the brake saver relays (K23 and K31, Illustrationthe brake saver relays (K23 and K31, Illustration
9C-2), taking away the 12 VDC signal from the9C-2), taking away the 12 VDC signal from the
forward and reverse solenoids of the control valveforward and reverse solenoids of the control valve
that is located on the transmithat is located on the transmission. ssion. This inhibiThis inhibitsts
the operator frothe operator from drivim driving through the brakes. ng through the brakes. TheThe
air pressure gauge in the instrument panel willair pressure gauge in the instrument panel will
assist in determining if the low air switch (S13) isassist in determining if the low air switch (S13) is
defective, if a short exists on wire #31, or if a lowdefective, if a short exists on wire #31, or if a low
air pressurair pressure problem existse problem exists. . If the air pressureIf the air pressure
gauge indicates above 75 psi, remove wire #31gauge indicates above 75 psi, remove wire #31
from the low aifrom the low air pressurr pressure switce switch (S13). h (S13). If the lowIf the low
air light (DS7) does not go out, wire #31 is shortedair light (DS7) does not go out, wire #31 is shorted
out.out.
Brake Fault lightBrake Fault lightī(DS8, Illustration 6-16) (ifīī(DS8, Illustration 6-16) (ifī
equipped)equipped).. īThere are three signals in parallelīThere are three signals in parallel
capable of illuminating the brake fault light (DS8).capable of illuminating the brake fault light (DS8).
It is imperative that the technician understand howIt is imperative that the technician understand how
these switches work to isolate the source of thethese switches work to isolate the source of the
failure. failure. The simplThe simplest way to trest way to troubleshoot is tooubleshoot is to
remove the signal wire #32 from the switchesremove the signal wire #32 from the switches
(S23(S23, S24, and S27), one switc, S24, and S27), one switch at h at a time. a time. StarStartt
the engine and see if the brake fault light (DS8) isthe engine and see if the brake fault light (DS8) is
illumilluminated. inated. By removiBy removing the signal wirng the signal wire #32 frome #32 from
the switch (S23, S24, or S27), the switch will bethe switch (S23, S24, or S27), the switch will be
elimieliminated from the circnated from the circuit. uit. Once the defectiveOnce the defective
circuit has been removed, the brake fault lightcircuit has been removed, the brake fault light
(DS8) wi(DS8) will not illumill not illuminate. nate. Next, the techniNext, the techniciancian
must determine if the switch is bad or if the circuit,must determine if the switch is bad or if the circuit,
the switch (S23, S24, or S27) is monitoring, is atthe switch (S23, S24, or S27) is monitoring, is at
fault.fault.
Troubleshoot the switches (S23, S24, and S27) asTroubleshoot the switches (S23, S24, and S27) as
follows:follows:
1. 1. The power The power cluster cluster overstroverstroke indioke indicator scator switchwitch
(S27) is located on the end cover between the(S27) is located on the end cover between the
shell assembly and the tube cylinder of theshell assembly and the tube cylinder of the
power clusterpower cluster. . The overstrokThe overstroke indicator swite indicator switchch
is a normallis a normally open switcy open switch. h. A visual insA visual inspectionpection
of the switch will confirm overstroke condition.of the switch will confirm overstroke condition.
The brass colored indicator rod will be protrud-The brass colored indicator rod will be protrud-
ing from the end cover approximately 3/4ā toing from the end cover approximately 3/4ā to
1ā. 1ā. The indicatoThe indicator rod must be physr rod must be physicallyically
reseated. reseated. The most The most common ccommon cause of theause of the
brake fault light (DS8) illuminating is when anbrake fault light (DS8) illuminating is when an
overstroverstroke condition oke condition has has occurroccurred.ed.
2. 2. The brakThe brake coolant e coolant temperaturtemperature swie switch (tch (S24,S24,
Illustration 6-16) is located in the brake coolingIllustration 6-16) is located in the brake cooling
valve. valve. The brake coolant tThe brake coolant temperature semperature switchwitch
is a normally open switch that will close atis a normally open switch that will close at
190190
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FF. . The simplest way to check this swThe simplest way to check this switchitch
is to turn the truck off and allow the truck tois to turn the truck off and allow the truck to
cool. cool. With an ohmmeterWith an ohmmeter, measure the resis, measure the resist-t-
ance between the posts on the switch and theance between the posts on the switch and the
chassis chassis ground. ground. The ohmmetThe ohmmeter readinger reading
should indicatshould indicate infinity (open cire infinity (open circuit). cuit). If theIf the
ohmmeter reading indicates a short, then theohmmeter reading indicates a short, then the
brake coolant temperature switch (S24) mustbrake coolant temperature switch (S24) must
be replaced. be replaced. Care musCare must be taken to it be taken to insurensure
that a good chassis ground is obtained.that a good chassis ground is obtained.
3. 3. The brakThe brake coolant e coolant pressure pressure switswitch (S23,ch (S23,
Illustration 6-16) is located in the brake man-Illustration 6-16) is located in the brake man-
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