Single-Pole, Single-Throw, Maintain ContactSingle-Pole, Single-Throw, Maintain Contact
Switches.Switches. A switch is designed with the purposeA switch is designed with the purpose
of controlling an electrical circuit by completing orof controlling an electrical circuit by completing or
opening the circuitopening the circuit. . With an ohmmeterWith an ohmmeter, check the, check the
resistance between the contact points of theresistance between the contact points of the
switswitch. ch. With the swiWith the switch closed (comtch closed (completing thepleting the
circuit), the ohmmeter reading should indicate 0 -circuit), the ohmmeter reading should indicate 0 -
40 ohms. 40 ohms. With the swiWith the switch open (opening the cirtch open (opening the cir--
cuit), the ohmmcuit), the ohmmeter reading should be infinieter reading should be infinityty. . IfIf
the above checthe above checks are good, the swiks are good, the switch is good. tch is good. IfIf
any of the above checks fail, the switch is bad andany of the above checks fail, the switch is bad and
must be replaced.must be replaced.
Single-Pole, Double-Throw, Momentary RockerSingle-Pole, Double-Throw, Momentary Rocker
Switches.Switches. This type of switch operates on theThis type of switch operates on the
principle that the circuit is closed only when theprinciple that the circuit is closed only when the
switswitch is held in the closch is held in the closed state. ed state. Once the switOnce the switchch
is released, the ciis released, the circuit wircuit will open. ll open. This swiThis switch istch is
checked like an On-Off switch with the exceptionchecked like an On-Off switch with the exception
that the switch must be held closed to completethat the switch must be held closed to complete
resistance checks.resistance checks.
Solenoids.Solenoids. A solenoid is an electrical compo-A solenoid is an electrical compo-
nent. nent. When electrWhen electricity icity is applied to the coiis applied to the coil, thel, the
solenoid wisolenoid will form an elll form an electromagnetectromagnet. . The magnet-The magnet-
ic field will pull or push an armature into the coilic field will pull or push an armature into the coil
(based on appli(based on application). cation). The armatThe armature can be con-ure can be con-
nected to a switch in electrical circuits to turn thenected to a switch in electrical circuits to turn the
swiswitch on or off. tch on or off. An armatAn armature can also be used toure can also be used to
open or close valves.open or close valves.
Solenoids employed as electrical switches can beSolenoids employed as electrical switches can be
troubleshtroubleshot with an ohmmeterot with an ohmmeter. . Remove the twoRemove the two
wires from the two larger posts of the solenoid.wires from the two larger posts of the solenoid.
Energize the solenoiEnergize the solenoid. d. With an ohmmeterWith an ohmmeter, check, check
the resisthe resistance between the two ltance between the two larger posts. arger posts. TheThe
ohmmeter should indicate between 0 - 40 ohmsohmmeter should indicate between 0 - 40 ohms
nominally.nominally.
Solenoids employed as hydraulic switches areSolenoids employed as hydraulic switches are
used to open and closused to open and close spools of vale spools of valves. ves. TheThe
simplest way to prove the solenoid coil good is tosimplest way to prove the solenoid coil good is to
energize the solenoid and then, with a metalenergize the solenoid and then, with a metal
object, touch the nut that secures the coil to theobject, touch the nut that secures the coil to the
cartricartridge. dge. The magnetic fThe magnetic field generated when theield generated when the
coil becomes an electromagnet will be significantcoil becomes an electromagnet will be significant
enough to pull the metal object to the nut (someenough to pull the metal object to the nut (some
solenoids employ a metal nut encased in plasticsolenoids employ a metal nut encased in plastic
and will require removal in order to detect theand will require removal in order to detect the
magnetic fimagnetic field). eld). This wilThis will prove the coil good; how-l prove the coil good; how-
everever, the armature may be stuck, the armature may be stuck. . If the hydraulicIf the hydraulic
circuit is still defective at this point, remove the coilcircuit is still defective at this point, remove the coil
and cartriand cartridge. dge. Now energiNow energize the coil, the armze the coil, the arma-a-
ture insiture inside the cartridde the cartridge should shift. ge should shift. If the arma-If the arma-
ture inside the cartridge did not shift and the coil isture inside the cartridge did not shift and the coil is
magnetizemagnetized, d, replace the replace the cartricartridge.dge.
Exercise care not to reverse polarity becauseExercise care not to reverse polarity because
some solenoids employ internal diodes which cansome solenoids employ internal diodes which can
be destroyed when the polbe destroyed when the polarity is rarity is reversed. eversed. TheThe
solenoids employed on the transmission controlsolenoids employed on the transmission control
valve contaivalve contain diodes. n diodes. The black wiThe black wire of the coilre of the coil
connects to the ground side of the circuit while theconnects to the ground side of the circuit while the
red wire goes to the positive side of the circuit.red wire goes to the positive side of the circuit.
DiodesDiodes(Illustration 6-10)(Illustration 6-10).. Diodes are one-wayDiodes are one-way
conductorconductors that provide isolats that provide isolation. ion. Current floCurrent floww
thrthrough a diode is froough a diode is from anode to cathom anode to cathode. de. TheyThey
are easily proven good by using an ohmmeter.are easily proven good by using an ohmmeter.
When using the ohmmeter, place the leads of theWhen using the ohmmeter, place the leads of the
ohmmeter on the opposite ends of the diode.ohmmeter on the opposite ends of the diode.
Observe the ohmmObserve the ohmmeter reading. eter reading. Then reversThen reverse thee the
ohmmeter leads on the ends of the diode.ohmmeter leads on the ends of the diode.
Observe tObserve the ohmmeter he ohmmeter reading. reading. The ohmmetThe ohmmeterer
readings should indicate a higher ohm resistancereadings should indicate a higher ohm resistance
in one direction opposed to the other directionin one direction opposed to the other direction
because the current generated by the ohmmeterbecause the current generated by the ohmmeter
is sufficient enough to forward-bias the diode.is sufficient enough to forward-bias the diode.
CCaatthhoodde e AAnnooddee
CurrenCurrent t FlowFlow
IllustIllustration ration 6-10. 6-10. DiodeDiode
To Next Page
Loading...