| Free A.S.E. Study Guide An ASE Study Guide for the Automotive Technician |
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| Automotive Relays |
In an automobile it is undesirable to have high current running through headlight and
horn switches, the PCM, etc. We do not want this high current running through sensitive
devices like the PCM and other sensitive circuits. The vehicles PCM (powertrain
control module) uses its low current signal or ground to activate the fans, idle control
devices, and other actuators. Yet high current is necessary to drive so many different
things. This is where the relay comes in.
Electromechanical relays are a simple mechanical switch that use a low current either
coming from a switch or the PCM to control a high current device such as a fan or your
headlights. They use an electromagnet (coil) that controls an arm that swings between
two contacts. When current is passed through the coil a magnetic field is created that
draws the arm towards the high current circuit.
When low current is run through the coil the armature or arm is pulled away from its
park position to activate whichever higher current device it controls. A standard has
been developed for the plug of a mechanical relay. These include either four or five
pins: 85. 86, 30, 87, 87a or 85, 86, 30, 87 (no park).
They will use numbers 85 and 86 as the electromagnet control. This coil will usually
contain a diode to protect the contact and other circuits from the sudden collapse of
current when the switch is turned off. On these relays (the most common) it’s important
not to reverse the 85 and 86 (coil) pins.
The number 30 pin is connected to the high current source and number 87a is the
prong where the arm parks (off). The 87 pin is used to activate the desired device (on).
A solid state relay is different than an electromechanical relay. These relays have no
moving parts and are considered to be more reliable. You will not hear the audible click
that comes from its older counter part. They are faster using logic gates activating the
switch in only milliseconds. Of course with extra speed and longevity comes the price
tag: they cost a lot more and are becoming a lot more common.
horn switches, the PCM, etc. We do not want this high current running through sensitive
devices like the PCM and other sensitive circuits. The vehicles PCM (powertrain
control module) uses its low current signal or ground to activate the fans, idle control
devices, and other actuators. Yet high current is necessary to drive so many different
things. This is where the relay comes in.
Electromechanical relays are a simple mechanical switch that use a low current either
coming from a switch or the PCM to control a high current device such as a fan or your
headlights. They use an electromagnet (coil) that controls an arm that swings between
two contacts. When current is passed through the coil a magnetic field is created that
draws the arm towards the high current circuit.
When low current is run through the coil the armature or arm is pulled away from its
park position to activate whichever higher current device it controls. A standard has
been developed for the plug of a mechanical relay. These include either four or five
pins: 85. 86, 30, 87, 87a or 85, 86, 30, 87 (no park).
They will use numbers 85 and 86 as the electromagnet control. This coil will usually
contain a diode to protect the contact and other circuits from the sudden collapse of
current when the switch is turned off. On these relays (the most common) it’s important
not to reverse the 85 and 86 (coil) pins.
The number 30 pin is connected to the high current source and number 87a is the
prong where the arm parks (off). The 87 pin is used to activate the desired device (on).
A solid state relay is different than an electromechanical relay. These relays have no
moving parts and are considered to be more reliable. You will not hear the audible click
that comes from its older counter part. They are faster using logic gates activating the
switch in only milliseconds. Of course with extra speed and longevity comes the price
tag: they cost a lot more and are becoming a lot more common.
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