Toyota COMPUTER-CONTROLLED SYSTEM Training Manual page 46

F*
ELECTRONIC CONTROL SYSTEM - Vehicle Speed Senso r
4 . MRE (MAGNETIC RESISTANCE
ELEMENT) TYP E
This sensor is mounted on the transmission or
the transfer and is driven by the drive gear of
the output shaft .
Speed sensor OHP 3 1
This sensor consists of an HIC (hybrid integrated
circuit) with a built-in MRE (magnetic resistance
element) and a magnetic ring .
OHP 3 1
OPERATION
The resistance value of the MRE changes accor-
ding to the direction of the lines of magnetic force
applied to it .
Thus, the direction of the lines of magnetic force
is changed by the rotation of the magnet fitted to
the magnetic ring with the result that the output
of the MRE becomes an alternating waveform as
shown in the illustration on the above right .
The comparator in the speed sensor converts the
alternating waveform into a digital signal, which
is then inverted by the transistor before being
sent to the combination meter, as shown in the
illustration at right above .
The frequency of the waveform is in accordance
with the number of poles of the magnet fitted to
the magnetic ring . There are two types of
magnetic ring (depending on the vehicle model) :
the type with twenty magnetic poles, and the
type with four magnetic poles . The 20-pole type
generates a 20-cycle waveform (i .e ., twenty
pulses for each rotation of the magnetic ring),
while the 4-pole type generates a 4-cycle
waveform .
Constant voltage circuit
,
+ B
20-POLE TYPE SPEED SENSO R
Magnetic ring
(rotating )
MRE output
N S N
Comparator 1
output 0
Speed 12 V~
sensor 0 V
output
OHP 3 1
In the 20-pole type, the frequency of the digital
signal is converted from twenty pulses for each
revolution of the magnetic ring to four pulses by
the pulse conversion circuit in the combination
meter, then the signal is sent to the Engine ECU .
(See electrical circuitry at right . )
In the case of the 4-pole type, there are two
different kinds : in one type, the signal from the
speed sensor passes through the combination
meter before going to the Engine ECU ; in the
other type, this signal goes directly to the
Engine ECU without passing through the
combination meter . (See electrical circuitry at
right . )
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