Toyota COMPUTER-CONTROLLED SYSTEM Training Manual page 140

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DIAGNOSIS - General, Principle of Diagnostic Syste m
The test mode is used only by the technician for
troubleshooting the engine control system .
Compared to the normal mode, it has been given
a greater sensitivity . For example, in the normal
mode, the ECU will light the "CHECK ENGINE"
lamp and register the problem in memory if the
same trouble is detected two times in
succession ; in the test mode, however, the ECU
will light the "CHECK ENGINE" lamp and
register it in memory if the trouble is detected
even once .
The test mode is made operative by the
technician by means of a predetermined
procedure .
The method of reading the diagnostic codes in
the test mode is the same as in the normal
mode . The methods for utilizing the normal
mode and test mode is explained in the
TROUBLESHOOTING section (page 149) .
The items which cause the ECU to light the
"CHECK ENGINE" lamp and the items registered
in memory when the ECU detects trouble differ
depending on the mode as well as on the vehicle
model . Please refer to the repair manual for the
affected vehicle . See CHECKING AND
CLEARING DIAGNOSTIC CODES (page 159) for
the normal mode and test mode setting
procedure, code output methods, and code
clearing methods .
In addition, various types of information are
output by the VF terminal of the check
connector, depending on the state of the T or
TE1 terminal and the state of the IDL contacts of
the throttle position sensor . For details, see VF
OR VF1 TERMINAL OUTPUT in this section (page
136) .
PRINCIPLE OF DIAGNOSTIC
SYSTEM
The signal level that signifies to the ECU that an
input or output signal is normal is fixed for that
signal .
When signals for a particular circuit are
abnormal with respect to this fixed level, that
circuit is diagnosed as being abnormal . For
example, when the coolant temperature signal
circuit is normal, the voltage at the THW
terminal is in a fixed range between 0 .1 to 4 .9 V .
This signal circuit is diagnosed as being
abnormal when the THW terminal voltage is less
than 0 .1 V (a coolant temperature of 139°C
[282°F] or greater) or greater than 4 .9 V (a
coolant temperature of -50°C [-58°F] or lower) .
5
~+r
Abnormal rang e
4
THW Normal range
/ for engin e
Normal range / ( Normal rang e
for diagnostic system for diagnostic syste m
3
2
1
0'
-~
~
Abnormal range
i
-50 139
(-58) (282 )
Coolant temperature °C (°F )
-50°C~ / r ~
~(-58°F~/ Normal range
139°C
(282°F
Abnormal
4 .9 V
OHP 9 2
132