Volvo 819 E - Page 68

Group 28 Ignition systems
Design and function - System descriptions
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A
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Signels to diegnostic unit (Picture 0 )
In
this mode, the diagnostic circuit in the control unit reads the selected test function program and determines , as ap-
propriate,
which fault/acknowledgement code is to be displayed by the LED to report a fault stored in the memory or
confirm operation of a switch.
(The
arrows in this picture indicate that the direction of signal flow is from the control unit to the diagnostic unit.)
The diagnostic circuit controls the operation of the signal transistor . To produce a flash, the circuit activates the tran -
sistor, connecting
terminal 1 on the control unit to ground across terminal 20. Current then flows through the ignition
switch and diagnostic unit to control unit terminal 1and to ground across terminal 20, causing the LED to light (i.e. to
produce one flash
of the particular digiti . Immediately after this, when the LED has been extinguished , the signal
transistor opens the terminal 1 ground c onnection and the cycle is repeated until the three-digit code for the partic-
ular fault
or acknowledgement code has been completed .
If
the self-diagnostic function has been selected, the di agnosti c circuit must be suppl i ed w i th information on when
the next fault code is to be disp layed. The c ontrol unit receives this informat io n in the form of the new signal which is
initiated
when the voltage at term inal 1 falls to zero as pushbutton A is again depressed.
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