Fiat Bravo Service Manual page 384

Bravo
!
§ i ** Engine
Fuel system
10.
SPEEDOMETER SENSOR
The speedometer sensor (vehicle speed sen­
sor) it is composed of a Hall effect sensor and
is located at the differential outlet.
The sensor transmits a signal to the control
unit where the frequency varies according to
the speed of the vehicle.
The control unit uses this information to im­
prove the management of the engine idle ad­
justment actuator and for the CUT-OFF strat­
egy.
B
The speedometer sensor signal is also
processed to calculate and memorize
the mileage travelled in the control
unit; this information can be read us­
ing the Fiat/Lancia Tester.
In order to achieve a good compromise between the high performance in terms of power at high speeds
and good torque at low speeds, a phase transformer (electronically and hydraulically operated) is fitted
for the inlet camshaft.
This device makes it possible to alter the timing diagram (inlet phase) according to the engine load con­
ditions; this parameter is processed by the MOTRONIC control unit on the basis of the electrical signals
received by the air flow meter and the rpm sensor and sent to the phase transformer solenoid valve.
The construction of the device involves a main assembly fitted on the inlet camshaft which has the task
of altering the angular position of the actual shaft in relation to the drive pulley.
In addition there is a valve, operated by an electro-magnet, both of which are on the inlet manifold and
connected hydraulically to the main assembly by appropriate ducts.
The operating principle is as follows:
- with the temperature of the coolant below 40 °C and when the engine is idling or the speed exceeds
4800 rpm, the electro-magnet (1) is de-energized, therefore the valve (2) thrust by the opposing
spring (3) remains raised not allowing the oil which is arriving from the duct (A) to reach the trans­
former.
In this case the timing of the inlet valves remains unaltered.
With the temperature of the coolant above 40 °C and with the engine speed above idle and below
4800 rpm with the butterfly angle greater than about 8°, the electro-magnet (1) is energized, thereby
thrusting the valve (2) downwards. In this position the oil, coming from the duct (A), enters the pis­
ton chamber (B) and from here flows via a special opening into the duct (C) inside the latter.
The oil can only leave the above mentioned duct via the upper port (in contact with duct (D) supply­
ing oil to the transformer) because with the valve (2) lowered, the lower port is not in contact with the
discharge duct (E).
The oil reaches the chamber (G) through ducts (D) and (F) moving the piston (4) axially towards the
engine; because this piston has helical teeth on the outside this axial movement causes it to rotate in a
clockwise direction (as seen from the timing side).
This rotation is transmitted, by means of a straight toothed splied profile, to the pinion (5) which,
bolted onto the threaded end of the camshaft (6), transmits the rotation to the shaft, thereby varying
the timing of the inlet valves by an advance of 9°.
When the electro-magnet is de-energized, the valve (2) returns to the original position, interrupting the
flow of oil under pressure to the chamber (G), but allowing the return of the oil to the exhaust, thanks to
the force of the opposing spring (7).
PHASE TRANSFORMER
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