Volvo 819 E - Speed;Position Pick-Up and Flywheel Configuration

Group 28 Ignition systems
D esign and function - Speed and crankshaft position information
Inductive speed/p osition pick-up
L .I-.-\ 4
'~7 090
Function and location
The purpose of the speed/position pick - up (1) is to s upply the control unit (3) with information on engine speed and
crankshah
position. The pick -u p is mounted close to the flywheel (2) (or carrier plate on automatic models), the
periphery of which is provided with a series of drilled holes. The location of the pick -up is such that it is influenced by
this con figuration of holes .
Construction
A. The pick- up (1) cons ists of a permanent magnet (4 ). a pole tip (5) which co ncentrates the magne t ic fi eld at the end
of the device and a coil (6 ). The magnet creates a magnetic field whi ch varies in strength depending on whether a
hole or 'too th ' is passing the tip .
Operation
An alternating voltage is induced in the pick-up coil when one of the pro jec tions or 'teeth' on the fl ywhee l passes the
tip. This variable signal is transmitted by a lead to the control unit.
In effect, therefore. th e pick-up supplies t he control unit with a unique al t ernating vol tage - an electrical ' picture ' of
the flywhee l from which the unit can determine both the engine speed and crankshah position.
Signal
B. The frequency (7) of the signal (i.e. the period between two successive voltage peaks) is a measure of the engine
speed. To pr ovide a reference signal for the crankshah position , one or more 'teeth' (one or two in the case of 4-
cy
linder engines and three on 6-cylinder un its) on the flywheel /carrier plate are made longer than the othe r s. The
con tr ol unit detects the c rankshaft position by sensing the interruption in signal (8) which occurs as a long tooth
passes the pick-up. This reference signal is used to determine the top dead centre (TOC) position.
"