The additional heater control unit
M75
receives
a supply controlled by the ignition (INT/A) at pins 15 and U from
the line protected by the fuse
B10
up to 2/99 or fuse
F6
of
the additional fuse box
B98B
from 2/99.
The direct supply arrives from the battery
(maxifuse
J/B1
of
B5
up to 2/99 or
B99
from
2/99) at pin TL through fuse
F10
of junction unit
B1
:
this signal allows the battery voltage to be checked, i.e. the level
of electrical power available.
Pin 31 of
M75
is earthed.
Pin N of
M75
receives the engine
rpm signal coming from the engine management control unit
M10
.
Pin TW, on the other hand, is connected
to the special temperature sensor
K80
which measures the
temperature of the coolant near the additional heater.
Relays
J36
and
J37
control
the engagement of the resistors
O25
.
The supply at the contact of the relays
arrives from the battery from the line of the specific maxifuse
B38
up
to 2/99 or from the HEAT maxifuse, located directly on the battery
A1
-
connector D from 2/99, while the coils are energized by the control
unit
M75
, which operates only relay
J36
to switch
on plug
O35 A
, only relay
J37
to switch on plugs
O35
B
and C, and both relays for all three plugs
O35
.
The safety relay
J38
interrupts the
supply of the two above mentioned relays and consequently the heater
plugs if the contact to earth is not correct: the coil for
J38
is
connected on one side to earth and on the other to the metal frame for
the device containing the heater plugs: this is connected to the
engine earth
C1
by means of a special earth loom
C41
:
if this connection is broken, relay
J38
is energized and
immediately stops the current supply to the heater plugs.