402
Thermo AC/DC
4 Functions of the heater
4.2
Functional sequence of the heater
4.2.1 Switching on
In order to switch on the heater, 24V DC respectively the
switch on signal must be connected according to the valid
wiring diagram to the AC/DC control wiring harness for
external control by an operating or control element (for
example, switch, relay, pre-selection timer) installed on
the vehicle.
This power is fed to the load contactor, the control ther-
mostat and the overheat protection (see chapter
constitute the control loop of the AC/DC control head.
The temperature guards interrupt the current path for
activating the contactor when their switching thresholds
are reached.
If all of the following three conditions are met:
1. 24V DC is applied to the external control input or the
switching contact is closed,
2. The control thermostat has not yet reached the upper
switching threshold / has fallen below the lower swit-
ching threshold,
3. The overheating protection has not yet reached the
upper switching threshold,
the contactor is activated, the moving core attracts and
closes the main contacts.
In this way the heating cartridges connected to the
contactor are powered. Each heating cartridge now con-
verts the electrical energy into heat.
4.2.2 Heating operation
The control thermostat opens its contact at 75°C and
closes it again at 68°C.
In this way, the heating operation is regulated.
The overheat protection opens its contact at 125°C and
must be mechanically reset by service personnel.
ATTENTION:
After the overheating protection has been triggered,
the cause of the overheating must first be removed
before it is reset.
4.2.3 Switching off
When the 24V DC control voltage / switch on signal at the
external control terminal is interrupted by a vehicle
installed control element (e.g. switch, relay, pre-selection
timer), the load contactor opens and the voltage supply of
the heating cartridges is switched off.
The circulating pump in the coolant circuit must continue
its operation at least 120s.
4.3
Malfunctions caused by overheating /
tripping of the temperature limiter
If the heater is overheated, the temperature limiter is
tripped and the heater goes into a fault lock-out.
The circulating pump in the coolant circuit must continue
its operation at least 120s.
After cooling down and remedy the cause as well as the
inspection of the heater for possible damage, in particular
the wiring, the temperature limiter and the control ther-
mostat, the button of the temperature limiter must be reset
manually.
NOTE:
The Chapter 5 contains approaches for troubleshooting.
Summary of Contents for Thermo AC 070
Page 47: ...memos...
