Additional coolant heater for the TDI (Articles)

At sub-zero temperatures, the low fuel consumption of TDIs reveals its downside: they produce significantly less heat loss compared to many other engines, and therefore reach their usual operating temperature noticeably slower.
If further heat is then drawn from the room for heating, the warm-up phase will be extended even further.

At a slow speed and with the heater and fan running at full power, a balance between heat loss and heat dissipation through the heater can already be achieved below the normal operating temperature: for example, the engine temperature does not exceed 70°C.
When the engine load increases due to speed or gradient, the temperature rises further. Conversely, on longer inclines with predominantly forward motion, the temperature can even drop further.
Since the symptoms are very similar to those of a thermostat that doesn't close properly or opens too early, it shouldn't be replaced immediately. Instead, it should be checked to see if the supply hose to the radiator, along with the radiator inlet area, are consistently warm/hot, even though the engine is still at a low temperature. Only then should the replacement of the thermostat be considered.

To mitigate the issue somewhat, most TDIs come with auxiliary heaters from the factory.
Older models have a fuel-powered auxiliary heater, while newer models (from around 1995) have 3 heating elements in the engine's coolant outlet, which connect to the heating system's heat exchanger.
These heating glow plugs are switched on by the MSG via relays when the intake air temperature at engine start is below approximately 5°C and the temperature from the main temperature sensor (Golf 4 sensor part number: G72) or the coolant temperature is below approximately 40°C.
The actual temperature thresholds can vary depending on the MSG data, and are also shifted up or down by the tolerances specified by the supplier.

Someone who starts their TDI on a frosty morning in the (slightly warmer) garage may, under certain circumstances, be able to get by without the help of the electric heater, as decreasing intake air temperatures (when exiting the garage) do not trigger a new decision about the on/off conditions.

As a water pump relay, a double relay is usually installed in older TDIs.
The double relay (a small, black box, about the size of a hand, with 3 thin control cables, 3 thicker screwed-in load cables, and sometimes a fuse under a plexiglass cover) is usually located in the engine compartment near the brake booster.

At one end of the double relay, there are 2 candles connected in parallel. At the other end, there is the 3rd candle. This allows for 1, 2, or 3 candles to be turned on, depending on the need.
Due to the increased diesel consumption during the generation of heat for the candles, additional waste heat is created, which further accelerates the heating of the engine.

Depending on the vehicle type, individual relays can also be installed and housed in a "normal" relay box.

To prevent the alternator from being overloaded due to the power consumption of the heating candles, the MSG constantly monitors its utilization by tapping into the DF terminal. When the utilization approaches 100% (e.g., during the afterglow phase with the headlights and rear window heater turned on, and at a low engine speed), the candles are switched off.
Due to the MSG's switching strategy, the spark plugs are only re-activated when the engine operating conditions and the alternator load during the last approximately 10 seconds indicate that they do not need to be immediately switched off after being initially activated. This reduces the overall load on the switch relay, which could otherwise lead to premature contact wear due to frequent and "short-sighted" on/off switching every few seconds.

Once the water temperature reaches approximately 40°C, the heating elements will be permanently switched off.

To enable the desired system monitoring, it is recommended to install 2 (different) sensors inside the unit, for example, a 3 mm LED for the individually controlled candle and a 5 mm LED for the 2-candle group.
The voltage for the controls can be tapped from the outputs of the aforementioned double relay. This is usually a small, black box, about the size of a hand, with 3 screw connections, a 3-way connector, and a fuse holder under a transparent cover.
The controls then indicate when the heating elements receive power. However, this does not necessarily mean that they are actually heating – for this, the candle must also have a passage and not be burnt out.

For a thorough diagnostic check, as with the actual light bulbs, the current from the main headlight bulb should be passed through the bulb(s). To do this, with the engine running, connect a cable from both relay outputs to the bulb, one after the other, and keep the other connection of the bulb to the battery positive terminal. The (intact!) candle should shine brightly, with the individual candle being only slightly dimmer.
Alternatively, you can check the sensors in a quiet environment (!!) and with the engine running, by briefly connecting them to the battery (+). The heating power of an intact candle causes the surrounding water to boil instantly, which can be heard as a hissing/bubbling sound.

Anyone who has a VAGCOM can also easily perform the "test run" using the gear shift position diagnostic: in cars with hearing problems, you can even hear the noise inside the passenger compartment.
Also, the interior lighting can also be used as a kind of control when the engine is idling: When the heating is active, it becomes a bit darker.
However, whether both or only 1 candle heat in the large compartment is difficult to determine, either by hearing or by looking at the interior light.
In case of doubt, check the wires and fuses individually (i.e., disconnect the plugs of the other fuses), if necessary. Repairing cables or replacing faulty fuses.


Air-source heat pumps

In newer vehicles (e.g. Polo 9N (production from the end of 2001) will no longer use incandescent bulbs in the cooling circuit, but PTC heating elements are integrated into the fan housing, which immediately heat the air circulating through the ventilation system.
Therefore, the freezing driver no longer has to share the electrically generated heat with the engine, and the heater provides warm air more quickly after the engine starts - however, the engine heats up more slowly than when using a glow plug-water heating system.

Here, the heating elements are also controlled by the MSG via load relays, again separately for small and large heating power.

Due to the PTC characteristic, the heating elements adapt to the required power:
If a large amount of cold air flows through, its resistance decreases, the power consumption increases, and the air becomes heated more strongly.
With milder outdoor temperatures and/or the beginning of heat dissipation from the water-to-air heat exchanger (which is located before the PTC heating element), the higher ambient temperature causes the resistance of the heating element to increase, and the power consumption decreases.

An additional activation condition (in addition to generator load and engine temperature, as in the water-electric heater) is the heating requirement triggered by the driver via a switch contact in the area of the hot-tap connection of the temperature control knob, or via a temperature request to the air conditioning system.
Because turning on an auxiliary heater when no heating power is required would be pure energy waste, while in the case of the electric coil heater, the motor still benefits from the heat that is not diverted to the passenger compartment.

An electrical test of the heating elements should be possible (subject to the author's own experience not yet being available), similar to the testing of incandescent bulbs, by bridging the input and output of the relay using the main headlight bulb. If the bulb lights up brightly and then gradually becomes dimmer (due to the current limitation of the heating element), it suggests that the heating element is intact.
When turning on the heater fan, the light should shine brighter again, etc.