Glow plug heater modification (for AFN and similar engines) (Articles)

I would have liked to program a higher activation threshold for my AFN glow plug heater (which is monitored with LEDs), approximately 5°C higher. However, the engine control unit does not offer an adaptation channel for this purpose.

So, once again, the only option was the old method of sensor signal manipulation – in this case, by tampering with the charge air temperature sensor, which also determines the outside temperature for the glow plug heating system.

The measured temperature can be lowered, for example, by connecting a resistor in series with the sensor.
To make the TDI-EDC sensors appear to read 0°C instead of 5°C, you would need to connect approximately 1.2 kΩ in series with the sensor.
However, in the upper measurement range, for example, a real charge air temperature of 50°C might be displayed as approximately 25°C, which would prevent a potential performance reduction triggered by excessively high charge air temperatures. I didn't want to face those consequences, especially since they come with no performance gains or other benefits.

In the MSG (presumably a circuit board), I found internal SMD pull-up resistors with a value of 750 ohms connected to a common 5-volt (likely) wire for pins 14, 63, and 64 (sensor inputs for motor, fuel, and intake air temperature).

A quick calculation using the target values for the sensors at various temperatures showed that changing the pull-up resistor shifts the apparent temperature across the entire measurement range by a fixed number of degrees, either upwards or downwards. A reduction of 5° would be achieved with a 630 Ohm resistor instead of the current 750 Ohm.
So, I replaced the 750-ohm pull-up resistor for the charge air temperature sensor with a tiny 644-ohm resistor that I salvaged from an old, defective L-Jetronic control unit (it's good to not throw things like that away).

A subsequent temperature comparison of all sensors using VCDS/VAGCOM, with the engine cooled down, revealed:
Water +2.7°, Diesel +4.5°, Charge air -1.8°.
So, for the air temperature, it was 5.4°C below the average of the other sensors. And those same 5° errors in the upper measurement range don't seem critical to me.

Shortly after the engine started, my tractor also dutifully activated the glow plug heater, which it usually didn't do under such "mild" conditions.

For those who want to replicate this at MSG 028 906 201 GH:
The pull-up resistor, with its purple-green-black-and-something color code, is located in the upper left corner of the circuit board, on the soldering side, when viewed from the connector panel.
If you follow the trace from pin 64 (the last row of pins, counting backwards from 68 starting from the left), you will find it after just a few centimeters.

The effects of using the next most common standard values instead of 644 ohms as a pull-up resistor:
680 ohms reduces the apparent temperature by approximately 3°C, while 560 ohms reduces it by about 8°C.

"Soldering a 3.9 kOhm resistor in parallel with the 750 kOhm pull-up resistor reduces the apparent temperature by approximately 5 °C."

Glow plug heater modification (for AFN and similar engines)
ulf 12-06-2003, 19:22
I would have liked to program a higher activation threshold for my AFN glow plug heater (which is monitored with LEDs), approximately 5°C higher. However, the engine control unit does not offer an adaptation channel for this purpose. So, once again, the only option was the old method of sensor signal manipulation – in this case, by tampering with the charge air temperature sensor, which also determines the outside temperature for the glow plug heating system. The measured temperature can be lowered, for example, by connecting a resistor in series with the sensor. To make the TDI-EDC sensors appear to read 0°C instead of 5°C, you would need to connect approximately 1.2 kΩ in series with the sensor. However, in the upper measurement range, for example, a real charge air temperature of 50°C might be displayed as approximately 25°C, which would prevent a potential performance reduction triggered by excessively high charge air temperatures. I didn't want to face those consequences, especially since they come with no performance gains or other benefits. In the MSG (presumably a circuit board), I found internal SMD pull-up resistors with a value of 750 ohms connected to a common 5-volt (likely) wire for pins 14, 63, and 64 (sensor inputs for motor, fuel, and intake air temperature). A quick calculation using the target values for the sensors at various temperatures showed that changing the pull-up resistor shifts the apparent temperature across the entire measurement range by a fixed number of degrees, either upwards or downwards. A reduction of 5° would be achieved with a 630 Ohm resistor instead of the current 750 Ohm. So, I replaced the 750-ohm pull-up resistor for the charge air temperature sensor with a tiny 644-ohm resistor that I salvaged from an old, defective L-Jetronic control unit (it's good to not throw things like that away). A subsequent temperature comparison of all sensors using VCDS/VAGCOM, with the engine cooled down, revealed: Water +2.7°, Diesel +4.5°, Charge air -1.8°. So, for the air temperature, it was 5.4°C below the average of the other sensors. And those same 5° errors in the upper measurement range don't seem critical to me. Shortly after the engine started, my tractor also dutifully activated the glow plug heater, which it usually didn't do under such "mild" conditions. For those who want to replicate this at MSG 028 906 201 GH: The pull-up resistor, with its purple-green-black-and-something color code, is located in the upper left corner of the circuit board, on the soldering side, when viewed from the connector panel. If you follow the trace from pin 64 (the last row of pins, counting backwards from 68 starting from the left), you will find it after just a few centimeters. The effects of using the next most common standard values instead of 644 ohms as a pull-up resistor: 680 ohms reduces the apparent temperature by approximately 3°C, while 560 ohms reduces it by about 8°C. "Soldering a 3.9 kOhm resistor in parallel with the 750 kOhm pull-up resistor reduces the apparent temperature by approximately 5 °C."

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