Car troubles: Audi a6 C4 2.5TDI fault code due to led tail lights

Car troubles: Fault code due to led tail lights

Car troubles are never fun, especially when you're dealing with an older vehicle. If you own an Audi A6 C4 2.5 TDI with AAT or AEL engine you may have come across a frustrating fault code and check engine light related to your taillights.

As it turns out, the brake light circuit in these engines can have too much resistance when aftermarket LED tail light bulbs are installed. These bulbs may have a higher resistance value compared to the factory bulbs, which can cause the circuit to "float." This can lead to the brake pedal switch position sensing circuit to stop working altogether. 

When the circuit floats, the brake pedal position sensing circuit stops working and the vehicle's computer will not be able to detect the brake pedal position. This will appear as a check engine light turning on when driving for a while and turning off by pressing the brake pedal.

But fear not, there is a relatively simple solution to this problem. By adding a 1-watt 207 Ohm resistor between the brake light positive and ground inside the tail light, you can effectively remedy the issue and get your vehicle back on the road in no time. This is a quick and easy fix that can save you from costly repairs or replacements down the road.

So, if you're dealing with a fault code related to your taillights on your Audi A6 C4 2.5 TDI AAT AEL, and have recently switched to aftermarket LED tail light bulbs, don't hesitate to try this simple solution. It just might save you a lot of headaches in the long run. Keep in mind that the resistance value might be different depending on the brand or model of the bulbs you are using.

Appropriate resistor to use as an pulldown resistor on the Audi A6 C4 brake light circuit to prevent random check engine lights

As you can see in the image, the resistor is a through-hole component that has a color code on it. The color code is used to identify the resistance value of the resistor. The first band is red, the second is black, the third is violet and the fourth is gold. This indicates the resistance of the resistor is 207 ohms with a 5% tolerance. It's important to double check the resistance value of the resistor before installing it as the power resistors might have non-standard colour coding.

To identify the first band, it's important to note that the first band is always the one on the left side when looking at the resistor. The left lead is the one that is closest to the band with the color code. The right lead is the one that is farther away or has only one band close to it. This way you can make sure that you are reading the color code of the resistor correctly. Additionally, the accuracy band, which is the fourth band, is usually located on the right side of the resistor which makes it easier to identify the right and left side of the resistor. This band is usually gold or silver in color and is used to indicate the tolerance level of the resistor, which is the acceptable deviation from the actual resistance value.

Taillight bulb holder of an Audi A6 C4 Avant 1995-1997 modified with additional brake light circuit drain resistor

As you can see in the image, the resistor is protected by a heat shrink tube and soldered between the ground and brake light conductor in the taillight bulb holder. This ensures that the resistor is protected from any potential short circuits, and is properly connected to the circuit. 

To calculate the current and power of the resistor, we need to know the resistance value and the voltage across it. We know that the resistance value of the resistor is 207 ohms and the voltage across it is usually a normal alternator output voltage around 14.4 volts.

Current (I) = Voltage (V) / Resistance (R)

I = 14.4 / 207

I = 0.07 Amperes or 70mA

Power (P) = Voltage (V) x Current (I)

P = 14.4 x 0.07

P = 1.008 Watts

It's important to note that the current and power will change depending on the voltage across it. Additionally, it's worth mentioning that the original BAY15d p21/5w bulb consumes more power, and the rear light enclosure is more than capable of dissipating the lower heat output of the resistor and the wiring is fully capable of carrying the lower current. This means that the addition of the resistor will not affect the performance or safety of the brake light circuit.

Let's calculate the surface temperature of a resistor in a brake light circuit just to be safe. We need to calculate the thermal resistance of the resistor. The thermal resistance (Rθ) of a through-hole resistor is typically around 60-135 °K/W. It can be calculated using the formula: Rθ = (Tj-Ta)/P, where Tj is the junction temperature, Ta is the ambient temperature and P is the power dissipation.

Now we can calculate the junction temperature (Tj) using the formula Tj = Ta + (P * Rθ) with an assumed ambient temperature of 25°C

Tj = 298.15K + (1.008W * 60K/W)

Tj = 358,63K = 85,48°C

The maximum temperature for the polyolefin heat shrink is 135°C and the temperature is lower. It's important to note that this is the temperature at the junction of the resistor. However, the surface temperature of the resistor will be lower than this value as the heat will dissipate through the metal leads and the component body.