Schlechtes Abblendlicht, Spannungsabfall --> RELAIS !!! | Posts 16+

Georg_G wrote:

Hmm, yes, that's silly. I was kind of thinking about using a diode. I'll try experimenting with that.

Well...

According to my knowledge, the control unit measures the cold resistance of all light bulbs when the ignition is turned on. If the relay contacts are open, the error lamp should always illuminate.

I also experienced something similar with my car's headlight system. It seems that the voltage drop when turning on the lights is being measured. The headlight monitoring isn't triggered during ignition or after the engine starts, but only when the low beam headlights are turned on. If the voltage drops below a certain threshold, you get the "headlight defective" message. Unfortunately, with some types of bulbs, the alarm can trigger even when everything is working correctly. This seems to be more common with long-life bulbs from GE or Philips EcoVision.

matthiasTDI96 wrote:

(...)if you already have 4mm² there, just use that.

It might not be strictly necessary. The specific resistance of copper is 0.0178 ohms per meter and square millimeter.

"That is, with a 2.5mm² cable and a length of 2 meters (which would be the approximate distance between the battery and the furthest lamp), we get: 0.0178 * 2 / 2.5 = 0.014 Ohms. With 5A per bulb, this results in a voltage drop of 0.014 * 5 = 0.07V across a 2.5mm² cable. A higher voltage drop will occur at the fuse . Therefore, you definitely don't need to choose anything thicker than 2.5mm²..."

As the previous speaker mentioned, the old lamp control devices directly measured the current flow (or the resulting voltage drop across a small resistor). With the relay solution, you won't draw significant current through the check module. (The current for the relay coils is much lower.) Therefore, the lamp failure detection will report a defect. You can either live with it, or you can bypass the low beam on and off connections at the check module and bridge the two cables together. This way, the module thinks the low beam is off and won't draw any current (you might need to add a small resistor to the output connected to ground).

Or you can replace the sense resistor in the TCU with a higher-value one, so that the current to the relay coil is sufficient.

Greetings.
Guste

Third possibility - if technically possible - would be the software-based deactivation of the lamp monitoring system. I had this done on my car because of LED taillights and side turn signals used for brake lights, reverse lights, and direction indicators. Except with the , it was practically but not really a problem. The low beam headlights are still monitored, but they could also be deactivated if needed. Could this perhaps work on a Volkswagen Skoda as well?

The result of the relay circuit is described at the top.

Is it already installed? Can you see the difference?