Thread: Trailer light blinking

Originally Posted by fourteen8

I was picking up my friend and his family. I unfolded the rear seats and realize after that the trailer light blinking when us the indicator either left or right.

Did I hit something by mistake or sensor fault or something?

Anyone can tell me how to switch it off?

Mine is D4 2014 Sdv6


Thanks

Assuming you don’t have a trailer attached, look for a switch in the rear cargo area. Someone has probably installed a trailer resistor setup to tell the car there’s a trailer attached for when the trailer has LED lights (car can’t detect the trailer in that case). You may have accidentally flicked the switch and enabled it to think a trailer is attached.

Originally Posted by DiscoJeffster

Assuming you don’t have a trailer attached, look for a switch in the rear cargo area. Someone has probably installed a trailer resistor setup to tell the car there’s a trailer attached for when the trailer has LED lights (car can’t detect the trailer in that case). You may have accidentally flicked the switch and enabled it to think a trailer is attached.

Yes no trailer attached.

Ok. I will check tomorrow morning for the switch. How come the car cannot detect if there is trailer attached? My D2 can detect if there is a trailer attached

Thanks

Originally Posted by fourteen8

Yes no trailer attached.

Ok. I will check tomorrow morning for the switch. How come the car cannot detect if there is trailer attached? My D2 can detect if there is a trailer attached

Thanks

The car can't detect a trailer with new low resistance LED lights, hence the extra switchable resistor.

Originally Posted by RANDLOVER

The car can't detect a trailer with new low resistance LED lights, hence the extra switchable resistor.

Whilst seemingly accurate, to try and elaborate a bit : LED's are a "high resistance" compared to a bulb. The cars are designed to detect a trailer by the current consumption of the indicators.

On the D3 it tests frequently (many times per second). On the D4 it tests when the drivers door is opened and closed. Compared to a 21W indicator bulb, a LED draws bugger all current, so an additional load resistor is required to make the car detect a trailer. On the D4 I believe it's only required on one Indicator. On the D3 it's a bit more complex.

Some people install the load resistor in the car and connect it to a switch that they turn on when they have a LED trailer attached. If the trailer isn't attached and the switch is turned on, the car still thinks there is one present, causing your issue.

Your D2 detects the trailer in a similar but more agricultural manner. Put a LED only trailer on and it won't know unless you have the same load resistors. On the D3/D4 it also disables the rear park sensors and changes the shift points (if you have a slushbox). I can't comment on the smarts the D2 may or may not contain because we've only owned a D1 and D3 and both had/have 3 pedals.

Originally Posted by BradC

Whilst seemingly accurate, to try and elaborate a bit : LED's are a "high resistance" compared to a bulb. The cars are designed to detect a trailer by the current consumption of the indicators.

On the D3 it tests frequently (many times per second). On the D4 it tests when the drivers door is opened and closed. Compared to a 21W indicator bulb, a LED draws bugger all current, so an additional load resistor is required to make the car detect a trailer. On the D4 I believe it's only required on one Indicator. On the D3 it's a bit more complex.

Some people install the load resistor in the car and connect it to a switch that they turn on when they have a LED trailer attached. If the trailer isn't attached and the switch is turned on, the car still thinks there is one present, causing your issue.

Your D2 detects the trailer in a similar but more agricultural manner. Put a LED only trailer on and it won't know unless you have the same load resistors. On the D3/D4 it also disables the rear park sensors and changes the shift points (if you have a slushbox). I can't comment on the smarts the D2 may or may not contain because we've only owned a D1 and D3 and both had/have 3 pedals.

I think you'll find that LED's are very low resistance, not high. This is why you may need to add a load resister in the circuit to enable the trailer detect feature. With a D4, the "detection" signal is sent down the indicator circuit at door opening and also when either indicator is used, (as I'm led to believe). For my van, which uses a 12 pin flat plug, I made up a small "dongle" that plugs into the unused 12N round socket. It's a 12N plug, a bit of polypipe and 2 load resisters wired up to the 2 indicator circuits. Just plug itin when towing van. Works a treat.

Originally Posted by Briar

I think you'll find that LED's are very low resistance, not high. This is why you may need to add a load resister in the circuit to enable the trailer detect feature. With a D4, the "detection" signal is sent down the indicator circuit at door opening and also when either indicator is used, (as I'm led to believe). For my van, which uses a 12 pin flat plug, I made up a small "dongle" that plugs into the unused 12N round socket. It's a 12N plug, a bit of polypipe and 2 load resisters wired up to the 2 indicator circuits. Just plug itin when towing van. Works a treat.

No he won’t. He is completely correct. They are high resistance. This is why LED use less current than an incandescent bulb for the same lumen output, and why you apply a load resistor to simulate an incandescent load.
Try it for yourself. Go and get an led and compare it to a filament globe on your multimeter.

Originally Posted by DiscoJeffster

No he won’t. He is completely correct. They are high resistance. This is why LED use less current than an incandescent bulb for the same lumen output, and why you apply a load resistor to simulate an incandescent load.
Try it for yourself. Go and get an led and compare it to a filament globe on your multimeter.

Mea Culpa. You are correct. My apologies to Brad. Learn something everyday.

Originally Posted by Briar

Mea Culpa. You are correct. My apologies to Brad. Learn something everyday.

Actually, you are/were correct if we take the definition of a LED as just the diode. They have a characteristic voltage drop and will happily take all the current required to pull the supply down to that voltage until the magic smoke escapes. They are used in an assembly with a current limiting device (a regulator or resistor commonly) which reduces the current to that which the LED won’t pop. As they are not actually an ohmic device, the terms low and high resistance don’t really apply, but if you think of it from the perspective as on a constant voltage, the lower the load resistance the higher the power draw. A led assembly draws considerably less than a 21W bulb, so the bulb presents the equivalent of a lower load resistance.

Regardless of the electrons, the car detects the trailer indicator by measuring the current the bulb draws. Same way it knows there is a bulb out.