How does the connection of a Trailer/Caravan get sensed?

I was hesitant to mess around with the car's wiring, so instead built my own trailer LED adapter. It's a simple 12 pin male to 12 pin female, with a Narva resistor running from the left indicator pin on one plug to the ground on the other. The whole lot is then reinforced with liberal doses of electrical tape.
This means, when towing a trailer the Disco doesn't recognize, I add this adapter between the trailer cable and the car. Suddenly, no more reversing beeper, the gearbox adapts, and the little trailer indicator lights up. The resistor is out in the wind behind the car, meaning it should stay quite cool too.
If towing something the car does recognise, the adapter stays in the glovebox.

My car came pre-wired with an Anderson plug at the back, which is actually very handy - i used it just this weekend, as i have also wired my air compressor into an Anderson plug (ie, removed the battery clamps). Now, when running the compressor, the bonnet can stay shut, and I simply plug it in to the rear. The old clamps are also wired into an anderson plug, meaning I can still use the compressor direct from a battery if needed.

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Originally Posted by orville

I have wired up a Caravan, boat trailer and box trailer. The last was the box trailer. I did it because I have a long driveway and when reversing the noise was annoying. I did it in a few minutes. I had some cheap resistors from the net (ebay). I stripped the wire near the left and right indicators. I soldered a resistor green one end and white the other on right side and on the left (other side) yellow and white. This gives the resistance required. Not fancy but works.

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Does this mean the car thinks there is always a trailer attached? And at the risk of teaching to suck eggs, does the resistor you used have a heatsink? They get pretty hot, and you sure don't want a potential fire hazard in there!

Thanks to all respondents. I'm experimenting with load resistors to see what works on my D4 MY13. So far I've got the beeping to shut up by connecting 6 ohm 50W resistors across LH indicator, RH indicator, tail-light and reversing connection; a total of 4 resistors. The reversing camera still operates. According to the local Land Rover service agent (Cheltenham, Victoria) it is necessary to fit parallel resistors to the indicators and tail lights; a total of 3 resistors. I'll report back the eventual findings.

On a slightly different topic, the service agent said that for MY13 models, fitting the diode on the braking system is unnecessary (for blocking back-feed into the Body Control Module) because it is only on MY12 and earlier that need that. He also said that the MY16 had entirely different wiring to earlier models and the LED trailer lights were appropriately sensed. LR could sell and fit a black box to overcome LED light (lack of) sensing problem for about $300. The box would include a switch to select whether a trailer has LED or incandescent lamps. I'm starting to think that I should have said yes to that and save all the hassles I'm having...

For what it's worth, the resistors get extremely hot. I'm currently nursing a burnt hand with heat sink stripes across the palm!

In my earlier post today I was adding resistors directly to a bare plug (minus trailer). To make the testing more realistic, I connected up to the trailer (caravan) and added a 6 ohm 50 watt resistor to the LH indicator only. In reverse the beeping shut up, the camera stayed on and I got a little green trailer displayed on the instrument panel for left hand indicator operation. No other effects were noticeable and all lights on car and van worked correctly. Success!

I then fitted resistors to RH indicator and tail lights. Result = similar to before; no beeping, camera works and green trailer displayed on both LH and RH indicators. Success, except for very hot tail light resistor. Decision - put two resistors in metal box on the trailer, each in parallel with RH and LH indicators respectively. Nothing else. Unless someone can tell me a benefit for wasting more power through a tail light resistor, I will not fit it. Indicators are less frequently used and are intermittent in operation resulting in cooler resistors and I am not relying on only one resistor to tell the car that there is a trailer attached.

Incidentally, I found that the "reversing signal" Pin 2 on the D4 black socket is actually an "always-on auxiliary power" outlet - hence the burnt hand previously reported. Note that all the information above is what works (or not) with my specific D4 and JB Caravan. DISCLAIMER - It may be different with other MY13 Disco 4 and other trailers.

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Originally Posted by StewG

In my earlier post today I was adding resistors directly to a bare plug (minus trailer). To make the testing more realistic, I connected up to the trailer (caravan) and added a 6 ohm 50 watt resistor to the LH indicator only. In reverse the beeping shut up, the camera stayed on and I got a little green trailer displayed on the instrument panel for left hand indicator operation. No other effects were noticeable and all lights on car and van worked correctly. Success!

I then fitted resistors to RH indicator and tail lights. Result = similar to before; no beeping, camera works and green trailer displayed on both LH and RH indicators. Success, except for very hot tail light resistor. Decision - put two resistors in metal box on the trailer, each in parallel with RH and LH indicators respectively. Nothing else. Unless someone can tell me a benefit for wasting more power through a tail light resistor, I will not fit it. Indicators are less frequently used and are intermittent in operation resulting in cooler resistors and I am not relying on only one resistor to tell the car that there is a trailer attached.

Incidentally, I found that the "reversing signal" Pin 2 on the D4 black socket is actually an "always-on auxiliary power" outlet - hence the burnt hand previously reported. Note that all the information above is what works (or not) with my specific D4 and JB Caravan. DISCLAIMER - It may be different with other MY13 Disco 4 and other trailers.

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I have done pretty much the same as you have described. I bought a blister pack of 2 x Narva 21W load resistors as below.

https://www.aulro.com/afvb/images/im...017/04/652.jpg

I mounted these together on the side and right at the front of the draw bar on my trailer, and wired directly into the trailer plug - one to each indicator feed and both returning to a common earth.
I have not found that they get hot.
This is all that is needed (in fact, you only really need to use one resistor, wired up to only one indicator circuit). There is no benefit to be gained in adding another resistor to your tail light circuit - the D4 uses the indicator circuit/s to sense when a trailer is connected.

Mine hardwired into the camper trailer. As I had two, I connected one to each indictor though as mentioned, only one is required.


https://www.aulro.com/afvb/images/im...017/04/651.jpg

Beware that the resistors MAY stop the ECU going to sleep and causing battery drain if left parked for significant time.Our ESTEEMED spannerman/auto elec at work put them on our Merc prime mover after fitting LED lights to correct the indicator speed.You can here a click noise when its parked,the ECU looking for contacts.If its parked for longer than a week it needs a jump start.This may or may not be the case with a Landy ECU.The resistors do get hot!!!!
Andrew

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Originally Posted by LandyAndy

Beware that the resistors MAY stop the ECU going to sleep and causing battery drain if left parked for significant time.Our ESTEEMED spannerman/auto elec at work put them on our Merc prime mover after fitting LED lights to correct the indicator speed.You can here a click noise when its parked,the ECU looking for contacts.If its parked for longer than a week it needs a jump start.This may or may not be the case with a Landy ECU.The resistors do get hot!!!!
Andrew

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I can't see how the Disco 4 could suffer the same problem because the purpose of the resistor(s) is to make the LED-lighted trailer look the same as an older incandescent-lighted trailer. The resistors are (ideally) mounted on the trailer and having any trailer connected (say for a week) should not prevent the D4 from shutting down properly and conserving power. However, your warning is heeded and I'll keep a watch on any aberrant D4 behaviour.

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Originally Posted by StewG

I can't see how the Disco 4 could suffer the same problem because the purpose of the resistor(s) is to make the LED-lighted trailer look the same as an older incandescent-lighted trailer. The resistors are (ideally) mounted on the trailer and having any trailer connected (say for a week) should not prevent the D4 from shutting down properly and conserving power. However, your warning is heeded and I'll keep a watch on any aberrant D4 behaviour.

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Was only refering to if the trailer was left connected for a long period,say whilst touring.I would be disconnecting,just in case.
Andrew

My D3 keeps sending pulse's down to the trailer when shut down. I originally had my LED box wired in with a couple of change over relays that had the trigger wire supply from a ign on 12v. When I turned the engine off my 12v trigger shut down and the pulsing at the trailer LEDS started. Cant recall if it stopped when the car was locked, but from memory if kept going for hours if I left the car unlocked. I ditched the relays after that