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Discussion Starter · #1 ·
I figured I would post this as a problem-solver for others. I've seen others post a hitch here, but I didn't see any wiring on that one ...
Problem #1 is that the Jetta V's taillights are the same filament as the turn/brake lamps but at reduced brightness. Problem #2 is the diagnostic system for burned out bulbs. Here is how I got around this without using any signal converters and while still keeping the wiring as compatible as possible.
1. On the trailer, install TWO brake/taillamp housings on each side.
2. Install LED replacements for the 1157 bulbs in all of the trailer taillamp housings. Around here, you can get those at Canadian Tire in the performance parts section. If there are any running lamps or license plate lamps, those must also be LED. All of the incandescent bulbs have to be gone, and if you go crazy with the running lamps and use too many of them (I have none) there's no guarantee that the current limit won't be exceeded, and I have no idea what the current limit is.
3. Wire one taillamp housing on each side of the trailer so that the taillamp wire (generally "brown") is connected to the taillamp filament, and the brake lamp filament on that bulb is not connected to anything at all.
4. Wire the other taillamp housing on each side of the trailer so that BOTH wires for the housing are fed from the stop/turn wire for that side of the trailer.
5. On the vehicle side, connect the feed for "brown" (taillamp) from the wire that feeds the side LED bulb in the taillight housing. This is a continuous 12-volt source whenever the headlights are on. (See an installation note below)
6. Connect the ground and stop/turn wires of the vehicle-side trailer harness as usual.
7. It is a good idea to make up a little tag and attach it to the vehicle-side plug, that says "LED BULBS ONLY" or something to that effect, as a reminder to not connect this system to a standard trailer.
The outcome:
When plugged into the MkV, both trailer taillights will be on as taillights on each side. One taillight signal is being fed from continuous +12 and the other has its stop/turn signal being fed from reduced voltage, but they appear identical. The stop/turn functions operate using one trailer lamp on each side, same as on the car. It works FINE.
Hints and tips:
If you have the harness unplugged from the taillight while trying to figure out which wire does what (unfortunately I did not write it down - I do recall that the orangeish wire is ground), you are going to find out that you only have a few seconds before the vehicle's bulb-diagnosis system thinks the bulbs are burned out, and shuts off the power. No problem, just turn off the switch and turn it on again.
The easiest way to make the connections is to take the taillight housings out of the car and make the connections from the back. It is a hassle, there is not much wire. I scraped the insulation off the wires that I needed to tap into without actually snipping the copper, and soldered my wires to that, taped everything up and zip-tied the trailer harness in place so it's less likely for vibration to pull anything apart.
To hide the wire that goes from the left to the right side of the car, remove the trim panel that surrounds the latch hook (it's easy), and then you can work the wire between the inner trim panels and the sheet metal by feel on each side.
If you have played around with European turn signals separate from the brake lamp functions, you are on your own, this solution will not work.
My taillamp circuit only draws about 50 milliamps. The running lamp on each side (inside the stock taillight housing) draws about 40 milliamps. If you have running lamps on the trailer that draw more current than that and it causes trouble, you could try unplugging the running lamp on the side that you made the wiring connection. If your trailer tail/running lamp circuit draws way more current than that, I wouldn't wire it to the taillight supply circuit - run the wire to the front of the car and feed it from an incandescent bulb, such as the front fender side marker lamps. I did not have to do anything of the sort - mine works fine the way it is.
 

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Hey I see you're somewhat local to me. I got a price today on a hitch, $190CDN and 60 for the wiring. Did you pay about that?
I'll analyze your post in a few, when I have time and my boss may not walk by and see my screen...
 

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Discussion Starter · #3 ·
The hitch that I bought was about C$180 from Hitch City in Mississauga, and I installed it myself - 3 bolts plus you have to temporarily lower the exhaust (easy) and you have to make a couple of clearance notches in the bumper cover and the plastic tray underneath the right side. It took me about an hour.
I bought a generic car and trailer wiring harness with about 25 feet of cable from Canadian Tire for $16, and did the wiring myself. I did not want to leave this up to someone else.
For the trailer side, the LED bulbs were about $11 each (4 required) from Canadian Tire, and the taillight housings are about $10 each (2 required) depending on what style you want.
The only thing I still need to do is buy and install a new hitch ball, and then I'm all set for towing.
 

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Discussion Starter · #4 ·
One other thing - compatibility. With the trailer wired in this fashion, it will work fine when connected to a different vehicle. The brake lamp will not be illuminated at reduced voltage as a tail lamp, but the one that's wired as a tail lamp will still work fine (and that's why I installed it).
If you have to hook your wired-as-such MkV up to someone else's trailer, swap out the 1157 bulbs for the LED replacements and disconnect any unnecessary running lamps, and it should be good enough get by. The taillights will be too bright because they'll have both the regular taillight circuit on and the part-voltage brake lamp circuit, but the brake lamp will still be brighter than the tail lamp, so it'll do. If you really want to get fancy, you could install on the vehicle side a switch or bullet connector in the brown wire to disconnect the normal tail light circuit to get around this (and use the brake lamps as reduced-voltage tail lamps, same as the car). You'll lose any running lamps that are connected to this circuit also, if you do this. Again, it's something to get by.
 

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Re: Trailer wiring without a signal converter (GoFaster)

I plan to buy a MKV Golf/GTI, and to tow a trailer with it. Therefore, the initial post in this thread is of interest to me.
However, I don't understand the problem created by the Jetta lights. Maybe the workings inside the light housing are different from other cars, but doesn't the Jetta have the same wiring coming into the housing as any other car? So why can't you wire in the normal way, and tow a trailer with the normal trailer lights?
I must be missing something!
 

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Discussion Starter · #11 ·
Re: Trailer wiring without a signal converter (sbwriter)

Quote, originally posted by sbwriter »
I plan to buy a MKV Golf/GTI, and to tow a trailer with it. Therefore, the initial post in this thread is of interest to me.
However, I don't understand the problem created by the Jetta lights. Maybe the workings inside the light housing are different from other cars, but doesn't the Jetta have the same wiring coming into the housing as any other car? So why can't you wire in the normal way, and tow a trailer with the normal trailer lights?
I must be missing something!

I already explained the problem in the first post, but I'll try to do it again in different words and I'll start with how a normal taillight system works.
With a normal car that has combined stop/tail/turn lamps in a single bulb (red turn signals that also work as brake lamps - NOT amber turn signals, NOT a completely separate bulb from the brake lamp), this is done using a dual-filament bulb. The most common dual-filament bulb is the good old 1157 bulb, same one that most normal trailers use.
With this type of bulb, there are 3 connections to the taillight housing: "Ground", "stop/turn", and "tail". The vehicle puts 12 volts between "tail" and "ground" to make the dim taillight filament come on. The vehicle puts 12 volts between "stop/turn" and "ground" to make the bright stop/turn filament come on. With me so far?
At first glance from OUTSIDE, the A5 Jetta looks like this is the way it works. But it doesn't.
The outer bulb - the one that flashes turn signal and is also a taillight - is a SINGLE FILAMENT BULB. It only has a "bright" filament in it. There is no separate "tail" and "brake" filaments. The car gets a taillight function out of that bulb by sending it a reduced voltage. It sends 3 or 4 volts to make that filament come on as a taillight. It sends 12 volts to make it come on as a turn signal or brake lamp.
Set aside the bulb-diagnostics system for a moment - I'll get to that below; for now pretend it doesn't exist (although in reality it will screw you up if you're not careful). If you wired the system up "normal", and you hook up your trailer, and switch on turn signals or brakes independently without the headlights on, it will look like all is well. But when you switch on the headlights, the taillights in the trailer will be getting the +12 signal from the brown wire to the normal "tail" filament, PLUS the low-voltage signal from the car's stop/turn-as-taillight signal being fed to the trailer's "stop/turn" filament, resulting in the taillight being too bright (because the taillight filament is on "full" and the normally-brighter brake light filament is on "partly". It will "sorta" work, but the taillights will be too bright.
But in reaility it will NOT work. The A5 Jetta has a bulb-diagnostics system in it. It measures the current going to the incandescent bulbs. If the current is outside an allowable range, it gives a "burned bulb" warning lamp on the instrument panel, and it switches off the function in question as protection against over-current damage.
My solution gets around this by using LED bulbs that draw so little current that the car won't even notice that they are there, and by separating the "tail" and "stop/turn" taillight housings entirely. The "stop/turn" taillight will be on at reduced voltage when the headlights are on, but this is not a problem, because it will look exactly like the reduced-voltage single filament bulb on the car and act as a combined stop/tail/turn bulb.
Do you get it now? If not, what am I missing?
By the way, I don't think we know how the Golf / GTI taillights are going to work on the North American models yet. It might be the same as the Jetta, or it might be using separate turn and brake bulbs, in which case you can hopefully use a normal signal converter ... but if the taillights are single-filament bulbs using the same voltage trickery as the Jetta, you'll have the same hurdle to cross.
 

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Discussion Starter · #12 ·
One other thing - a design variation, if you will. If your trailer has one taillight per side and does not have any running lamps, you can make it work by hooking up the ground, left stop/turn/tail, and right stop/turn/tail circuits as usual on the car, and leave the brown wire (taillight) completely unconnected on the car side. The trailer lights will look OK because the stop light element will be used at reduced voltage as the taillight, just like on the car. BUT ... this way there's no separate circuit for running lamps or license plate lamps. If you have running lamps or license plate lamps, they won't work. Remember, LED bulbs only in the trailer, to slide under the bulb-diagnostics system. I'd suggest hooking up the brown wire to the side-marker LED, as I originally posted, but through a switch or bullet connector, then you can have it work either way.
 

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Re: (GoFaster)

Quote, originally posted by GoFaster »
One other thing - a design variation, if you will. If your trailer has one taillight per side and does not have any running lamps, you can make it work by hooking up the ground, left stop/turn/tail, and right stop/turn/tail circuits as usual on the car, and leave the brown wire (taillight) completely unconnected on the car side. The trailer lights will look OK because the stop light element will be used at reduced voltage as the taillight, just like on the car. BUT ... this way there's no separate circuit for running lamps or license plate lamps. If you have running lamps or license plate lamps, they won't work. Remember, LED bulbs only in the trailer, to slide under the bulb-diagnostics system. I'd suggest hooking up the brown wire to the side-marker LED, as I originally posted, but through a switch or bullet connector, then you can have it work either way.

In Europe, VW advertises the Golf as having a fairly high towing capacity. So VW must have a device that gets around this problem.
 

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Re: (GoFaster)

Quote, originally posted by GoFaster »
One other thing - a design variation, if you will. If your trailer has one taillight per side and does not have any running lamps, you can make it work by hooking up the ground, left stop/turn/tail, and right stop/turn/tail circuits as usual on the car, and leave the brown wire (taillight) completely unconnected on the car side. The trailer lights will look OK because the stop light element will be used at reduced voltage as the taillight, just like on the car. BUT ... this way there's no separate circuit for running lamps or license plate lamps. If you have running lamps or license plate lamps, they won't work. Remember, LED bulbs only in the trailer, to slide under the bulb-diagnostics system. I'd suggest hooking up the brown wire to the side-marker LED, as I originally posted, but through a switch or bullet connector, then you can have it work either way.


I guess I am unclear about the wiring of the Jetta. Does it have two wires in addition to ground going to the taillamps, or only one wire in addition to ground? I think it must be the latter? If so, there is something up front on the vehicle that varies the voltage that is fed into a single wire?
Thus, I guess wiring that single Jetta wire to all the trailer lights would make the taillamps work fine, but the running (side) lights would operate the same as the taillamps rather than having a constant brightness level? So the problem is that if you wired the usual way, the running lights would brighten every time you used the turn signal or the brakes? I suppose that might be illegal, but otherwise is there a disadvantage?
 
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