B4 Trailer Wiring (4-FLAT) Made Easy

Dave, this is AMAZING. Thank you. Saved me the trouble of hunting down the right wires, and avoided the need to parallel split in, which I hate doing.

A quick note: I am constantly a last-minute upgrade and repair guy so I did not have time to order the VW connectors. Instead I used to three of the round barrel-type snap connectors in the 22-14 gauge. A little pressure from some pliers crushed them flat and provided a very strong connection to the blades in the tail light housing. I would have used the blade-type connectors but they were too wide. The round ones, when crushed, fit perfectly even between the three blades on the bottom connector.

I didn't ground on the tail light bolt because there was too much paint. Instead I grounded nearby on a clip that attached some plastic interior panel to the sheetmetal body.

Thanks again for the tip. Easiest upgrade ever.

Thanks for posting!

Tony

Does this apply to the A3 Jetta as well?

You tell us. =)

Access to the A3's driver's side tail light assembly is, I think, through a panel in the trunk. Pretty easy to get too, it's how you change your bulbs. If your assembly has the same empty sockets as the B4 odds are it has the exact same pinouts for trailer hookups.

VWannabe said:

Does this apply to the A3 Jetta as well?

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That's a possibility, but knowing how VW does things, a very very distant possibility. I don't like to assume... the outcome is all too predictable.

In fact, speaking of which I may have a connection error in my first post...I have no marker lights at night. Brake, parking and turn signals work fine, just no marker from the shared brake lights. First with one trailer I used 90% of the time during daylight and no longer own, and now the same crap with rented u-haul trailers. Something is going on herethat I'll have to drop everything to figure out.

Slam, following these instructions do you have functioning marker lights at night? Not separate markers mind you, ones that run through a shared brake light (the bulb with two filaments in it???

Hey Dave. If I recall correctly my trailer only has 3 wires (L, R, and stop). In order to get functioning running lights at night I would need to buy a "3 to 4" wiring harness adaptor and try to get power from one of the two spots you mention. I haven't bothered; I only needs my motorcycle rarely and during the day.

slam said:

Hey Dave. If I recall correctly my trailer only has 3 wires (L, R, and stop). In order to get functioning running lights at night I would need to buy a "3 to 4" wiring harness adaptor and try to get power from one of the two spots you mention. I haven't bothered; I only needs my motorcycle rarely and during the day.

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I have seen these and the trailer is grounded through the ball, which is very inefficient and sporadic. Better to run a separate ground wire back to the car body.

Sorry, I meant only three hot wires. The fourth wire is indeed the ground wire.

But there's no separate wire for running lights, so if you want both brake lights and running lights you need an adaptor box.

A year later I've stumbled across the photos I took.

Here's the barrel-type connector crushed to fit the blades:


Here's the wiring attached to the driver's side taillight block:


Edit: can't see to embed Flickr images. Image set is here

Good write up but my question is where did you get the hitch and how difficult was it to install? I need a hitch to carry my bikes and a hitch-haul for extra luggage.

I think I got my hitch from a U-haul rental yard. It was expensive, $200-$300 or so.

Update: I missed the second part of the question. No, it was not hard to install. The rear bumper comes off with four bolts. I test fitted the hitch by holding it in place with clamps. Used a fat drill bit to drill holes through the spare wheel bay and the rear box. All easy.

My biggest issues was that the internal cross-bar that runs across the inside rear was missing. The front of the hitch mounts in the spare wheel with bolts and fat washers, but bolts on the rear of the hitch is supposed to pass through the hitch, then the sheet metal of the body, and then through a cross-bar that runs essentially the full width of the rear of the car. This part was missing. I hunted around some metal yards to find an appropriate bar. For extra credit I painted with with some Dupli-color that matches my car (Candy White, I think). Looks great.

Putting the rear bumper back on is also easy, but I have to remember to work the black part of the bottom of it over the hitch. It is possible to re-install the bumper covering the hitch.

Resurrecting this great thread with some info about how i did it, and a question.

I was in a rush to install the hitch and get the wiring ready, so i could not wait to order the connectors.

By studying the original photo above, i simply took three narrow female spade connectors, added a length of wire to each, wrapped a few thicknesses of electrical tape around each, and connected them to the terminals as in the photo - one to the single-pin port, one to the upper pin of the two-pin port, and one to the middle pin of the three-pin port. To each wire i spiced the appropriate color-coded wire of the flat 4-pin trailer connector, as explained in the original post. These spade connectors will not hold on quite as securely as the click-in housing, but they will be snug enough. I will simply check it frequently during each extended trip.

So here's my question.

After i made the above attachments and before i connected it to trailer wiring, i tested the lights. They did not work right. The blinker lights worked only partially, a turn signal indicator faintly lit on the dash when i pressed on the brakes, etc. Something was already not right.
Then i connected it to the trailer lights, and only some functions worked - one blinker, one brake light, no tail lights, as i recall

I thought i had missed an important piece of the puzzle.

Then i drove the car without changing anything (except disconnecting the trailer and its harness) with the car's 4-pin flat connector still attached as described above , and after a short while ( maybe immediately?) all the car's lights and blinkers worked perfectly. And then i hooked it up to a trailer harness, and the trailer lights also all worked perfectly !!

So it worked. Just came back from 300 miles of towing, and lights were fine.

But .. why was it all messed up at first ? It is as if it needed to somehow get broken in, by driving a bit. Or needed some time for the computer to reset, or something like that.

Any ideas??

Thanks,
Barry

Not sure why, I've had something close to that happen to me as well. To be quite honest, The original post leaves something to be desired, like illumination at night. The lights all should work fine, except the running lights. I'll be installing a 5-4 adaptor and running power directly back from the battery and that's what I would suggest as well. Your trailer should have some kind of lights on when the tow vehicle's headlights are on. With the original post, that is not the case, hence the need to run power from *somewhere*. I would prefer the battery, but we'll se how that works out to power the adaptor.

I hooked mine up in the same manner, just using spade terminals and plugged them in to the appropriate places and have had no issues in many years, not even at the beginning. It would be nice to have snap in ones, but mine have not gone anywhere. If memory serves, I bent the terminals little by hand to make a more secure connection. I did my own wire tracing with a test light and connected them appropriately, so I'm not sure if I have them connected in exactly the same spots, but I did not have to run a separate power from the front.

barryfromvt said:

Resurrecting this great thread with some info about how i did it, and a question. ... Any ideas??

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Sounds like a crossed connection or short to me. Did you use insulated barrel connections or are they bare? if bare, they might have been touching a neighboring pin, causing a short or cross-connection. If one of your connectors had a stray wire outside the barrel and if this wire was touching another pin, this too would cause the issue.

Similarly if the makeshift harness had bare wires touching metal or paint somewhere, you could have been shorting to ground. This would explain the dim lights and poor performance; most of the juice is going straight to ground, only a little is getting to the bulbs.

While surfing German ebay looking at Passat 35i parts I saw several OEM looking trailer hitch farmes, some with B4V rear bumper covers still attached. Many had the wiring harnesses also still present.

Were these OEM?
Did VW offer OEM hitches in the US for B4 / B4V?

I am only familiar with after market hitches and had installed such hitch with no wiring on my B4 to use with a bike rack.

So I’ve decided to scrap my first post and completely revise and update it with what I’ve done recently.

I have found that the BEST way to run trailer lights is by running power from the battery back to the left taillight assembly and using a 5-4 trailer wiring adaptor. Experience has taught me that stealing power form the taillights just blows fuses, causes the car’s lights to function wonky, not to mention the trailer lights as well...if at all. Even the car’s left taillight will appear noticeably dimmer than the one on the right when it does work.

I bought an adaptor from Ebay, but they are available at most trailer stores and even rental truck places I see will carry them. Some will have a lead for power, others will not. We’ll be using the ones WITH power.

OK, so...first order of business is to pull power from the battery, fuse it and get it out back. There may be locations on the fuse block where you can get fused power, but I do not like monkeying with the fuse block at all, besides plug and play replacement of fuses/relays. It’s really not the area I’m good at so I’m not going to suggest it.

I found it easier to run the wire front to back rather than the other way, so starting at the battery, get a 12 ga. wire fuse, splice it onto about 18.5-19 feet of red 12 ga. wire and follow this route. MAKE SURE to leave this DISCONNECTED from the battery while you're running the wire. (I know you know that, but I feel obligated to say it)


It was nice of VW to put that little notch there, like they knew I was coming

Up and around the strut tower


remove the coolant overflow vessel for the next move:


Pierce this and get the wire through, again...thanks VW. This will put you about 1" outboard of the clutch pedal, the insulation behind this spot is already clearanced for wires/whatnot.


Feel free to take liberties with your routing if you find something works better, this way did good for me.


Get the wire behind the left side kick panel and into the door sill/wire chase. I found you don't even have to remove this panel, just remove the one phillips screw, slide it in against the carpet and it'll pop right in.



Next, carefully pry off the entire door sill panel....removing the seat is NOT necessary.


You'll find these wireway stantions under the door sill, they come in handy- there is a spot on each one that is perfect for holding 12ga. wire.


**Continued on next post***

This is another easy slide-under deal


Next, under here you'll find a pocket where the harness for all the taillights goes trough, you can push the wire through....


....then snake it along the trunk floor sides right on top of the existing harness


Next, remove this fastener and snake the wire around and behind the carpet, up to the light assembly.


Feel free to mount your converter anywhere you like, even next to the hitch outside. I chose this location only because I like connections like this to be out of the weather and soot. Go ahead and run your ground to the taillight mounting stud/nut and start splicing in the individual connections and lastly connect the power.


I'm not going to specify wire colors, because I've found not all manufacturers follow that convention. so, whichever 5-4 converter you end up using, follow the labels they provide for what each wire does, and splice in your VW terminal ends and snap them into the corresponding size/locations here:


As far as the connectors that you'll need are concerned, here's a short list of what you’ll need to interface to the tail light assembly:

(1) 191-972-702 A [2-lead brown connector] *If you want a white connector, change suffix to "E", for black like the rest, drop suffix off entirely.
(1) 191-972-701-D [1-lead blue connector]
(1) 1J0-972-924 [3-lead black connector]
(2) 000-979-133-E [yellow repair wire with slide-in factory crimp-ons]

You'll have to cut both of the yellow wires to produce 4 ends to work with (they come with slide-in terminals on both ends) Cut the length to your taste, but remember to leave yourself enough to work with a butt splice connector or room to solder/shrink wrap whichever you prefer. If the wire ends do not slide deeply into the connectors and seat with a smart little 'click' don't force it! it's in wrong- pull it out and rotate 90 and try again. That's it you've got it. At $8.40 (2011 price) for each wire from the dealer, you'll want to be careful.

It should be noted as well, If you find yourself sourcing the connectors from a donor car's wiring, do not confuse the 1-and 2-lead connector with their larger cousins, 191-972-751 and 191-972-752 respectively. Check against the part numbers referenced above to make sure you've got the correct sized connectors as they are very easy to mistake since they are almost visually identical and also in the case of the 2-lead connector you will find it will not fit the tail light.

Q: So where do I find these connectors if i’m at a salvage yard?
A: In the case of the 3 pin connector, if you’ve found a B4- the button that pops the trunk remotely will have one, under the e-brake console. for the 1 and 2 pin connectors, those can be found hiding behind the fuse block. Also, this is a good time to collect the special crimped wire ends to avoid buying them at the dealership. (I’ve used a used one in the picure above, it’s the brown lead going to the 3 pin connector)


OK, so that's it...we're done....and I mean it this time. Go put a 10amp fuse in by the battery and get towing

And just like that, the pictures are back. PB came to it's senses...only took them well over a year.

TDIDaveNH said:

And just like that, the pictures are back. PB came to it's senses...only took them well over a year.

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Glad to see the pics are back, my conversion thread photos are all back and working too!

Steve