DIY Trailer Wiring

[spongerich]

I've been searching for an OEM wiring harness and I've been striking out completely.

In digging about, the only real 'trick' part appears to be the TCM which is widely available. Beyond that, it looks like it wouldn't be all that hard to fab up a suitable connector to plug into the wiring.

Anyone know what the pinouts are on the factory connector?

The part I'm thinking about building is this one:

[TerminatorX5]

The WDS is very murky on this particular wiring - since the final plug is different from country to country, the diagrams do not list the US 7-pin plug, but rather a 13-pin Euro (or Aus) plug...

there are only 4 "active" wires on the US plug - one is ground, it is brown wire.

The other ones are left/right turn signals, reverse light and brakes.

The actual TCM supplies rear fogs to trailer, receives rear fogs from LCM, receives iBus line... none of those are present on the outside plug at all...

GE/BL - reverse lights
BL/RT - left turn signal
BL/GE - right turn signal
GR/RT - brakes
BR - ground

take these colors with grain of salt, as I do not guarantee their correctness - if you can test it, please do...

I don't remember all german abbreviations for the colors, but RT is ROT, red... BR is BRAUN, brown... BL is BLAU, blue...GR is GRUN, green...

[spongerich]

Thanks!

Does the WDS show any detail for accessories? I've found 61130412965 which is the connector for a 5 wire US light trailer, but it's hard to tell what the other end is. If it plugs into the harness, then there's my "kit" The pigtail and TCM should get me up and running. I'd love to have the complete factory setup, but I rarely haul more than 1000 lbs and I really can't wait for an OEM hitch to show up. If I can get something workable without hacking the wiring, It'll be reasonably easy to swap the hardware for a factory receiver if/when I manage to find one.

I'm guessing that the TCM tells the LCM to send power to the harness? Otherwise, it would seem rather pointless to have such a fancy/expensive module just to turn off the park sensors.

[LVP]

Some have said they get a "tow mode" on their dash. I don't get that with mine, but maybe it happens on the 2000-2003 models?

[mam4.6]

I've had the same question for awhile, since I occasionally pull an enclosed trailer around. What is supposed to happen when you connect the trailer plug to the vehicle harness? I have an OEM hitch with factory wiring. How are trailer brakes set, or does the TCM not do any braking? I've never seen the TOW MODE when hitched up.

[LVP]

It definitely doesn't do braking. Many have had to hack into other sources for the signals to do this. There is a wireless unit that can save a lot of hassle and uses the signals in the existing connector. Just plug and go (more or less). As for Tow Mode, I haven't seen it, nor did I really notice a difference when towing my sleds. It did disable the PDC though.

[spongerich]

I couldn't find anything specific to E53, but pages from the E70 WDS that I found seems to say that the TCM does a couple of important things.

• It both connects and isolates the trailer electrically, so a fault/short doesn't propagate from the trailer into the vehicle.

• Deactivates reverse light, rear fog light, PDC

• Sends and stores Check Control messages when trailer faults are detected.

Also has circuit breakers/overload prevention and detection.

The WDS doesn't say directly, but I would guess that the TCM also somehow announces ACHTUNG!! TRAILER_MODE_ACTIVATE on the CAN so that at least 42 other control modules ranging in function from transmission management to steering wheel heaters can now do 'The Right Thing' for towing.

Seems like once the TCM brings power back to the connector, the rest of the wiring is pretty straightforward. If 61130412965 connects directly to the vehicle then I'm a happy guy.. that's only about $40. Add a decent eBay TCM and it's boat shopping time.

As for the hardware,well, I'll live with an aftermarket receiver for now. Hell, I towed well over 1000 lbs pretty regularly with my little MK4 GTI VR6 for years and that receiver fastened primarily to the sheet metal on the passenger side of the luggage compartment and spare tire well. Not the most elegant design, but it worked. She towed up to and past the rated max effortlessly and at surprisingly high speed.

I can't believe that the X5 is going to be anything less than leaps and bounds better, even with a "2nd best" receiver. I think it'll easily handle all that my 5x7 landscaping trailer can throw at it.

[JCL]

The TCM is recognized by the vehicle, and can then advise the vehicle when a trailer is connected to the external trailer connection.

The TCM monitors the external socket and if it detects a trailer, then advises the vehicle, which disables the rear PDC, and activates another mode in the DSC algorithms that is called trailer stability control. I presume it also tells the LCM, because bulb monitoring then extends to trailer lights. If there is a rear camera (some models) it can change the view to facilitate connecting the trailer.

[mam4.6]

Wonderful information, thanks fellas...

[semcoinc]

Originally Posted by spongerich

I've been searching for an OEM wiring harness and I've been striking out completely.

In digging about, the only real 'trick' part appears to be the TCM which is widely available. Beyond that, it looks like it wouldn't be all that hard to fab up a suitable connector to plug into the wiring.

Anyone know what the pinouts are on the factory connector?

The part I'm thinking about building is this one:

I too was searching high and low to find this wiring harness to match up to an aftermarket hitch off ebay for ~$110!

Presto, I just found it, (I think), It is Westfalia PN 303169351001 or KWES061 01 or 303169300117

Westfalia 303169351001 KWES061 01 303169300117 Trailer Wiring Harness Kit New | eBay

Been looking nearly a year to find this and all the junkyards I've called didn't have one.







Good Luck Folks.

Mike