2009 Jetta TDI trailer wiring

How does one wire up a trailer light harness for a 2009 Jetta? The wiring
is really weird, each bulb operates like a dual filiment bulb but in
actuallity its a single filament bulb. They use pulse width modulation to
vary bulb brightness and to save wiring, brilliant but at the same time god
help you when you need to diagnose bulb problems. So how does one wire up
to this system? Can i just splice in and will the computer doing the pwm
automatically compensate?

On Sat, 4 Apr 2009 19:52:16 -0700, "Fred" >
wrote:

>How does one wire up a trailer light harness for a 2009 Jetta? The wiring
>is really weird, each bulb operates like a dual filiment bulb but in
>actuallity its a single filament bulb. They use pulse width modulation to
>vary bulb brightness and to save wiring, brilliant but at the same time god
>help you when you need to diagnose bulb problems. So how does one wire up
>to this system? Can i just splice in and will the computer doing the pwm
>automatically compensate?
>

IIRC, there is an adapter you can get to do these. Be very, very
careful about splicing into the existing harness, the computer is not
designed to drive the trailer directly.

On Apr 5, 8:07*am, PeterD > wrote:
> On Sat, 4 Apr 2009 19:52:16 -0700, "Fred" >
> wrote:
>
> >How does one wire up a trailer light harness for a 2009 Jetta? *The wiring
> >is really weird, each bulb operates like a dual filiment bulb but in
> >actuallity its a single filament bulb. *They use pulse width modulation to
> >vary bulb brightness and to save wiring, brilliant but at the same time god
> >help you when you need to diagnose bulb problems. *So how does one wire up
> >to this system? *Can i just splice in and will the computer doing the pwm
> >automatically compensate?
>
> IIRC, there is an adapter you can get to do these. Be very, very
> careful about splicing into the existing harness, the computer is not
> designed to drive the trailer directly.

Further to this, putting even the lightest of trailers on a Jetta -
almost any VW or similarly sized *small* car - is just not a good
idea.

Peter Wieck
Melrose Park, PA

wrote:
> On Apr 5, 8:07 am, PeterD > wrote:
>
> Further to this, putting even the lightest of trailers on a Jetta -
> almost any VW or similarly sized *small* car - is just not a good
> idea.
>
> Peter Wieck
> Melrose Park, PA

Codswallop! I towed a tent trailer for thousands of miles through Canada
and the US, through mountains and freeways with a bone stock 1990 Mazda
MX-5 Miata, with no problems at all! Also put many miles towing
trailers with a 1971 Datsun 810 s/w.

There are dire warnings about improper connections to the Jetta wiring,
and we were assured that only the dealer is the place to have it installed.

Stuart H.

On Apr 7, 1:36*pm, "Stuart H." > wrote:

> Codswallop! I towed a tent trailer for thousands of miles through Canada
> and the US, through mountains and freeways with a bone stock 1990 Mazda
> MX-5 Miata, with no problems at all! *Also put many miles towing
> trailers with a 1971 Datsun 810 s/w.

That is as it may be. But the brute fact of the matter is that a light
vehicle hasn't the mass to handle even a small emergency with a
trailer - entirely apart from transmission loading, suspension loading
and so forth. That you are lucky to-date does not make what you choose
to do a good idea.

Peter Wieck
Melrose Park, PA

On Tue, 7 Apr 2009 17:31:44 -0700 (PDT), " >
wrote:

>On Apr 7, 1:36*pm, "Stuart H." > wrote:
>
>> Codswallop! I towed a tent trailer for thousands of miles through Canada
>> and the US, through mountains and freeways with a bone stock 1990 Mazda
>> MX-5 Miata, with no problems at all! *Also put many miles towing
>> trailers with a 1971 Datsun 810 s/w.
>
>That is as it may be. But the brute fact of the matter is that a light
>vehicle hasn't the mass to handle even a small emergency with a
>trailer - entirely apart from transmission loading, suspension loading
>and so forth. That you are lucky to-date does not make what you choose
>to do a good idea.
>
>Peter Wieck
>Melrose Park, PA

So I have yet to hear anyone explain the physics of this. an 80,000
pound rig is pulled by a 20,000 pound tractor? Purely designed to tow
so there is one difference. Or a 6,000 Dodge has a combo weight of
18,000. Again designed to tow. Now my 2,800 pound Toyota was rated to
pull 5,000 pounds. So I do not get why I could not tow a 1,000 pounds
behind my Jetta. Car is rated to tow that load. Any rig used to tow
has a big difference in acceleration and braking. When was the last
time you saw an 80,000# rig on the race track. Now look again and you
will see 10 wheel tractors on the race track.

I am a huge fan of brakes on trailers which is where many people screw
up. Trailer brakes should have enough power to stop the trailer and
then some. My equipment trailer was much easier to use with two axles
of brakes then it was when it had just one set of brakes.

I have seen overloaded trailers wagging the tail of a pickup truck on
the interstate. Been there and done that at relatively slow speed.
Note to self. Calculate load of trailer and check tire pressure before
towing.

> wrote in message
...
On Apr 7, 1:36 pm, "Stuart H." > wrote:

> Codswallop! I towed a tent trailer for thousands of miles through Canada
> and the US, through mountains and freeways with a bone stock 1990 Mazda
> MX-5 Miata, with no problems at all! Also put many miles towing
> trailers with a 1971 Datsun 810 s/w.

That is as it may be. But the brute fact of the matter is that a light
vehicle hasn't the mass to handle even a small emergency with a
trailer - entirely apart from transmission loading, suspension loading
and so forth. That you are lucky to-date does not make what you choose
to do a good idea.

Peter Wieck
Melrose Park, PA

uhhh...my 03 Jetta TDI wagon is rated to tow about 1,000 pounds, and tongue
weight of 165 ISTR. However, it seems the exact same vehicle is rated for
nearly 1,000 kg--about 2000 pounds-- in Germany. Recently talked to a Smart
owner who had a very small trailer--mobile blacksmith--hooked to his diesel
Smart. In Canada, the 40 hp diesels had no stated towing capacity, but in
Germany the official government road safety figures were about 750 pounds.
Seems that in North America, fear of litigation is the deciding factor, not
safety per se. Towing a reasonable sized (to the tow vehicle) trailer is not
a problem...if you always take it into account and adjust your driving
speed, distances, and habits accordingly. A modest aluminum tent trailer
weighs about 1,500 pounds, which is still a lot less than the mass of even a
small car. However, if you tow a fibreglass house trailer (Boler 13ft, 760
pounds, aerodynamic) like I do, then no problems.at all, even in emergency
stops and accident avoidance. If a car can handle four adults and their
luggage in the normal course, then it can handle one adult, a dog, and a
light trailer. Just don't try to tow a 20+ footer weighing 3,500 pounds with
your Golf.

Jim Bartley
PEI

Please note the interpolations:

On Apr 7, 11:07*pm, Jim Behning

> So I have yet to hear anyone explain the physics of this. an 80,000
> pound rig is pulled by a 20,000 pound tractor? *Purely designed to tow
> so there is one difference.

That, and the tractor has - at those weights - 10 tires on the ground,
a driver trained and licensed to operate it (consider a multi-engine
commercially licensed pilot *Qualified" on and handling a 777 vs. a
civilian pilot in a single-engine Cessna 140) and yet for all that
they still screw up with spectacular results on-occasion.

> Or a 6,000 Dodge has a combo weight of
> 18,000. Again designed to tow. Now my 2,800 pound Toyota was rated to
> pull 5,000 pounds. So I do not get why I could not tow a 1,000 pounds
> behind my Jetta.

There is a significant difference between *can* and *should*. You can
jump off a high bridge into a river or onto concrete. Perhaps you
*should* not.

> Car is rated to tow that load. Any rig used to tow
> has a big difference in acceleration and braking. When was the last
> time you saw an 80,000# rig on the race track. Now look again and you
> will see 10 wheel tractors on the race track.
>
> I am a huge fan of brakes on trailers which is where many people screw
> up. Trailer brakes should have enough power to stop the trailer and
> then some. My equipment trailer was much easier to use with two axles
> of brakes then it was when it had just one set of brakes.

Brakes are helpful for stopping in a straight line. They are not
terribly *helpful* if you are stopping otherwise, especially if your
vehicle has anti-lock brakes and you trailer does not - and, BTW, most
combinations extend the anti-lock system to the trailer brakes as well
these days. If the trailer has strong brakes which grab on a slippery
curve, you will tend to straighten out rather than follow that curve.
That could be acutely embarrasing under certain conditions. Kinda
being caught between whiplash and tighteing the bow...

> I have seen overloaded trailers wagging the tail of a pickup truck on
> the interstate. Been there and done that at relatively slow speed.
> Note to self. Calculate load of trailer and check tire pressure before
> towing.

AMEN to that. Worse than towing at all - overloading. Then you are
betting everything against the house rather than merely playing close
odds. The problem with physics is that it really does not care about
nameplates and ratings very much, but about the immediate conditions
when its various laws and forces come into play. Comparing a VW jetta
TDI at a curb weight of ~3,000 pounds to a Freightliner Cascadia with
a curb weight of ~20,500 pounds has you agreeing only on the fuel
used. As if a 3,000 pound general-purpose passenger vehicle is rated
to pull 12,000 pounds of trailer - based on the same 4:1 ratio.

Sorry - driving is dangerous enough without taking everything to the
margins.

Peter Wieck
Melrose Park, PA

On Apr 8, 7:24*am, "George Mills" > wrote:
> If a car can handle four adults and their
> luggage in the normal course, then it can handle one adult, a dog, and a
> light trailer. Just don't try to tow a 20+ footer weighing 3,500 pounds with
> your Golf.

Small quibble he
Four adults and their luggage (and possibly their dog) distributed
about the center-of-gravity. Not a trailer added as an appendage
hanging off the back with all the tongue weight on the back.

Peter Wieck
Melrose Park, PA

> wrote in message
...
On Apr 8, 7:24 am, "George Mills" > wrote:
> If a car can handle four adults and their
> luggage in the normal course, then it can handle one adult, a dog, and a
> light trailer. Just don't try to tow a 20+ footer weighing 3,500 pounds
> with
> your Golf.

Small quibble he
Four adults and their luggage (and possibly their dog) distributed
about the center-of-gravity. Not a trailer added as an appendage
hanging off the back with all the tongue weight on the back.

Peter Wieck
Melrose Park, PA

I'll quibble back. You raise an extremely good point. There are many
misguided people out there who don't know or won't pay attention to
limitations. I specified a Boler fibreglass trailer at 760 TOTAL pounds, and
NOT 760 on the tongue, more like 100 or so. Remember all trailers MUST have
some positive tongue weight! They are also aerodynamic, nor generally
affected by cross winds, and have real car sized tires, on a big steel frame
so they have a balanced and very low centre of gravity. I suggest that
people Google Boler to find out about these remarkable trailers and their
history. I have towed it very successfully and safely with an ancient
automatic Corolla with 300,000+ km, with about 2/3 of its nominal 108 hp
left. No appreciable fading on the drum brakes either, even in hilly (4-15%
grades) country.Within reason, a(n) (aluminum) folding tent trailer is
similar, but I never considered one because they tend to have smaller,
skinnier tires, more weight, and poorer areodynamics than a Boler or similar
type of trailer like a Casita or Trillium, and so tend to bounce around a
bit more. I also specified NOT some freakishly large and long trailer that
exceeds the length and weight of the towing vehicle. The second scenario
will waggle the tow vehicle, even if it could pull it, and unbraked it would
be a downhill disaster waiting to happen. Cross winds would make it so easy
to jackknife as well. The first is quite safe, as long as you account for it
being behind you. They were designed to be safely towed by small economy
cars.

Jim Bartley
Prince Edward Island