Trailer electics - faulty Trailer prep [Resolved]

[Willam]

I have just fitted a Westfalia towbar and 7 pin Trailer Prep. electrics (from PF Jones) to my 2019 T6 Transporter.
All good and all electics working except for the near side trailer side light.
I have traced the problem to lack of voltage (0.1 V only) at pin 5 of the 12 pin socket supplied as part of the Trailer Prep. package.
The next stage seems to be to trace back 'upstream' to find the source of the fault.
Can anyone help, please, with a wiring diagram for the Trailer Prep. so I can check connections upstream?
Also, are the trailer side lights fused individually and, if so, where is the fuse? (I have looked under the passenger seat but have not found a blown fuse.
Thanks for any help.

 

[Deaky]

Check the connections through the D Pillar. There’s a wiring diagram in the downloads section but you’ll need VIP membership to download it.

T6 - Trailer Coupling Wiring Diagram

Factory Fitted Towbar Circuit Diagram VIP Membership is required to download this document

www.t6forum.com

 

[Willam]

Thank you Deaky. Will do, and will upgrade to VIP for detailed tech content.

 

[Willam]

Wiring diagram was helpful.
Have done further checks.
Electrical continuity is good all the way from the trailer socket (7/58 L) to the trailer module (Socket T16n, Pin 10, Grey/Black wire).
Pic of Trailer Module included below.
With Side Lights On, there is only 0.08 V at Pin 10, T16n (i.e not zero, but far from the 12 V needed).
(I also checked the equivalent Right side light (Pin 4, T16n) and that has 12.7 V, as expected as this side light is working on the trailer.)

My thinking is that the lack of trailer left hand side light is due to one or more of the following:
- Fault in the Trailer Module (but could this affect just one of the two side lights?)
- Incorrect coding (again, could this affect just one of the two side lights?)
- Fault in wiring 'upstream' of the Trailer Module.
- Blown fuse, but I cannot find any fuse that would affect just one trailer side light?

Anyway, I am just guessing, so further advice appreciated please as to what to check next. Thank you.

 

[Vanderlust]

I had a fault with my T6 towing electrics when the offside position and indicator stopped working. vw dealer ran diagnostics and said the van was in order. Subsequently found the 20 amp fuse in the middle of the three in the photo under the passenger seat was blown replaced and all working. The fuse was for one of 3 12 volt feeds to the trailer control module so have no idea why only certain lights were affected.

 

[Willam]

Thank you. Have just checked all fuses under the passenger seat (15, 20, 20, 30, 40 Amp) and all are good.

 

[roadtripper]

Actually I would expect a blown fuse to only effect one side, sidelights are on separate circuits for that reason so you don't lose every marker light.

 

[Willam]

With the trailer connected, the van side lights are working fine; it is just the trailer left side light that is not working.
So, is there a specific fuse for each trailer side light (independent from the van side lights)?
If so, please tell me where it is - I cannot find one on the trailer wiring diagram.
Thank you for helping.

 

[roadtripper]

Yes trailer sockets are always separately fused from the vehicle and should have 2 separate circuits for the lights. That's not just a transporter thing.

I have a 6.1 and OE towbar and fully wired, my 4 30amp fuses are along the front of the passenger fuse box:

I mapped them out here:

https://www.t6forum.com/threads/which-towing-fuse-does-what.44984/

 

[roadtripper]

Have you checked the trailer independent of the van, either continuity between pin 3 (ground) and pin 5 (right) and pin 7 (left) or on another vehicle.

Reason I ask is the OE trailer module does trailer detection by looking for 2 continuous circuits out of the 4 side light and indicator circuits. When I built my trailer simulator for the 12v EHU system you can see the module pulse then to check them. So testing for voltage on the van side may not be reliable as it was in dumb analogue days, the module may have detected a bad circuit, powered it down and is pulsing it to detect when it is fixed.

 

[roadtripper]

Looking at the T6 diagram your fuses should be SH 4, 5, 6 and SH 15, but the T6 uses a separate relay for the switched voltage supply and that's clearly from SH15 so that will not be feeding lights.

 

[Willam]

Excellent and comprehensive information. Will digest then act on it tomorrow, in the light of day...
Thank you!

 

[Willam]

Responding to helpful information above:
Fuses
Checked fuses - SH 4, 5, 6 all OK.
SH 5 protects RH trailer side light (confirmed by pulling out and seeing the trailer light go out)
Logically, as it sits next to and is the only other matching SH fuse, SH 6 protects LH side light.

Trailer module
Checked voltage at input to Trailer Module J345:
Pins 2, 9, 11, 12
Respective voltage with Ign off 0, 12, 12, 12
Respective voltage with Ign on 12, 12, 12, 12 (all live, good)

Fuses SH 5 and 6 connect to Pins 12 and 11

Trailer wiring (tested on two devices - a free standing Trailer Board and a Thule bike rack):
Plug pin 3-5 continuity good (approx 2 Ohm resistance)
Plug pin 3-7 continuity good (approx 2 Ohm resistance)

Conclusion so far:
It seems that the fault (no voltage to trailer LH side light) must lie within the Trailer module or vehicle coding related to the module.
The van is UK registered but was originally registered abroad (in Australia, though manufactured in Germany) so I am wondering if it still has non-UK coding. Perhaps this might affect the trailer module?
I have checked the Westfalia trailer electrics supplied to Australia and it shows the same 7 Pin designation as for the UK, but other sites show possible variations from UK pin designation.
I have only a basic ODB reader, so cannot currently check coding settings.

Unless I have misundertood / drawn incorrect conlclusions, I think my next step is to get hold of a VCDC device (or similar) and read the vehicle codes and correct if needed. If that does not solve the problem, then replace the Trailer Module.
Does this all look sensible please?

Thanks again for everyone that has helped above - this is an excellent and very useful Forum.

 

[roadtripper]

Ah Australia might be a clue, they use a different 7 pin wiring as they have electric operated brakes on pin 5.

If it's your right hand side lights that don't work, try stepping on your brake pedal?

You may also find your trailer foglights come on with your reverse lights, as they change pin 2.

 

[Willam]

Repeated tests and confirmed that all the trailer functions are working as per standard UK wiring for a 7 pin trailer socket, except for one fault:
No voltage at Pin 7 when there should be 12V (i.e Left Hand side light not working)
(Stepping on brake pedal gives 12V at Pin 6 only, as it should. Selecting reverse gear gives 12V where expected (to optional Blue/Yellow wire in D pillar) and not to Pin 2.)
So, perhaps the most likely cause of the single fault is the Trailer Module rather than a coding error?
Not sure if I can test the Trailer Module independently of the vehicle?
If not, suspect that I will have to assume it is faulty and replace it.
Comments / further advice welcome, of course.
Thanks.

 

[roadtripper]

I was going to say is it worth testing the output directly on the trailer module pins but I think you've already done that?

I think you've also tested 2 distinct items plugged into the trailer socket which makes wiring/lamp faults less likely.

At this point the balance of evidence sounds like a failed drive transistor in the trailer module, I can't think of any region that uses UK wiring but that one pin. Also I note that a couple of our resident coding gurus @mmi and @n10n have looked at the thread and I'm sure they'd have chipped in with any obvious coding faults.

One final thought, do you get any lamp faults reported when you have the trailer or board plugged in? At least on the T6.1 module it uses both sides and both indicators as trailer detection, so I'd sort of expect if there was a fault on the sidelight circuit you'd also get a lamp faults reported...

 

[n10n]

Never say never but doesn’t sound like a coding issue. Here’s all what OBD11 shows available - I have the same module part number.

* Country just has North-America, Australia and ROW.
* No idea what Transporter compatibility function means

There wasn’t anything about coding in the fitting instructions?

 

[Willam]

Thanks roadtripper.
Answers to your questions in order:
Yes,
Yes,
Third paragraph; strongly agree.
I have looked at coding info on this Forum and have found nothing that indicates the possibility of coding one side light independent of the other, however, I guess this might be applicable for when one of the side lights is used as a parking light, controlled by the turn indicator?

Last paragraph; I don't get any lamp faults anywhere. Have used my basic OBD device to scan and it reports no fault codes.

Unless further information on coding is forthcoming I will be looking out for a replacement Trailer Module as the solution.
Thanks again; your time and expertise is appreciated.

 

[Willam]

Thanks n10n, just seen your post after replying to roadtripper.
I did not get any coding instructions with the towbar. I think the van must have been coded when VW fitted the Trailer Prep at the factory because, when the 7 pin plug is connected, the van responds as expected (trailer icon shown on dash, parking sensors disabled etc).

 

[mmi]

Unless further information on coding is forthcoming

Although not likely to be coding issue but what's the current coding?

What does the controller itself think about missing lights? Does it "see" them as active?
OBDeleven > Control units > 69-Trailer control > Live data > Channel 4

• Value 1 = left turn signal
• Value 2 = right turn signal
• Value 3 = left tail light
• Value 4 = right

Please post what's in channel 9? Values 1 - 4?

EDIT: Apologies, I somehow thought you had OBDeleven at hand.