Front interior light issue

[veearrsix]

I have recently taken delivery of a 2019 T6 shuttle. It is a salvage vehicle, but the only damage was light fire damage to each front corner. I have had to repair/replace the front bumper loom and the connections from the main loom. I don't think these fixes are relevant to the interior light issue, but wanted to add, just in case.

The centre front interior light refuses to come on, the doors do not trigger it and the switch near the light does not switch it on. The switch does however switch on the interior lights in the rear rows. The reading lights either side also work.

Using No. 75/3 from the T6 wiring diagram, I have attempted to validate the switch, I get continuity across the correct pins when the switch is in the relevant position, so I feel like the switch is working (still not totally convinced though). I have also bypassed the switch by bridging the Pin 5 and Pin 8 from the switch plug to force the light to come on, it does.

So I'm stumped, the switch appears to work, the light module definitely works and all wires appear to have the correct continuity, however when I plug it all back in, the interior light still refuses to illuminate. I assume there is some additional connection (a144) so that the switch can control the rear interior lights as well, does this mean it is a possibility that the front interior light has been coded out? Or is there a hidden fuse that I have yet to check (I did have to replace a bunch of fuses due to the damage to the plug in the front bumper having shorted out a few circuits)

 

[veearrsix]

Does anyone know which channel in the Central electrics BCM the interior lights will be on, I want to try and iew live data with obdeleven

 

[veearrsix]

Turns out, somehow I failed to notice that the lights on the steps are also not coming on, so it's not just the central interior light.

 

[roadtripper]

Generally the interior lights are switched on the negative side - one side through the door switch circuit to come on with the doors, the other to a local earth for turning it on. The positive feed is controlled by the BCM and will turn off after about 20 minutes to save power (one hour in some cases)

So it sounds like your issue is in the door switches

 

[veearrsix]

When I open the doors, the rear ceiling lights do come on. Also, viewing the live data I can see the doors opening and they also show as open on the dash. If it's the door switches, would that mean all of them are broken?

 

[roadtripper]

Have you got a picture of your light panel in the front?

If some are working with the door switches and they show fine on the dash then the fundamentals seem fine, do they time out after about 20 minutes (in which case the BCM feed is fine)

Have you checked the lamp itself is fine?

 

[veearrsix]

So I have done a little more checking, I am lucky enough to also have a 2014 T5.1 shuttle. This has the same style of light unit, so I decided to do a quick swap to see if it made any difference, it didn't. This is the style of light unit:



I have ruled out the bulbs them selves as I have tested them. What is interesting though is I decided to measure the voltage getting to the light, just to see if it was getting anything. I measure the black/yellow earth and the power coming in as shown:



So the voltage across these wires for the T5.1 (which works fine) when the switch is set to be always on regardless of if the door is open or not is just under 12v



But the voltage on the T6, when the switch is in the same position is much much lower, but still registering some voltage.



What I also noticed, is when the switch was set to operate when door opens, the T5.1 was just under 12v, then you could see the voltage drop as the light was supposed to dim, the T6 also did this, just from the much lower voltage. So the T6 appears to be sort of working, just never getting max voltage.

Also, the reading lamps either side of the interior light work as expected on the T6, copying the behaviour shown on the T5.1.

 

[roadtripper]

If you are measuring using DC your "lower voltage" may be actually a pulsed voltage. With slow pulses that's used to monitor the circuit but with fast pulses (PWM) it's used to dim as that method works with both traditional incandescent lamps and led.

It might be interesting to try measuring it as AC, though to really see the pulses you need an oscilloscope.

 

[roadtripper]

It might be worth measuring the resistance of the door switch circuit pin at that connector and a known good ground.

It should be very high with the doors closed but very low when a door opens.

I'm wondering if you have some damage or corrosion on that line.

 

[veearrsix]

I tried measuring the circuit using AC instead of DC, not sure what I was supposed to see, but I think it was about 0.1v. I tried measuring the voltages getting to the switch. I noticed that, if I measured across the solid brown (shared ground I believe) to the constant power (red/black) I get the 11.5v as before. I then measured across from the brown/white from the dash panel (a144), I get 11.5v when the door is open, which goes to 0 as it dims. Confirmed both of these with a test lamp.

I then plugged the switch back in and back probed the same plug and the same wires and got 0v across brown/white and the constant power when the door was open, then rain stopped play. Not sure if this makes sense, doesn't to me, but I'm not an auto electrician. I got side tracked by this before I had a chance to measure the resistance.

Does anyone know what the brown/red wire is supposed to do?

 

[veearrsix]

The brown and red wire that goes to pin 6 of the light switch plug, should this be live or ground? As far as I can tell with my limited knowledge, it looks like it should be ground.

 

[roadtripper]

I think that line has a number cut off in the diagram and is actually "B129 Connection Interior Wiring Harness"

As I mentioned above the VW lights are positive timed supply from the BCM and then 2 switched earths, one permanent (for lights on manually) one switched on the door circuit (for door operation)

Ground is usually brown and you can see that on pin 8 column 25-27 and column 19 looks like it's the incoming BCM positive supply (the 7 in the box means it comes from column 7 on the other diagram page)

Looking at the switches I would say B129 is the door switch ground.

I can't be certain as the diagram I've got to hand is the van/Kombi one and a little different.

 

[veearrsix]

I'm learning more about how to read these diagrams, I had no idea the numbers at the bottom were relevant!.

So I believe the brown/white wire (col 16) is the switched door circuit, this is the one that behaves the way I would expect when the door is opened, going to 11.5v when opened, and ramping down to 0v after waiting once the door is closed or when the ignition is switched on. The wire from B129 (col 22) is the one I'm confused by. The reason I asked if it should be live or not is because it IS live, but looks as though it shouldn't be.

If I use a test lamp from the brown and red wire to ground (body of van) it lights up and measured it is 11.5v. Sorry for the poor focus.



I also had a look where else B129 is used...





These are each of the other lights that don't illuminate when they should. I have checked continuity to the left front door step light from the red/brown wire in the overhead socket and they are indeed connected as expected. However when I use the test lamp across both of the inputs in this connector for the light (both brown/red and red/black)



The test lamp lights up regardless of which input connection I use. Again, this to me looks incorrect. Just looking for confirmation really, that I understand what I'm looking at is the problem. If it is, then at least I will know why the lights aren't coming on, even if I don't know why B129 is live when perhaps it shouldn't be.

 

[veearrsix]

Well, I found a problem



Unfortunately, repairing this hasn't resolved the issue

 

[veearrsix]

Well I am now truly stumped. Trying to convince myself I will find the problem without removing the dash, but its looking like the only way to find the problem.

 

[veearrsix]

I have solved the issue, it was caused by the damage to the wiring I found, plus one of the fuses (that I was sure I had checked many many times) had also blown.

Still confused how the blown fuse was causing the brown/red wire to be live when it shouldn't have been, but now it's sorted.

 

[dksolo95]

Hi i seem to having the same issue word for word on my T6 kombi. Are you able to confirm which fuse this is that you blew?

Also what wire will cause this issue?

 

[veearrsix]

Hi i seem to having the same issue word for word on my T6 kombi. Are you able to confirm which fuse this is that you blew?

Also what wire will cause this issue?

I can't remember to be honest, I wasn't checking them by number/location, I literally checked every fuse in the fuse box in the middle of the dash for continuity. I have a fuzzy memory of it being one of the mini fuses, maybe 7.5amp or 10amp.

 

[dksolo95]

I've tested all those fuses for continuity 3 times now and all of them passed. All I can think of now, is a wire is damaged. Any tips on how I can trace this myself, even though im no auto? lol

 

[veearrsix]

Using the wiring diagrams, they usually indicate a wire between 2 plugs, sometimes those plugs are easy to get at and you can test the continuity between them. I won't sugar coat it, I had the seats out, carpet out, door steps and all of the bottom dash panels out to try and trace stuff. I also knew that the corner I found the issue in had been affected by an external fire, I just couldn't tell how badly until I lifted the carpet.