Trying to find out what this fuse does, it's blown twice now

N

nonlineartom

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VIP Member
I've had this T6 for about 5 months now. Whilst driving last month It suddenly acted like the key had been taken out the ignition, lost engine, MFD, power windows e.t.c only thing left working was hazards, headhunt, interior lights, stuff that still works with no ignition. Breakdown found a 10A fuse in slot 26 to be blown, suggesting it was for the body control module. Replaced with another 10A and then it was fine for 800 miles, until it did it again today out the blue. Again fuse 26. Replaced and it's working again. I can't work out what is causing it to pop, the fuse diagram on this forum suggests it's rear wiper / washer but the Haynes references the AA used said it was control module. The suspicious piggy bagged fuse I can only assume is for the 12v socket in the back that was an aftermarket install years ago.
worth noting these issues only started after the door speakers were replaced, but the electrician who installed them said he didn't go near the fuse box, they are direct replacements no amp. Image shows the fuse that went. It's a 2015 T6 t28, not sure if that makes a difference.

P.S it's pretty scary to just lose engine, power brakes, and power steering when you're driving at 50mph in the snow. Don't recommend it, not fun.

IMG_5658.jpg
 
further info. The fuse card document on the VIP section lists fuse 26 as:

- Onboard supply control unit -J519- (T73b/51) 1)
- Interface for external use 6-pin connector -T6bn/5-

J519 seems to be the body control unit? it also lists it as a 5a fuse, so not sure what a 10a is doing in there. Anyone else had this issue?
 
It's a 2015 T6 t28, not sure if that makes a difference.



has it got stop start?
 
Dellmassive said:
Any leisure electricals?
Click to expand...
nope, this isn't a camper, only non standard electricals are 4x small LED spots in the headlining, which I can only presume are wired into the main cabin light as they only come on when the doors are opened/ van unlocked in the same way the main cabin interior light comes on.
There is a single 12V socket on the non-sliding door and a USB port, camper van style. Which I assume is the piggy backed fuse seen in the photo. Rear speakers were added (at the same time the door speakers were replaced) just before the problem first appeared but they are wired direct into the head-unit by a professional fitter. cabling goes up windscreen pillar and above headliner. Really appreciate you taking the time on this, I'm very green with auto electrics
 
The reason it says BCU is because a number of items are controlled by the BCU but the system it is controlling has its own supply fuse.
there are normally smaller fuses for the control switches that signal the BCU then the BCU switches the output to the consumer feed by an appropriate size fuse.
Fuses can be sized different depending on the options fitted to the van. The fuse card is based on my 2016 California as it has nearly all the options you can have so as complete as I could get it.
I’ll have a fresh look at the diagram as there are variations.
 
Loz said:
The reason it says BCU is because a number of items are controlled by the BCU but the system it is controlling has its own supply fuse.
there are normally smaller fuses for the control switches that signal the BCU then the BCU switches the output to the consumer feed by an appropriate size fuse.
Fuses can be sized different depending on the options fitted to the van. The fuse card is based on my 2016 California as it has nearly all the options you can have so as complete as I could get it.
I’ll have a fresh look at the diagram as there are variations.
Click to expand...

Thank you, any insight is greatly appreciated, as I say, it acts like the key has just been removed when the fuse pops, you get the "check safe lock" on the MFD for a few seconds then the MFD blacks out, only lights up if you open a door to show which door is open, inserting and turning the key doesn't do anything at all. No control of central locking or windows, but interior lights come on when the door is opened. AA guy tried replacing fuse 26 with another 10A which blew instantly, that was with key set to ignition on, he removed the key and put a fresh fuse in and that one didn't blow.
...Really don't want to be going on any smart motorways with this timebomb
 
Yea on your earlier model you have fuses in sc21 and or sc26 where mine(on the card ) does not.
if you use the fuse part of the circuit diagrams page 669 you’ll see all the circuits that are linked.
there are some joints and it is basically the supply for running and starting.

2B4430F3-997F-4A11-92DC-F12598879F87.jpeg 4FE85114-206C-425C-9196-3310AE65DE09.jpeg
 
Loz said:
Yea on your earlier model you have fuses in sc21 and or sc26 where mine(on the card ) does not.
if you use the fuse part of the circuit diagrams page 669 you’ll see all the circuits that are linked.
there are some joints and it is basically the supply for running and starting.

View attachment 191438 View attachment 191439
Click to expand...


so do you have any idea what might be causing it to pop seemingly at random? both times it went I was just cruising on B roads with nothing changing input wise, no indicators or light changes or anything before it went, it just... went. Dead short or cable insulation breaking down?
 
Still struggling with this and every garage and auto electrics guy is booked up for the next 3 weeks minimum. Scared to drive it in the meantime as the fuse has popped twice now (800 miles apart between instances) and I don't know when it will do it next. Is there any further diagnosis I can do myself with a multimeter?
 
You could try removing the fuse that blows.

Then use the multimeter set to continuity from the output side of the fuse and ground.

Then follow the loom and wiggle and bang it about.... If you find a rub, chaff, or screw damage to the loom the multimeter will read a short or very low resistance.


..


Post some general pics of the inside of the van for us to see.
 
Dellmassive said:
You could try removing the fuse that blows.

Then use the multimeter set to continuity from the output side of the fuse and ground.

Then follow the loom and wiggle and bang it about.... If you find a rub, chaff, or screw damage to the loom the multimeter will read a short or very low resistance.


..


Post some general pics of the inside of the van for us to see.
Click to expand...
I can't get the multimeter probes into the space because of the trim in the way, looks like it's a bit of a mess to remove that trim? Will upload some more general photos now
 
As far as I can tell the only non VW electrics are these 4 led spots in the roof, that are connected to the main interior light system, then there is a 12v socket on the second row of seats, which I assume is the piggy-backed fuse seen in the fuse box, the white cable seen going over the front of the fuses is screwed into the metal behind the trim, presumably as an earth. Dashboard shows this is a standard T6 blue motion (euro 5) last photo shows some loose loom I found sliding around under the passenger seat, I assume this is supposed to be mounted to the seat base itself? the metal jack was being stored under the passenger seat, knocking up against these connectors, could that be causing an occasional short which would blow fuse 26?

IMG-5727.jpg

IMG-5728.jpg

IMG-5729.jpg

IMG-5730.jpg

IMG-5742.jpg
 
Update for anyone reading this and having similar issues:
Took the van for diagnosis at an auto electricians, they found fuses 5,6 and 7 were 10 amp and should be 30a fuses, these fuses apparently deal with the ignition. He replaced them with 30a fuses.
note these fuses weren't the ones that were blowing, it was the 10a fuse for the body control module that popped twice.

I thought that fixed it, then yesterday, whilst half way round a roundabout the engine just cut out, acted like I'd pulled the key from the ignition, I was able to coast to the side of the road and checked the fuse box, the BCM fuse hadn't popped this time and I was just able to restart the van as usual

Any idea what could cause it to just think the key has been removed and then magically fix itself straight away?
 
Mate I feel your pain. I've some minor electrical gremlins and know how frustrating it can be. Might be time to bite the bullet and get VW to check it over.
 
I have the Blue Motion T6 28 (2016) and the automatic stop and start causes me the secondary issue you are having "whilst half way round a roundabout the engine just cut out, acted like I'd pulled the key from the ignition". If I always disable (switch off) the auto-stop start every time I start the engine, the fault never occurs. When it does occur if I forget to switch off the autostart it is very scary. Nearly always happens when on a roundabout or when changing down whilst slowing in a sharp corner. Still trying to find an answer. A complete lack of Volkswagen Van dealers in my area means I am still researching a solution. I remember once reading on the forum that it was a problem for which there is a solution but I do not know what that solution is. I am beginning to think that the system finds it hard to cope with Double D'Clutch gear changes. I know they are not necessary any more but it is very hard to change a driving style that was the norm for such a large part of my life. I approached three local auto electricians asking if the Auto Stop/Start can be disabled but they are all so backed up with work that they are not interested in looking at the issue.
 
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