Headlight out on one side, bulb tests ok. Voltage seems…

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Discussion Starter · #1 · 10 mo ago

I got no response in the electronic forum so I figure I might get a better chance here…

I thought my LED headlight bulbs finally gave up the ghost (passenger side is out, had these on since the third day of owning it in 2016). So I went and ordered some new ones. To my surprise, the bulbs (tried both) didn’t work. I thought I may have blown a fuse. Looked at the manual and it doesn’t mention a headlight fuse. Upon further searching, it seems that it’s buried in the BCM and the circuit may control more than the headlight itself? All other passenger side lights are working, front and rear, signals etc. Has anyone experienced this? Just the headlight is out?

It got dark so I gave up on replacing them or testing the circuit for voltage for the night. I guess I’ll test the old bulb and the new bulbs and make sure I don’t have 3 dead bulbs. Edit: All bulbs work with 12V applied.

Do I need to take the lower dash apart to replace the fuse in the BCM? I’m hoping it’s just a fuse.. actually I can see fuse 11 from the steering column. Wonder if I should swap it with another 25A fuse to see if it changes/resets anything.

Update: I get 10.4V reading at the 2 pin H11 plug. The funny thing is I get this voltage even with the key out of the ignition since the night before. I took out the headlight and measured the same 10.4V at the 4 pin (The four pins are Ground, empty, Hi, Low) headlight plug. (So the headlight internal wiring is not the problem here) Started the vehicle, and I got 11.2V at one point. tapped power at the plug, the bulb is still not turning on. The 2 grounding point in front of the airbox on the frame horn seems pretty clean and they tested ok from the harness ground to these ground points.

Do I have a BCM failure? Are there any settings in Forscan to diagnose/reset this circuit? I have the dongle but I haven’t used it at this point.

I really need some help. I really want to go somewhere with this van. I don’t want to bypass the BCM to run the headlights at this point.

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You can buy used copies of the Transit wiring diagrams on ebay for $15 dollars, New copies from Helm Inc cost $86 dollars. The Service Manual will cost you about $180 dollars new.

Headlight out on one side, bulb tests ok. Voltage seems...

Find great deals on eBay for transit wiring manual -connect. Shop with confidence.

www.ebay.com

Looks like the Ebay prices have gone up since I last checked.

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Have you tried unplugging the BCM and plugging back in? How about removing the negative cable at the battery and reconnecting after 5 minutes? Are your relays all good? Check all the grounds around the headlight area and the bcm.

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Discussion Starter · #4 · 10 mo ago

You can buy used copies of the Transit wiring diagrams on ebay for $15 dollars, New copies from Helm Inc cost $86 dollars. The Service Manual will cost you about $180 dollars new.

Headlight out on one side, bulb tests ok. Voltage seems...

Find great deals on eBay for transit wiring manual -connect. Shop with confidence.

www.ebay.com

Looks like the Ebay prices have gone up since I last checked.

Thanks for the link. I will buy one.

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Discussion Starter · #5 · 10 mo ago

Have you tried unplugging the BCM and plugging back in? How about removing the negative cable at the battery and reconnecting after 5 minutes? Are your relays all good? Check all the grounds around the headlight area and the bcm.

Unplug the harnesses attached to the BCM and try again?

I have tried a 15 minute battery disconnection and it didn’t change anything.

Headlight out on one side, bulb tests ok. Voltage seems...

I cleaned up the grounds here just in case. They were relatively clean though. I’m in southern California so it’s not very wet here nor the roads get salty in the winters.

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Discussion Starter · #6 · 10 mo ago

I dug into the BCM fuses and both F11 and F15 25A fuses have continuity. Swapped left and right fuses and the problem persists. Is there something in Forscan to tell if anything failed in the BCM internally? There are 3 plugs in the BCM, 2 browns and a blue. I wonder if the blue/green striped positive 12v low beam wire at the passenger headlight harness is in any of these three plugs?

Headlight out on one side, bulb tests ok. Voltage seems...

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Discussion Starter · #7 · 10 mo ago

Have you tried unplugging the BCM and plugging back in? How about removing the negative cable at the battery and reconnecting after 5 minutes? Are your relays all good? Check all the grounds around the headlight area and the bcm.

BTW, which relay(s) are you referring to? Are they inside of the BCM?

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The relays are in the engine fuse box I would guess. Its been a few years since I last looked at the fuse box diagrams, Which are all on this forum in multiple places.

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Fuse box diagrams are on page 194 of my 2016 owners manual in the Linked Pdf.

(Relays too)

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Fuse box relays. Under the hood right hand side.

BTW, which relay(s) are you referring to? Are they inside of the BCM?

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This is in response to someone with low beam issues on a 2016 T350. I think I found somewhere else as well that there is a Service Procedure issued for it. The link at the site / in the pic posted doesn’t work so I haven’t seen the bulletin myself.

Headlight out on one side, bulb tests ok. Voltage seems...

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Discussion Starter · #12 · 10 mo ago

Thanks for the replies so far.

This is the only thing i found based on the headlight TSB. But it points towards the Connect. Transit Connect low beam

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Discussion Starter · #13 · 10 mo ago

Headlight out on one side, bulb tests ok. Voltage seems...

I finally got my Forscan registered and dusted off my new in box ELM327 clone with HS/MSCAN switch. This issue was a protection algorithm in the BCM kicked in. It had detected a short somewhere. I am guessing it could be the LED had failed and had a short and the BCM shut off JUST the passenger’s side low beam circuit to prevent further damage. So it does this to keep the rest of the lights working. This was a great learning experience as I have only dealt with older VW/Audi products and most were before the days of having a dozen control modules scattered all over, except my wife’s Audi. I use my VAGCOM occasionally for DTCs etc but had never played with Forscan. I hope this will help someone down the line.

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