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Discussion Starter #1
I tried replacing my terrible trunk light with a strip of LEDs, since this light is switched through the Rear Body Control Module (RBCM), I fried a transistor (MOSFET) inside of it by overpowering the circuit. The symptom of a failed transistor is that the trunk light just stays on all the time and never powers off.

Don't make the same mistake I did, only use a low powered LED if you replace it. The stock bulb is rated at 3W, I would suggest not exceeding that number since several people have fried theirs as well.

Below are some photos of the inside of the RBCM - one of the burned up chip, and the other of the chip removed and cleaned a little (and poorly at that). I can't see the writing on the MOSFET, but if someone is brave enough to open up their good RBCM, I might have a chance of replacing it with a bigger MOSFET, but there's some cratering of the board, so who knows - I'm not super skilled on a soldering iron.
 

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Thanks for sharing. I have heard that even when replacing burnt-out bulbs - like the trunk bulb - there is merit in disconnecting the negative post of the battery, making the swap, and then re-attaching the negative post. Sounds like a whole lot of work, but that BCM is apparently very, very sensitive.

I think there is a TSB out on the CX5... something to that effect....?
 

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Shorts on that circuit are VERY bad news, as you can see.... it's completely unprotected, which is insane -- but it is what it is.
 

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Discussion Starter #4
I would highly recommend disconnecting the battery, the bulb is tiny and apparently easy to short out. I dug around a few Mazda forums and it's a problem for 3s, 6s, and the CX-5 for sure - they all use the same RBCM.
 

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Shorts on that circuit are VERY bad news, as you can see.... it's completely unprotected, which is insane -- but it is what it is.
I'm fairly certain I overloaded it, but I agree, there should be some sort of cheap protection on a bulb, not a $125 module. If they had spent about 50 extra cents, they could have used a much beefier MOSFET that would not fail like this.
 

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I'm not sure what year 6 the OP has, but on my 2018, I accidentally shorted out the trunk light circuit a while ago when I installed my trunk net. Luckily, the worse thing that happened was that I blew a fuse in the panel under the dash on the driver's side (which also knocked out the interior lighting and my keyless entry system). Once I found and replaced the fuse, all was well again...
 

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If I remember, pull the ROOM fuse under the hood before replacing that dam bulb and save yourself the $125!
 

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Discussion Starter #8
I'm not sure what year 6 the OP has, but on my 2018, I accidentally shorted out the trunk light circuit a while ago when I installed my trunk net. Luckily, the worse thing that happened was that I blew a fuse in the panel under the dash on the driver's side (which also knocked out the interior lighting and my keyless entry system). Once I found and replaced the fuse, all was well again...
It's a 2015 GT. If your's blew a fuse in the dash fuse box then they redesigned the whole system. The RBCM in your car (part number TK52675Y0*) appears externally identical to the KD45675Y0*, but must have some updates to run the switched power up to the fuse block and then back to the trunk. All the power my RBCM comes from the ROOM fuse in the engine bay.

In case you are searching for a new RBCM, part numbers for likely nearly identical RBCMs are as follows (I'd recommend using a boolean modifier (*) to replace the last letter and catch them all):

KD45-67-5Y0A
KD45-67-5Y0B
KD45-67-5Y0C
KD45-67-5Y0D
KD45-67-5Y0E
KD45-67-5Y0F
KD45 67-5Y0G
 

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Discussion Starter #9
If I remember, pull the ROOM fuse under the hood before replacing that dam bulb and save yourself the $125!
This is correct - top left 15A fuse in the engine bay fuse box. No shortcuts, it's a fragile module!
 

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It's a 2015 GT. If your's blew a fuse in the dash fuse box then they redesigned the whole system. The RBCM in your car (part number TK52675Y0*) appears externally identical to the KD45675Y0*, but must have some updates to run the switched power up to the fuse block and then back to the trunk. All the power my RBCM comes from the ROOM fuse in the engine bay.



In case you are searching for a new RBCM, part numbers for likely nearly identical RBCMs are as follows (I'd recommend using a boolean modifier (*) to replace the last letter and catch them all):



KD45-67-5Y0A

KD45-67-5Y0B

KD45-67-5Y0C

KD45-67-5Y0D

KD45-67-5Y0E

KD45-67-5Y0F

KD45 67-5Y0G
Then it definitely is different for the '18s then, since the ROOM fuse was located in the dash fuse box (5th one down on the right side, IIRC...)

So yeah, pull the ROOM fuse before swapping out interior or trunk bulbs, whether the fuse is under the hood or dash.

Write that down.
 
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