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Discussion Starter · #1 ·
'07 X Prem w/AWP, 5MT

So... I decided to wire the front map lights to turn on with the door opening, and fade out as well. Of course I just HAD to dive in without checking diagrams or reading through the great threads here (the consolidated lighting thread).

I ended up burning the circuit that drives the lights. I smelled and heard the fry. Specifically, I made the mistake of not noticing the reversed wiring of the map lights up front. So I shorted power to ground through the driving circuitry. Result: no door operated lights.

Research showed the "body integrated unit," part number 8821SA140, runs around $300 at most dealers. But I did find it for $166 at SubaruPartsForYou, and an even better $158 from Subarupartspeople.

After nearly passing out, I decided to see if I could fix it. I followed the wiring diagrams to the BIU under the steering wheel, just to the right of the relay block, which is itself hanging on the fuse block. There are two block-style connectors into the BIU, which is held by a 10mm nut to a beam.

Opening it up revealed a fried trace and MOSFET. The FET's "drain" is connected to pin A5 of the BIU. The FET is an NEC 2KS3377, which is rated for 30W max (approx. 2.5 amps). So those worried about overloading the dimming circuit, be advised to keep below 30W total for all bulbs. The trace carrying current from the pin to the FET measures 1mm wide. The calculator here shows that a 1mm trace can support well more than 2.5 amps (about 16 actually). So it looks like the FET's 30W rating is the limiting factor. The existing 12V8WAL bulbs are unknown wattage, though. Presumably 8W? With 2 on the original circuit, each would have to be 15W or less to not exceed the FET's power rating. But if they are indeed 8W, then the maps lights added would give a total of 32W, slightly exceeding the stock FET's rating.

In my case, after wiring around the burned trace, the dome lights would come on with a door opening but would not dim off. I figured the FET was burned. I replaced it with an NTE 2987. It's rated for 105W. It was the closest I could find in terms of gate voltages at a local electronics house. The extra power rating would allow me to add the map lights to the circuit without needing the transistor setup outlined in the consolidated thread.

So I soldered it in place, carefully making sure the pinouts were the same. I affixed a new heatsink of sorts, a beefy old nut, and used thermal compound between them with a nut and bolt securing them. The original FET was attached to the circuit board as its heatsink. I plugged the unit back in and voila, it worked!!!

So now I should be able to drive the front map lights off the repaired (and power upgraded) BIU. I'll do it after I recover from this "experience."
 

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2018 XT Touring CVT
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Thanks for the great write up.
How would you bypass the FET?
Some LED modules should not be dimmed, it can shorten their life.
 

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Discussion Starter · #3 ·
So you mean have the dimming system go from on to off, with no steps.

Without cracking open and soldering inside the BIU, the simplest would be to add a relay off pin A5 of the BIU. Pin A5 is held at ground to light the lights and goes to +12V to dim the lights.

So... the lead from pin A5 is snipped. A5 from the BIU is wired to the magnet ground of the relay. The other side of the relay magnet is wired to +12V constant on (not ignition). A5 to the lights is wired from the output of the relay. The input of the relay is connected to ground. It's a good idea to add a quenching diode across the magnet of the relay, since the voltage spike from turning off the relay could damage the FET on A5 inside the BIU (although in this case the relay is slowly turned off, so there may not be a need for the diode, but costs little to add it). The connector here has the diode pre-wired. A relay like this one _should_ work fine (It's rated for 40A; you could drive big things off the door switch with it). It has 90 ohms across the relay, so the current draw is quite small on the FET.

I say "should" since there is an issue that could arise. When the doors are closed, the BIU raises the pin A5 voltage from ground to +4V. This causes the lights to dim, but remain on. When using a relay, that would mean the magnet has to stay energized on only 8V (+4v and +12V across the magnet). Bosch style auto relays like the one at that link generally stay energized with only 5V across the relay. But no telling when ordering a relay without specific info from a data sheet.

The wire at pin A5 on the BIU is blue. When unplugging the blue harness from the BIU, and with it turned to face you, it is the 4th pin from the right (5th pin from the left) on the top row. Top being where the locking tab is.

So, if you're looking at the sketch I attached, operation is as follows...

1, door is opened, A5/BIU goes to ground.
2, the magnet is energized, closing the relay.
3, ground is connected to the A5/lights wire. Lights come on.
4, door is closed, A5/BIU goes to +4V, magnet should remain on.
5, A5/BIU goes to +12V slowly, causing the magnet to go off at some point.
6, relay opens, lights lose their ground and go off instantly.
 

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2016 & 2018 2.5i Premium CVT
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Good piece of analysis. Sorry that you weren't able to check the info here first. My previously expressed concerns about overloading the modules were pretty much academic in nature at the time I wrote them, but we have now seen a few situations where those concerns were confirmed by reality.

If bashfulbernie wants to simply turn the lights on and off without dimming, I think he could bypass the module completely and just wire things the old fashioned way, straight to the door switches from the lights. None of this modern integrated stuff. Hard wire!
 

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Discussion Starter · #5 ·
There's a small problem when wiring to the door switches. The driver's door is wired separately from the other doors for some reason. All the door switches feed into the BIU, with the driver's door having a dedicated BIU input, and the other 3 doors and the liftgate sharing an input.

So, if you hard wired all the doors to the lights, you'd mess with the discrimination between doors that the BIU sees. You'd have to isolate them with some discrete parts.
 

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2006 Forester XT
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A change to some 1 watt LED 'bulbs' would let you run everything to the dimmer, and then some - with much more light in total.....

S
 

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Oznium.com is one source.
All LEDs are dimmable, but it can shorten the life of some types.
 

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Discussion Starter · #9 ·
Dimmable LEDs would be the way to go if you like that color tone, with no need to modify the dimming circuit. I prefer the incandescent tone, though
 

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2003 MY03 X
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Hi Dougr,
Sorry to dig up an old thread but i have had a similar issue with my 03 2.5x forester. I have courtesy lights under the doors the run off the ignition barrel light and recently there was a short. After the short, the barrel light no longer works and the cabin light does not work on the door setting. Does the 03 model have the same Body Integrated Unit as you mentioned and pictured in your first post?

If this is not the appropriate place for a response i have a thread open. in interior electrical however i am not alloud to link to it till i have 15 posts.
 

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2006 Impreza WRX with PPP Manual of course! ;-)
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Hi guys, this seems to be my case too after looking to see if I could power up some foot-well lights in my 01 SF5 from the ignition ring light wiring (I wasn't too comfy lying under the steering wheel and misjudged the connector with my electricians screwdriver, a small pop later, and I knew it was trouble).

The lights still work but don't dim any more, I've disconnected the ignition ring light and the dome light is now off all the time to stop the battery from draining.

Is there an 'easy' solution to this one? (read "cheap"!)
 

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Discussion Starter · #14 ·
It's been a while since I did all this work, but my recollection is the following: if the FET is fried it still works as an on-off switch without the dimming. Therefore the relay circuit would work. The easy way to know is this: if the lights still come on, the relay will work. If the lights are out, the relay will not work.
 

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2004 Forester XT Premium 4EAT w/ 4WD on demand
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i noticed something odd today while getting gas, my ignition light still comes on and when i tested the voltage yesterday i got 5v on the switched wire. Is there anything else b/t the BIU and the dome light i should be looking at?
 

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2005 Legacy GT Wagon 5MT
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Discussion Starter · #17 ·
I'm happy to see you made use of the info. That was the whole reason for posting! Thanks for the reply.
 

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2001 Legacy GT
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dougr,

I could really use your input on my issue. I have 2001 Legacy GT, however I have swapped a brand new 2004 Turbo Baja bulkhead harness to do a STi swap. The issue I am having is I get no illumination when switching my lights "on". I have a video showing the issue.



I have tried two used integrated units, but not a new one. I am fairly certain I have checked all fuses and they are not blown.

Any ideas or help in diagnosing this would be great. I have been stuck on this for months.

The only wiring I have changed is I spliced in STi plugs, but i only tapped into wires, so unless two pins are grounding in the plug I don't think there would be a reason this should affect the illumination?

The stalk on my 2001 is supposedly the same as the baja, I have tried a STi stalk and an 02 legacy stalk as well, same results. I can see from the wiring the harness itself is different but my 2001 harness used an illumination control module with no integrated unit and the baja just uses an integrated unit to control so I assume the difference is handled by the BIU.

thanks for the help!

Ben
 

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Discussion Starter · #19 ·
Ben,

Wow, that's way more involved than my issue. It's been a while since I did this whole project and I can't actually recall exactly what my BIU handled, and I can't find the service manuals on my laptop here. They must be on an archive drive I have laying around.

I don't know if I can be of much help! Sounds like a tedious tracing job. When you say you've used a 04 Baja harness, that means you still have the 01 Legacy BIU in place, correct?

Doug
 

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2006 forester auto
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Body inegrated unit, dome dimming circuit: data for and repair of

sorry to revive an old thread but im having problems with mine too..after doing this fix, it work for me for a few days then the dome lights stayed on and will only turn off if on the off position it will not turn off when the door closes or when u lock the door when in door position.. before i did all these i added wheel well lights and a key ring light. i have a 06 forester x..also another question is can i just buy a used biu from ebay theres a few that are under $30 but was wondering if i can just use any from 06-08 or does it have to be the same part numbers thats on the biu? mine is a U 88281 SA130.. theres a few SA150,SA120 and the first letter is different eg. like M 88281 SA150 from an 08 XT..any help would be appreciated.thanks in advance!
 
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