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Cutting the infamous "brown wire"

Where is this , and can I get a picture ??
 

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What year is your bike? Some years don't have it.
 

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mwnahas said:
What year is your bike? Some years don't have it.

2002

I found this , but no pic and still un clear... :confused:
Can someone explain the functional difference between cutting the brown wire (or pulling the cat code plug) and disconnecting the air box sensor? Do they accomplish the same thing?

If you DO have a two-map motronic and you're NOT using the secondary (upgrade) map, don't do another thing until you SWITCH TO THE UPGRADE MAP!!!. On your '02, just snip that little brown wire and you'll be smiling. The two items do different things. The Brown Wire/Cat Code plug change causes the Motronic to use either the primary or secondary FI map, depending on weather the Brown Wire is "connected" or not, either by loop or plug. This of course is if your LT has the correct Motronic unit.

The air box sensor is used for the Motronic to determine when the timing needs to be retarded according to air box temperature, no matter which map is being used. The primary map retards the timing around 86 degrees F., the secondary map either at a different temperature, less change, or not at all. I have not seen any post stating what the secondary map does relative to temp, so maybe no one knows. Rhine West probably does after their work on the replacement chip.

My dealer removed the cat code plug last summer when it got hot. This summer I still had a little hesitation, so I unplugged the air box sensor. All vestiges of hesitation went away. Some have reported a fuel mileage change when unplugging the sensor, I did not see this. I plugged the sensor back in a couple of weeks ago when working on my throttle cables, no hesitation (weather is cooler now), and still no noticeable fuel mileage change.
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With no brown wire, your best bet is to unplug the air temp sensor.

I unplugged mine without removing anything. Of course I had looked at it on another service when I had the fairings off and knew the "lay of the land", and just what to expect. With a little dexterity, you can push up on the wire retaining clip and pull the plug out toward the front of the bike. Getting a finger behind the plug while holding up on the retaining clip is the tough part, but if you try a little while, it will work.

As originally programmed, the LT leaves the factory with a Motronic program that retards the ignition timing when the ambient air temperature is above 86F and the rpms are below 3000. This is done to ensure that there will be no detonation even with low octane fuel.

The ignition retard manifests itself, usually unexpectedly if the rider's not aware of this "feature," by the engine not delivering as much power as it normally does below 86F -- not really a stumble, but when you're pulling out from a stop and leaned over, rolling on the throttle the "usual" amount doesn't result in the "usual" amount of power delivery -- this can be at best an unpleasant surprise, at worst the prelude to a drop or an accident if oncoming traffic is too close.

< One would assume with a build date of 11/02 it would have a fixed motronic. >

It does, but you likely haven't invoked the "fix." Earlier Moronics had the hi-temp, low rpm retard map as the primary map. The "new" Moronics still have the same map, and to ensure the bikes can always operate without detonation, the old map is active when the bike is delivered. The new Moronics also have a second "no retard" map. To invoke this map on your 11/02, you (or your dealer) need to: (1) cut the small brown wire loop located under the right subframe rail, just aft of the battery; and (2) always use premium fuel if you will be operating the bike in ambient temps over 86F.



Last item: the air temp sensor. None of us know *exactly* what the new Moronic's second (no retard) map looks like. It appears that it eliminates most, if not all, of the hi-temp, low rpm retard. *However* -- some have noticed that the new map hasn't cured *all* the hesitation feeling. I'm one of those, and I even have the Rhine West chip which is supposed to have the ignition retard programming completely removed from its optimized mapping. If you look inside a Moronic, there's a lot of processing "stuff" in there besides the fuel injection maps, and I suspect that there are circuits that make additional adjustments based on hi. temp. For those that still sense less than optimum performance at hi air temps, disconnecting the air temp sensor at the front of the air box eliminates any ambient air temp input to the computer, and seemingly eliminates the last traces of the hesitation.

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You can disconnect it on any year LT.

The sensor is mounted into the front top corner of the air box, directly behind the radiator cap and just under the left top portion of the gas tank. The connector has one of those "press-to-release" wire spring retaining clips.

While some folks have been able to reach in from under the front of the fairing and over the radiator to disconnect the plug, there's no way I could get my fat paws in there *and* press the clip in -- I could get a finger on the connector, but that's it. I just waited until the next time I had the left tupperware off to disconnect mine. Don't forget to wrap the connector with something (electrician's tape in my case) to prevent rain water intrusion.


Pulled the cat-code plug on my '00 long ago. since it was the early motronic and software, instead of changing the hot-weather ignition timing it just disabled the O2 sensor. It ran a bit better IMHO but it backfired a bit on deceleration. I removed the catalytic converter so that can fills with mixture when engine-braking, then BLAM it would blow. After waiting 3 years for the Motronic upgrade the bike just went out of warranty and I gave up on BMW and got a Rhinewest chip. I ordered it Monday and it arrived Tuesday. Really easy to install, since I already had the bike apart. I hope it works, because it sure was expensive!


Cutting the brown wire activates an alternate fuel map. It's located under the right-hand frame rail under the rider's seat. Rather than just cutting it, cut it and install a toggle switch that you can switch back-n-forth between maps and experiment what works best on your LT. Your LT has to be OFF when changing the position of the switch to activate the other fuel map.
 

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Mine is an '03 so your mileage may differ. Lift the seat. Under the right frame rail is a big bundle of wire. The brown wire is part of this bundle and has a few inches exposed and a loop near the preload adjuster.
 

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I have an 03 LT which is very sluggish a lower RPMs. I have raised the seat and there is a brown wire that goes into a two wire connector. The other wire is green with a brown strip. Is this brown wire the one to cut? :bmw:
 

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free2ride said:
I have an 03 LT which is very sluggish a lower RPMs. I have raised the seat and there is a brown wire that goes into a two wire connector. The other wire is green with a brown strip. Is this brown wire the one to cut? :bmw:
If I remember correctly, that would be your heated seat connector (Grn/Brn, Brn for ground). I believe you're looking for a different wire.
 

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Thanks very much. I found an old thread with a photo and went to my bike and there it was. Snipped it and will test it out tomorrow morning.

I love the bike and getting rid of the hesitation will be great. :bmw:
 

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Here's a photo from my '03
It's the thin brown wire not the fat one
in the photo I've attached white wires to a switch to the brown wire so in case I'm in a spot where I can only get regular octane, I can lift the seat and flip the switch.....(never had to )
 

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I'm new. Read the posts, deliberated for more than 6 months.

I cut the brown wire. Now I have a much different bike-- couldn't be happier (the bike, especially).
 

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Me too, Raindog. I didn't have any problems until it got really hot this past summer. Cut the wire, taped the ends, and now it's all good.
 
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