Hey Friends,
    I have a weird one for you. The electrical system on my 1985 K100RS is more or less stock. I did make one change, though: I connected the two leads for the altitude corrector plug to a switch, using the last remaining switch blank in the dashboard (After grip warmer and hazard lights). I even drew a tiny picture of mountains on the switch  . 
    I've never had to turn the switch off, as the five years and 16,000 miles I've done with this bike have all been above 3000 feet.
    Here's my question, though. If I do ever go below 3000 feet, should I turn the bike off to flip the switch or just do it on the fly? I've been looking at the wiring diagram and don't see any reason that flipping the switch while riding would hurt the bike in the slightest (I think it would need much more advanced electronics for that), but I'm not positive. So I thought I'd ask. Any thoughts?
    Thanks!
     -Jon

The altitude corrector shouldn't be needed below about 10,000 feet, IIRC. Highest I've been was just over 16K on a normally-apirated bike and that struggled badly from about 9K upwards. (Yes we did change the jetting but it still wasn't happy).

I've had my K75RT over 9,000 feet without the altitude plug and at other times been down to sea level with it plugged in.  I have never noticed a difference in the way the engine ran.  I don't bother with it anymore.

I suspect it's something to do with the emissions and not using it has no adverse effects on the way the bike runs.  I would just ignore it.

As far as operating your switch, I don't recall the manual saying anything about shutting the engine off to install the plug, so I would guess it doesn't make a difference.  You might want to double check your user's manual just to be sure.

I can't provide an answer to your question but, have you ever flipped the switch at the elevations you ride? If yes did it change the way the bike ran?
What I do know is that my K75 (92) will not idle very good above 10,000 feet but seems to run fine above 3,000 RPM. Both my K100RT (87) and K100LT (88) seem to run fine up to 15,000 feet. Never tried the plug in on any of them at lower elevations.
Of course none of this is any good to you as I suspect the RS has a different fuel map as the rest of the Ks.

You can flip the switch at any time - running or not.  The altitude jumper nearly provides a modified signal to the ECU telling it to enriched the mixture.

To clarify, we want a leaner mixture (less fuel to available air) at very high altitude because the sea level-plus mixture will be too rich, but running with the 'altitude plug' below 10K feet is probably running the bike leaner than requirements. Look at the spark plug tip ceramic after a hard run up a hill at high altitude after a quick shutdown.

It makes no difference to the relatively crude ECU if the signal changes with the flip of a switch (a good idea) while running.

Thanks TWB. The way the altitude plug is wired is that it simulates twisting the throttle wide open and engaging the second position on the TPS. This is a picture of the connection that plug is making:

It applies 12v (Green/Red) to pin 3 of the TPS which is essentially pin 3 of the ECU. I’m not exactly sure what that condition triggers within the ECU but I thought it normally would try to enriched the mixture to try and give the engine greater fuel at higher RPM (i.e. try to always keep the air/fuel ratio (Stochastic ratio) at around 14:1).This is born out in a section from the Bosch publication "Gasoline Fuel Injection System L-Jetronic - Technical Instruction" on page 25:

If I’m wrong, I would welcome being corrected.

Prolly not wrong, Rob, but it should not enrichen but in fact lean the mixture which is what you want at altitude. The way the function of the 'altitude' plug was conveyed to us in dealerships back in the day was that the round plug connection, part number 61 13 1 459 504, when connected, would be detected by the ECU through that pin you mention and open an approximately 10% leaner signal (how it did that was never explained in detail) for running at high altitudes and was a North American option only - for all 2 valve per cylinder K75 & K100 models. In fact, checking that number on ETK or realoem.com shows it as a US market part number only. Clicking the part number will show a link to realoem for those interested in obtaining the plug or for record keeping. The various harnesses may have the port in non-US models as well.

There are currently 3 (1 in CA, 2 in PA) of part number 61 13 1 459 504 'contact ring' in the US warehouses and 472 in the German DC and they're US$16.96 each.

Could it be that if the fuel injection computer gets a signal at pins 2 and 3 at the same time, this is interpreted as high altitude?

Dave, It's Bosch magic.

I'm wondering if the signal on Pin 3 causes the ECU to select another fuel map, where the region in low to mid throttle has a lean mapping and the region in full throttle has a rich mapping. That can then serve both purposes.

I suspect it's a placebo designed to get the EPA to approve the fuel injection system. Think of the VW Jetta diesel affair a few years back.

In my opinion, the barn door and throttle position switch do a pretty good job of measuring airflow and compensating for air density due to altitude. The lower density air at high altitudes won't deflect the barn door as much at a given throttle position and engine load, causing the ECU to inject less fuel as the altitude increases.

I've forgotten to install or. conversely, remove that plug often enough to feel pretty confident that whatever it does isn't very noticeable as far as drivability performance. Case in point, I forgot to plug it in before riding to the top of Pike's Peak and still had a very nice ride.

That also makes sense. So if the fuel injection computer gets a signal from both pins, might that mean it enriches the mixture by a lesser than normal amount under full throttle? (I'm thinking the only way the computer can get a signal from both pins at the same time is with the altitude plug installed)

There's nowhere in Australia to ride a bike over 10,000 feet, the highest mountain is ~7,300 feet and a bit, and not many people around there.

Point-Seven-five wrote:In my opinion, the barn door and throttle position switch do a pretty good job of measuring airflow and compensating for air density due to altitude.
  
I suspect it's a placebo designed to get the EPA to approve the fuel injection system.  Think of the VW Jetta diesel affair a few years back.

That is an astute observation; however, the Jetta Diesel affair was not down to a placebo but rather to software designed specifically to provide acceptable emission data during laboratory inspection operation that was impossible to duplicate during actual highway operation.

On the other hand, BMW's altitude contact ring—rather than being a placebo—is more likely its typical belt-and-braces approach to Brick engineering. Think of its threadlock recommendations.

Laitch, I was implying that there was a chance the plug was an attempt at gaming the bureaucrats to get approval for the Jetronic system much the same way the VW guys diddled the ECU software.  It's just an acknowledgement that sometimes you need to do things to protect a large investment in product design and engineering from the vicissitudes of "badge heavy" government morons.

I can see it now, some suck up government type who is a complete scientific illiterate recalls hearing someplace that at high altitudes the air is less dense and that was the reason why when they were a hippy back in the day their VW bus couldn't run worth a sh!t above 5,000 feet without having something about rejetting the carburetor or something.  

So when BMW comes to get a certificate of compliance said bureaucrat asks how to keep from causing global warming when somebody rides their K bike in the Rocky Mountains.  A technical explanation of how the ECU and the airflow sensor work to adjust automatically for altitude falls on deaf ears and BMW is required to provide a means of altitude compensation for the system.  An engineer wise to the ways of government agencies suggests a magic plug that tells the ECU to not make the mixture too rich.  That, along with a few bundles of cash to key members of Congress who make a few phone calls to the guardians of the environment, and the Jetronic system for the new K bikes is approved.

The result?  BMW gets to import their motorcycles, the bureaucrat is a hero and gets a nice promotion for screwing with a big motorcycle company and a few Congress critters are able to buy their girlfriends some expensive jewelry and nose candy.  We get to ride some nice bikes.  It's a win, win, win, win.

Truly excellent storytelling by Point-Seven-five.
As Einstein reportedly said "Imagination is more important than knowledge. For knowledge is limited ........"
Shep

Very, very limited, indeed.

Government corruption is the sole reason there is a plethora of "National Association of (fill in the blank)" with offices and huge budgets in Washington, DC.

Wow! I'm off the forum for a week and come back to find my weird little question has turned into a whole discussion. Cool!

Therefore, I figured I owed it to you guys to do some experimenting. I just got back from a ride done in the name of science. It definitely had nothing to do with the fact that it's a gorgeous, crisp early-fall Colorado morning. Air so clear it looks like the glaciers of the continental divide are 100 feet away, aspens just starting to change...  No, this was a selfless ride of science.

I rode between 5300 and 8300 feet. Once I determined that switching my elevation switch while the bike is running doesn't hurt anything (it doesn't), I started switching it back and forth at different RPMs and at different elevations. And the result is... Nothing. No discernible change whatsoever.

So... I don't know. Maybe it does nothing. It would make sense to have it lean the mixture for high elevation. Speaking of which, a few of you have mentioned using it only above 10,000 feet. What I've read says to use it above 4,000, which makes sense to me. I live at about 5400 feet and with carbureted vehicles here you need to jet them a little lean or they'll get sooty pretty quickly. Modern vehicles correct for altitude automatically, of course. Seems reasonable that BMW would give you a setting that would be a good average mixture for riding around the mountain west.

Or maybe it was just for emissions certification 

I live in Ireland..my K100LT has one on it. Not sure of the reason, its ex UK police bike and no roads above 3,000 feet in either country. Curious.

I had fiddled with it, as said it does nothing.

OK here's my guess, it only makes a difference at full revs. The air flow meter could detect the density at part throttle but not full load. If the computer gets a signal from pin 2 (full load) and pin 3 (altitude plug) at the same time, it knows that the altitude plug must be switched on and reduces the enrichment by 10%. If you have 10% more than you need it would be difficult to notice the extra, mostly wasted fuel. 

If you cut the engine straight after a full load at 5,000 feet, or better 10,000 feet, and looked at the spark plugs, you might just see a tiny, tiny difference. As a test, maybe an exhaust analyser might detect a difference.

Edit: once again, I didn't explain myself well. The point I was trying to make, with a signal at pin 3 (throttle at idle, or altitude plug connected) and pin 2 (throttle at full load) the throttle can't be at both positions at the same time, so that would mean full load at high altitude. Although the air temperature sensor helps with air density, it doesn't compensate for reduced density at altitude with full throttle.

Bikesmith wrote:
I rode between 5300 and 8300 feet. Once I determined that switching my elevation switch while the bike is running doesn't hurt anything (it doesn't), I started switching it back and forth at different RPMs and at different elevations. And the result is... Nothing. No discernible change whatsoever....
...Or maybe it was just for emissions certification 

Thanks for confirming my observations.

If indeed the plug only affects full load/wide open throttle operation, my observations make sense in that there aren't many opportunities for opening the throttle much beyond occasional blips in the places where I ride at high altitude.