Both my street 85 K100rs and my parts bike RT are equipped with the vacuum sensing switch mounted on the inside of the fuel rail and plumbed to the vacuum port on the #1 throttle body. I understand that it's signal goes to pin #15 of the Ignition Module. I also understand that it was "discontinued" by the end of the 1985 model year run.
It appears to be an "on/off" switch.
What is it's function? What will occur if it fails or works incorrectly?
I've read a lot of hours in these forums and I've not been able to locate the information. The FI troubleshoot section shows it clearly on pin #15 of the Ignition Module but makes no mention of its values or reasons?

It changes the ignition timing depending on the load on the engine. Remove it and feed it to a dumpster you will not even notice the difference. You wont get any change in power, torque or fuel consumption. BMW discontinues it because it did nothing.

Hi Ibjman

Have a look at the recent post 'Where does this go?' or similar heading.
Same question asked. Its in the

Too hard. HERE is the link

does anyone really know HOW it functions?........is it a simple on/off switch set at a certain value or does it change resistance with various vacuum readings.
One reason I ask is: if you disconnect & plug the vacuum line, must you also tear the fairing & airbox all apart (again) & disconnect the wire to pin #15?
I appreciate everyones recommendation to "just unhook it" but I'd like to be able to understand it's designed function (even though it proved in effective)

It's a simple on/off switch that turns at a preset vacuum. Best to remove the switch as well and tuck the wire up out of the way which involves removing the fairing. I would just make sure the port on the FI stub is capped and do the rest when you have the fairing off for other reasons as well.

The Vacuum switch is a N/O switch that closes with the presence of vacuum signal coming from the #1 throttle body (manifold vacuum) test port.

It is calibrated to 0.06 bar to close. I measured it in USA terms & found it closes at 3"Hg. The others are correct that it grounds pin #15 of the ign module.

It has been reported that this changes the ignition timing. It has also been reported that it has no verifyable function on engine performance and was discontinued after 1985.

I would guess that it's designed purpose was to "advance timing to normal (from a retarded start point????) as soon as the engine has any vacuum. Cranking vacuum? I still wonder what was the intended engineering idea of changing the engine ignition timing when the engine drops to less than .06 bar. In my experience.....even cranking vacuum will be higher than that?

Is it possable that the test port vacuum would drop below that point during conditions of W.O.T.

I have not tested vacuum at the #1 throttle body supply port during any conditions.

Who knows? I have never been a supporter of disabling any part. My vehicle is now running well with the vacuum switch in operation.

To the best of my knollege there is nothing detrimental about the switch so why stuff with it if it's working without any problems.

Could it be, that there may be something actually beneficial about keeping it (even if it may not be readily apparent)?

A lot of folks say that because they don't SEE any effect of whatever subtle timing adjustment it creates that there is none.

Also, when BMW discontinued it's use I think it's higly likely that they made some other adjustments to the fuel/timing overall management to offset the removal of the switches function.

My best guess is that it could have been intended to retard the timing a few degrees (anti-knock) during periods of very high throttle load at lower rpm ranges????? I guess I need to connect a vacuum gauge with a TEE connector to throttle body #1, hang it on the tank and go for a ride to see if I can create a situation where the vacuum actually drops below .06 bar while running.

This old thread came in handy. I used it to try and settle dispute. I still lost the argument but at least they could see I wasn't the only one disconnecting it and not noticing any change. Would love to know if ibjman ever did his test and the results. It will save me doing it.

I have a mix of K bikes and often find myself in this debate. What I do know is my earliest K, from 1983, is a darn site more lively than my two 84 Ks. These three all have the vacuum switch. On my 84 RT, I did not notice much difference but I haver kwept the vacuum switch functional. However my 83 RS, it has the engineering screws on the cam cover and also has the vacuum switch, this is a seriously quicker bike than the others and ther K1100LT I also have.

They will run perfectly if you just use a vacuum cap on the No 1 throttle body in place of the vacuum pipe.

On RT/RS models you only need the fairing lowers off, plus the air box bottom part out, to access it. First time you go in there use lots of copper grease. Your obstacle is likely to be the two Allen bolts inside the air box if they have never been disturbed.

Interesting, for a number of reasons.

1. Low Flying Brick started as a November 1st 1983 naked. No engineering screws but I know it's been down the road on the l/h side so the cam cover has been replaced.
2. I removed the vacuum switch the first time I rewired it and found no disernible difference
3. Later I added K1100 throttle bodies and advanced the timing by six degrees - that definately made a difference
4. While LFB was still wearing an RS fairing, I tested it back-to-back with a standard K100 (Brick 2, also with an RS fairing) and initially thought that there was a problem with Brick 2 because it was so sloowww


So, there's something different about the very early bikes? Most I've seen on the clock was indicating 112mph with (it felt like) quite a bit more to come.

Very interesting. I've always been happy with the performance of my 85 k100rt. I certainly not looking for more go. The only other bike I own that out performs it is the F800st. But that would be comparing apples and oranges. I've finally repaired the fairing and have dressed the bike back to an RT. I will re connect the vacuum switch just so I can say I have done it. I will put a vacuum guage on it aswell so as I know all is in order.
Good to know the 1100 throtle boddies fit and make a difference. Now I'm going off topic but can you or someone tell me why the holes are in nose cone. I'm thinking cop lights or antenna?

I was going to say that your upper fairing is off an LT and the hole is for the radio antenna, but the hole is in the wrong place.

I think that's where the blue cop lights were, did it used to be a cop bike?

A few times it's been mentioned that cop bikes had a computer with more go. They do have a different part number.

Porto wrote:This old thread came in handy. I used it to try and settle dispute. I still lost the argument but at least they could see I wasn't the only one disconnecting it and not noticing any change. Would love to know if ibjman ever did his test and the results. It will save me doing it.

What was the argument that you lost? Maybe you're right. Connecting a vacuum gauge would only test the vacuum, it wouldn't reveal if the switch worked. I did check the switch with a test light and posted the results on here (or another place called motobrick) unfortunately I can't find it back. An even better test would be to check the timing with a timing gun at different revs, and then repeat the test with the vacuum hose removed and plugged.

Thanks for the replies. Was never s cop bike, but my guess is that the nose cone is not the original. I noticed that when I first pulled it apart for a repair and respray. That was back in around 2011. Was probably replaced after a spill. Unfortunately I hit a Deer in the National park soon after the respray. I've been riding it as a naked ever since. I've only just found the time to repair the body work and refit it. It will have to stay in primer now as I dint have the time to do the top coat. To be honest I like the primer look. As for only testing the vacuum? Naturally I'm going to test that the switch looks as well :-). I found a pic online of a police model and the blue and red lights are mounted in spots I have the holes. If only the bike could speak.

Porto wrote:If only the bike could speak.

It's just as well it can't. Would you really want a Brick whose talking points were likely to be in the categories You see that school bus, don’t you? Your MOT inspection isn't overdue, is it? Don’t look at her, mind your riding! You're not in the correct lane position. Do I smell alcohol? Slow down, mate!

ibjman wrote:
Also, when BMW discontinued it's use I think it's higly likely that they made some other adjustments to the fuel/timing overall management to offset the removal of the switches function.

I'm thinking this too. Since engine's have a timing advance at revs, an additional vacuum advance fuel efficiency at Idle would be difficult to notice considering it would only be at idle for a small part of the overall time, for example. I would say BMW did their homework and wouldn't put something on if it was useless. 

I'm thinking the brick was the first BMW bike with a computer, superseding the carby, at least at the very early stages. I think vacuum control of the timing, and also fuel pressure, was due to convention (carby day's thinking) The computer has sensors for engine load, speed and many others, so vacuum control isn't needed. 

Think of all the ways this switch could fail, the wire could break or short out, the switch could get stuck on or off, the vacuum hose could get blocked or crack, leading to unmetered air in the engine. The computers can be programmed from the sensors and the vacuum switch deleted. I think this also explains why the K1100 fuel pressure regulator doesn't have a vacuum hose, but the K100 does.

Porto wrote: If only the bike could speak.

It can. One day I did a vin check on the brick in my avatar, the colour said police white. The original colour was white. It has many extra earth connections. The part number of the fuel injection computer is for a police bike. The part number of your fuel injection computer might show that it's a cop one.

Dai wrote:Interesting, for a number of reasons.

1. Low Flying Brick started as a November 1st 1983 naked. No engineering screws but I know it's been down the road on the l/h side so the cam cover has been replaced.
2. I removed the vacuum switch the first time I rewired it and found no disernible difference
3. Later I added K1100 throttle bodies and advanced the timing by six degrees - that definately made a difference
4. While LFB was still wearing an RS fairing, I tested it back-to-back with a standard K100 (Brick 2, also with an RS fairing) and initially thought that there was a problem with Brick 2 because it was so sloowww


So, there's something different about the very early bikes? Most I've seen on the clock was indicating 112mph with (it felt like) quite a bit more to come.

I didnt mention, my 83 RS and the 84 Sprint have the same 1983 engine manufacture dates. The Sprint started life as a naked [discovered when I registered it in Ireland in March 2023] and does not have the engineering screws that are on the RS!