Post by: Atabeach on June 27, 2026, 02:23:55 PM
HALL SENSOR...
Post by: Laitch on June 27, 2026, 04:56:38 PM
Click this link (https://motobrick.com/index.php?topic=11014.msg95717#msg95717) to the opening post of the Service Manual section here. Click on the link to Vogel's troubleshooting guide at the bottom of the opening post. Perform the tests of the throttle switch and the coolant temperature sensor explained on pp. 25–29. Let us know the results.
Post by: Atabeach on June 27, 2026, 08:56:01 PM
1) The post referenced non start issue regarding throttle switch. Page 7/8 I did not see how to test it. For historical purposes the bike is an 85 and …backfires. Maybe It should be changed. Please advise.
2) I did not see how to test coolant temperature sensor either. Please let me know the ohm value or how to test the unit ….
I did discover an oil leak coming from the bottom of the air box ( photo 1)
Thanks again.
Post by: Laitch on June 27, 2026, 10:22:49 PM
Thanks Laitch, So I checked the Grounds and Positive terminals and then spent about an hour reviewing Vogels detailed post. Very informative, thanks.The photo of the air box in your post is of the intake air system on an 4-valve engine, not a 2-valve engine as should be in an '85 K100. The 2-valve air box that connects to the throttle bodies is pictured below. What's going on, Florida Man? :laughing4-giggles:
1) The post referenced non start issue regarding throttle switch. Page 7/8 I did not see how to test it. For historical purposes the bike is an 85 and …backfires. Maybe It should be changed.
(https://motobrick.com/gallery/4/1601-270626221937.png)
(https://motobrick.com/gallery/4/1601-270626222121.png)
Post by: Atabeach on June 27, 2026, 10:38:17 PM
Can i assume since the fan clicked on the coolant sensor is good?...or is there a different sensor?
In Vogles post he stated that the hall sensor seemed to act up when heated...we are in Florida.
Post by: Laitch on June 27, 2026, 10:59:35 PM
Ok soooo no internal hose, Check!Check? None of that assembly belongs on a 2-valve K100 Brick so what's a photo of it doing in a thread about the performance of a 2V Brick engine? I don't know what you've got going down there in the steamy sub-tropics. Will you offer a comprehensible explanation? My next dose of coffee isn't until after 8am tomorrow.
177381
Post by: Atabeach on June 27, 2026, 11:08:46 PM
Post by: Laitch on June 28, 2026, 03:50:29 PM
I did discover an oil leak coming from the bottom of the air boxIs this a photo of one of your Brick's throttle bodies? If it is, what is the blue linear object in it?IMG_7491.jpeg (53.38 kB . 768x576 - viewed 331 times)
Post by: Atabeach on June 28, 2026, 04:40:11 PM
Post by: Laitch on June 28, 2026, 10:09:45 PM
Closer inspection is needed.
Post by: Atabeach on July 01, 2026, 05:48:58 AM
The post by Vogel specifically mentioned running rough when bike is hot would point to the Hall sensor. I tried to remove it from the bike to test it but the 5 pin connector crumpled miserably in my hand. I need to change it anyway so hopefully that was the issue.
Post by: Atabeach on July 03, 2026, 04:31:34 PM
Verify that the battery positive clamp, the battery ground clamp and the battery ground cable attachment on the transmission are tight.It was the Hall sensor. Vogel had it right...when the bike fails when HOT....
Click this link (https://motobrick.com/index.php?topic=11014.msg95717#msg95717) to the opening post of the Service Manual section here. Click on the link to Vogel's troubleshooting guide at the bottom of the opening post. Perform the tests of the throttle switch and the coolant temperature sensor explained on pp. 25–29. Let us know the results.
Post by: Laitch on July 03, 2026, 06:21:43 PM
Post by: Edward on July 03, 2026, 07:20:07 PM
https://www.chenyang-gmbh.com/en/hall-effect-vane-sensor-cyhme56c-output-npn-oc-power-supply-4.5-v-24-v-dc-alternative-for-2av54-2av16a-2av51a-2av56-honeywell.html (https://www.chenyang-gmbh.com/en/hall-effect-vane-sensor-cyhme56c-output-npn-oc-power-supply-4.5-v-24-v-dc-alternative-for-2av54-2av16a-2av51a-2av56-honeywell.html)
Post by: Atabeach on July 07, 2026, 09:39:05 PM
Post by: Laitch on July 08, 2026, 12:08:31 AM
The multimeter is set to Ω. The ignition switch is Off. The ground lead touches contact #1 of the fuel injection plug, the positive probe contacts pin #10. There's a photo of the plug with the contacts numbered in Vogel's a few pages after the Hall Sensor section. At ambient temperature the multimeter should read approximately 2.5KΩ.
If the values are close to that number, start the engine and point a thin stream of propane at the joint of the z-tube with the air box as shown in your photo of the oil leak. If the revs change during spraying, that's one problem uncovered. If okay, move on.
Post by: daveson on July 08, 2026, 08:36:50 AM
This short video only shows the test results for one output, the test should be done for both, the brown wire and the orange wire.
https://youtube.com/shorts/P3X0zvDY6Vw?si=V3UK2QuetQEQQroF (https://youtube.com/shorts/P3X0zvDY6Vw?si=V3UK2QuetQEQQroF)
If your brick is the earlier model of 1985, the coils aren't the best. You could use a timing gun to check for a good signal from the coils to the spark plugs, or even look at them for cracks. In your troubleshooting guide, there are also tests for the coils.
Post by: Atabeach on July 08, 2026, 09:47:19 AM
1) test temperature sensor & propane around z tube , thanks Laitch!
2) test coils ( I think I have spares from box of stuff )
3) remove the used hall sensor I bought and bench test
Thanks, I’ll post the results
Meanwhile in the garage… another barn find. $500, plus handling😳
Post by: Laitch on July 08, 2026, 09:58:29 AM
Meanwhile in the garage… another barn findThat's one of the rare BMW unicycles. Great find!
Post by: Atabeach on July 08, 2026, 07:04:42 PM
1) Quick look at spark plug wires no signs or arcing...Coils no cracks , will bench test shortly.
2) test value of female pin 10 to ground is 1.5...this is below 2.5! I will be replacing temperature sensor either way....
3) bench test hall sensor..pins 2 & 5 ....all appears good.
Q Should I apply heat ( blow dryer ) to hall sensor?
Q Is the value of the temperature sensor @ 1.5 so low as to keep the bike from running?...TIA
Post by: daveson on July 08, 2026, 08:59:38 PM
Give the electrical plugs a good clean with the help of some electrical contact cleaner.
If the situation is still that the problem is much worse when the engine gets warm, this would be the best time to do tests, as it doesn't have enough time to cool down compared with bench testing. Also it could pass a static bench test, but might not pass a dynamic test while cranking.
Here's another, even quicker test. The test in this twelve second video will verify if the Hall sensor is sending a pulsing signal to the computer, and also verify if the computer is sending a pulsing signal to the coil. Then repeat the test for the other coil. This could be done with the fuse for the fuel pump removed.
https://youtube.com/shorts/5VrtFtr07-4?si=M8VIfew8xUk24JT2
Post by: daveson on July 08, 2026, 09:14:45 PM
https://youtube.com/shorts/Hqttqo-y0xg?si=H2fvwdDZ2N2eMD-G
Post by: Atabeach on July 08, 2026, 09:30:09 PM
Post by: Laitch on July 08, 2026, 10:02:28 PM
Q Is the value of the temperature sensor @ 1.5 so low as to keep the bike from running?...TIAIf the value at the temp sensor was at 1.5KΩ when the engine was cold, that wouldn't be likely to activate enough fuel for a good start.
Post by: daveson on July 09, 2026, 12:14:34 AM
… In the 12 second video the probe enters the boot, does the wire lead go to ground…
The test light clamp is attached to battery positive. I posted more details in the comments section, I probably should have posted it in the description. Actually I should have shown it on the video, it would have only been a few extra seconds.
Post by: Atabeach on July 09, 2026, 05:40:57 PM
Some observations and my thought process:
When the problem occurs the front exhaust pipe tube and the rearmost pipe are significantly cooler. Using an infrared sensor confirms this….They are not firing as they should. I believe this is cylinder 1 & 4
Tests:
when the problem is occurring… I will use a test lamp at the back of Hall sensor connection , pin 2 and or 5 to negative terminal…).
When the problem is occurring I will test the black wire clip on coil 1 & 4 …to a positive terminal. Depending on results…
I will then bench test coil or swap them with extras I have.
Please let me know if this makes sense or if I am missing something
TIA
Post by: daveson on July 09, 2026, 08:05:01 PM
We don't know the quality of your spare coils. One option, swap the coils on your bike, if the problem transfers to cylinders two and three, it's probably a bad coil.
On my 1984 brick, both coils had cracked cases, but the cracks could only be seen after removing the coils. They both had open circuits, so that would probably be a quicker test than swapping them.
Notice in the Hall sensor video that the test wire is clamped to the battery negative post, not the positive. It is tricky. The pulsing signal from the Hall sensor is positive, but the pulsing signal from the computer is negative.
To muddy the waters even more, they came up with a real doozy. I would have loved to be in the meeting room when they designed this one, but I can imagine how it went. Someone in the meeting says "Hey, the two positive signals from the hall sensors, let's have an orange wire for one, and a brown wire for the other, because every other brown wire on the bike is negative" They all started laughing, then someone says "good one Fritz, let's go with that"
Post by: Atabeach on July 10, 2026, 05:59:30 PM
COILS GOOD.
The hall sensor was bench tested...good..UNTIL
The hall sensor connection was tested ON BIKE...( 12 SECOND VIDEO ) .Orange to lamp to negative terminal..No light! Same with Brown.
WTF! the bike starts!
Post by: Laitch on July 10, 2026, 06:01:50 PM
Post by: Laitch on July 10, 2026, 07:10:16 PM
so the coils were tested on bike and then removed..bench tested...2.6 ohmsWhat's the value at the secondary terminals?
COILS GOOD.
Post by: daveson on July 10, 2026, 07:36:57 PM
When I found out that both coils on my brick were open circuit, I said to myself "WTF, but the bike starts and runs" So I said it again, I said "Daveson, what the fuck"
It was very difficult to start, but when warm it would start easy and run well.
Here's my guess, which may apply in your case as well. I guess there was a crack somewhere along the circuit. Depending on the location of the crack, sometimes the crack gets larger when warm, sometimes it gets smaller. The electricity can jump the gap, as it does at the spark plug. The Hall sensor problem could be opposite to the coil problem I had, therefore when it gets warm, the gap gets bigger, and it struggles to cope. I think the situation with my coils was very unusual, because every other time I've heard about coil problems, they failed when they got warm.
Maybe it's just the hall sensor plug. I would repeat the test on both sides of the plug. If the test result is good on the input side of the plug, then you need a new plug, because the test result was a fail on the output side.
Post by: Laitch on July 11, 2026, 02:37:27 AM
It's good to know I'm not the only one who has WTF moments.Of course you aren't; we even have an emoji here for the occasion. :thisplacewhack
Post by: Atabeach on July 11, 2026, 07:24:59 AM
Laitch…OL reading across both secondary spark plug terminals in BOTH coils. I used two different multi meters. 2.8 across primary. The bike starts and runs.
Observations, With the bike running:
Test light from primary coil to ground illuminated test light but slowed idle. (Probably normal)
Test light from hall sensor female pins ( back of clip ) 2 & 5 to negative terminal did not illuminate small led or test lamp…
The fan clicks on normally when hot.
Continuity between hall sensor disk & wires / wire pins are good.
Not sure where FEMALE pins lead…please advise! I want to test continuity.
Question: I have 3 K 75 coils. Can I safely remove one primary wire & spark plug wire …and attach to one of the K75 coils?
Post by: Laitch on July 11, 2026, 08:51:03 AM
Laitch…OL reading across both secondary spark plug terminals in BOTH coils. . . . I have 3 K 75 coils. Can I safely remove one primary wire & spark plug wire …and attach to one of the K75 coils?:thisplacewhack
Post by: daveson on July 11, 2026, 11:16:46 AM
Of course you aren't; we even have an emoji here for the occasion. :thisplacewhack
Wow, I always thought of that as meaning "nothing to see here" or "boring"
I hope I haven't put any noses out of joint in the past, due to my misinterpretation of that one.
Post by: Laitch on July 11, 2026, 01:07:22 PM
Wow, I always thought of that as meaning "nothing to see here" or "boring"It was labeled by its creator (or enabler), "Thisplaceiswhack". :laughing4-giggles:
Post by: daveson on July 11, 2026, 07:39:09 PM
Test light from primary coil to ground illuminated test light but slowed idle. (Probably normal)
But was the light pulsing? I suppose it was. It's more rapid at idle than while cranking, so more difficult to see a pulsing.
Test light from hall sensor female pins ( back of clip ) 2 & 5 to positive terminal did not illuminate small led or test lamp…
With positive on both sides of the test light, the light will remain off.
ALERT! This morning started bike…ran rough ….TEST LIGHT from pin 2 & or 5 to positive shut bike IMMEDIATELY …test lamp does NOT illuminate
...the light...
Again, with positive on both sides of the test light, the light will remain off. This test provides constant power to the hall sensor, so the pulsing signal is lost, therefore the engine stops.
Continuity between hall sensor disk & wires / wire pins are good.
Not sure where FEMALE pins lead…please advise! I want to test continuity.
Don't test continuity because your multimeter supplies power to do this, Hall sensors are very sensitive and can easily be damaged. This is probably why Bert's guide warns us not to use an Ohmmeter for Hall sensor tests.
The Hall sensor signals go to the ignition control unit.
Question: I have 3 K 75 coils. Can I safely remove one primary wire & spark plug wire …and attach to one of the K75 coils?
I wouldn't, K75 coils are different.
One safe test would be to swap the coils to see if the problem transfers to the other two cylinders.
Post by: Atabeach on July 12, 2026, 11:49:48 AM
….both coils seem to provide good spark. The plugs are wet so for now I am hoping the new Bosch temperature sensor will solve problem.
Daveson, thanks again
My new addition is up and running ( see photo )
…another barn find 🙄🙄🙄
Post by: Atabeach on July 22, 2026, 08:13:10 PM
I did notice today that the voltage ( 12.4 at idle ) never climbed above 12.9 when increasing RPMs. The battery is new and tests "good" on my battery tester even under "Load"
Post by: Laitch on July 23, 2026, 08:58:19 AM
.What is the battery voltage when the ignition switch is Off?
I did notice today that the voltage ( 12.4 at idle )
Post by: Atabeach on July 23, 2026, 01:29:19 PM
I tested the TPS with test lamp and multimeter...zero ohms from pin 1 & 2....test lamp on until I turn the throttle.
Pressing the starter button at idle bogs the bike down instantly.
Post by: Laitch on July 23, 2026, 04:04:31 PM
What is the battery voltage when the ignition switch is Off?Your response to my query seems to indicate that some lights are on when the ignition switch is turned to Off. Is that the case?12.5 Volts. With some lights off she climbs to 13.2ish
Post by: Atabeach on July 23, 2026, 05:19:20 PM
Post by: Laitch on July 24, 2026, 08:02:41 AM
Post by: Duckbubbles on July 24, 2026, 09:39:09 AM
Frank
Post by: Atabeach on July 24, 2026, 10:24:03 AM
The battery seems weak and so does the alternator charging current. Has the battery light in the instrument cluster been intermittently flickering or faintly glowing when riding or idling?No. The alternator goes out and remains out unless she is about to stall. Agreed..13 volts low but should run the bike.
Post by: Laitch on July 24, 2026, 10:47:17 AM
Does the Brick start reliably? Does it ever start and idle until shut off?
Does it idle smoothly?
Can it be ridden any distance at all without conking out?
Post by: Atabeach on July 24, 2026, 02:26:10 PM
My thinking:
Although 13 v is plenty…I’m going to change alternator as it is probably getting tired.
I have a used fuel regulator from a k75…I’ll change it
While the airbox is out I will bench test the airflow meter.
….let me know if that makes any sense.
Post by: Laitch on July 24, 2026, 02:40:32 PM
While the airbox is out I will bench test the airflow meter.Is the airbox out now? If it isn't, why is it being removed?
….let me know if that makes any sense.
Post by: Atabeach on July 24, 2026, 02:44:19 PM
Post by: Edward on July 24, 2026, 02:54:30 PM
Post by: Laitch on July 24, 2026, 02:59:39 PM
Post by: Atabeach on July 24, 2026, 03:04:09 PM
Post by: Laitch on July 24, 2026, 03:27:02 PM
Am I mistaken to believe a faulty fuel regulator could cause a rich running condition? The temperature sender , TPS and coils seem to be fine. I’m just going with process of elimination. Does anyone think 13 volts is not enough to run the ignition system. Please advise.I don't believe an alternator outputting 13V while the engine is running is capable of producing engine performance reliably.
Certainly, a fuel pressure regulator be a cause of over-fueling, but so can a clogged fuel return line, a faulty fuel pump or a faulty fuel injection control unit, not to mention the air flow meter which can be tested in place rather than on a bench. Repair using the process of elimination can be too costly for many of us; that's why there's all the testing; however, replacing parts also can eventually improve performance, if the defective part—or parts—eventually are replaced. That seems to be the diagnostic strategy with this Brick. AI can make a great companion for that.
Post by: Atabeach on July 24, 2026, 05:08:34 PM
This model 1985 has a 33 amp alternator. Would anyone recommend a rebuild kit? Otherwise I am buying used off of Flea bay....
Post by: Laitch on July 25, 2026, 05:21:23 PM
Thanks Laitch, I will take your advice and change the alternator….That isn't my advice.
The battery reading should be ≈13.5V to 14.5V with the engine running. Resting voltage should be 12.6V or above. Why do your battery's figures differ? My advice is to determine the root of each condition one step at a time. Why is your Brick's battery at 12.5V instead of within the optimal range. One case here where the battery wasn't holding a charge was caused by the alternator's cable connection to the battery being loose at the alternator side. Another case involved a bad diode.
All electrical connectors and ground connections of a 40 year-old Brick are first suspects when engine performance is erratic. The components themselves are the next suspects.
Post by: Atabeach on July 25, 2026, 06:05:10 PM
The battery and grounds were removed cleaned and reinstalled…reply 1 ( Bike was sitting for a decade and all electrical connections were removed& cleaned.)
The battery…at 12.5 was tested using my battery load tester…showed good…re- assess as needed, 4 months old…
…The bike always seemed to charge and start right up. I never thought to test the charging system, when I did I still thought 13 Volts was sufficient to run…ignition. Guess it may not be.
So....now, I removed and bench tested alternator ( to clean all brushes & contact areas also )
Turns out she is tired. Ohms across the two brass armature rings should be 2.5….they read 5
Changing the original 40 year old alternator makes sense…as did the temperature sender ( ohm reading not perfect) and hall sensor….not big ticket items.
The coils and wires checked out ok and were not replaced.
I am following a logical order here not haphazardly changing stuff.
This is a $750 barn find…so I was expecting some costs….It still has the original fan!
Thanks for your …..help. 😳😳
Post by: Atabeach on August 06, 2026, 05:40:33 PM
Post by: Edward on August 06, 2026, 10:10:51 PM
Post by: Atabeach on August 06, 2026, 10:34:45 PM
Post by: Edward on August 07, 2026, 12:45:24 AM
Post by: Atabeach on August 07, 2026, 12:15:35 PM
Laitch had mentioned the air flow meter. I don’t know how to test this while component is on bike.
Lastly….mine is an early 85 and has this vacuum switch…( see photo ) ...anyone know whether it may be the problem….or how to test it ?
Post by: Laitch on August 07, 2026, 04:02:28 PM
so the coils were tested on bike and then removed..bench tested...2.6 ohmsYou've only done half the coil test—the input side. OL is not an acceptable value. Either your multimeters have been set too high and are not sensitive enough to make a reading, the test leads are not contacting the terminals, the leads are not well connected to the instruments or the multimeters themselves are not suited for the application.
COILS GOOD.OL reading across both secondary spark plug terminals in BOTH coils.What's the value at the secondary terminals?
Post by: Atabeach on August 08, 2026, 02:20:45 PM
Post by: Laitch on August 08, 2026, 05:09:13 PM
Based on your comparison with the K75, it's possible the coil can only supply enough current to allow the engine to idle. When the demand on the engine goes beyond an idle, high resistance within the coil cannot supply enough energy to meet it.
Do you have a spare coil pack or two?
Post by: Atabeach on August 08, 2026, 06:14:11 PM
Laitch...Reasoning 101: this happened suddenly. The bike was running flawlessly....Could BOTH coils get tired at the same time? ...or Could...the $30 multimeter be off?
The coils and wires are 40 years old...changing them is not a BAD idea. TIA
Post by: Scott_ on August 08, 2026, 10:22:21 PM
Post by: Atabeach on August 08, 2026, 10:29:24 PM
Post by: Scott_ on August 08, 2026, 10:51:05 PM
I have a set that is an early '93 and a set that is late '95.
93 set:
13.23 kohms on the plug side 0.7 ohms on the ecu side
12.89 kohms 0.7 ohms
95 set:
12.85 kohms on the plug side 0.6 ohms on the ecu side
12.87 kohms 0.6 ohms
I could sell you a set, however looking at your picture I don't think they would be a direct fit for the older k100. The plug wire set is different style as is the ECU connector is not a spade connection on the 1100's.
Post by: Laitch on August 08, 2026, 11:36:02 PM
Scott that would help. My inkling is what Laitch suggested…they may be tired. They are 40 years old . . .According to the photo in your Barn Find thread, the engine case reads 1984—42 years. It's likely the engine was completely assembled that year. The coils in the engines of the early models like that are notorious for becoming unreliable. That's well documented on this site and elsewhere. Later in production—late '85 or in '86—the coils were replaced by coils much more reliable. They could be identified by the orange tops of the secondary terminal towers.
The resistance of the K1100 coils is around 70% less than the primary resistance of the K1002V coils. Figures below are from the LE Jetronic Testing for K Models manual downloadable here.
My understanding is this particular difference can possibly lead to the overheating and failure of the ignition control unit. The failure wouldn't necessarily be instantaneous but definitely could be inconvenient or even dangerous depending upon where it happens. Maybe somebody who has done that swap can chime in. If not, you could be the pioneer! icon_cheers
Post by: Laitch on August 08, 2026, 11:57:43 PM
(https://motobrick.com/gallery/4/1601-080826235504.png)
Post by: Atabeach on August 09, 2026, 11:26:03 AM
in the meantime can anyone post the process to check the air flow meter…
Lastly, there is a fuel vacuum switch near the fuel regulator. It was eliminated in later models. What does it do? Should I leave it be?
Post by: Laitch on August 09, 2026, 01:02:40 PM
Lastly, there is a fuel vacuum switch near the fuel regulator. It was eliminated in later models. What does it do? Should I leave it be?Its effect is up for debate, except at BMW. It was removed because it took extra material and labor cost but it effects were debatable, even in Germany. The Removers won the debate. Leave it alone as an original production artifact, or remove the hoses and cap the source points of the hoses then remove the vacuum switch to give to your favorite grandchild to use as a mobile over its crib, or as a crackpipe in its bedroom.
Post by: daveson on August 09, 2026, 07:03:16 PM
this happened suddenly. The bike was running flawlessly....Could BOTH coils get tired at the same time?One possibility is that one coil was working well and the other coil was working, but not well. A crack could show an open circuit when tested with a multimeter, but the high current could jump the gap while running, which would be about zilch millimetres. It jumps a big gap at the spark plugs (one wasted) Now if the other coil fails, the problem becomes much worse.
I don't think the spark plug leads have been tested yet, that would be worth a look. Original leads from a K75 should not be used, they're different.
Post by: daveson on August 09, 2026, 07:33:52 PM
in the meantime can anyone post the process to check the air flow meter…
Lastly, there is a fuel vacuum switch near the fuel regulator. It was eliminated in later models. What does it do? Should I leave it be?
The air flow meter is dealt with in Bert's guide, which I think you have.
It's not a fuel vacuum switch, it's a vacuum switch. Check anyway the vacuum hose to the fuel pressure regulator, that it's not leaking, or kinked, or blocked, or partially blocked. Do these checks also for the hose to the vacuum switch.
Some people remove it because they notice no improvement in performance with it connected. With that reasoning you could remove a wheel nut and throw it in the bin, there would be no drop in performance but after a few years the disc might develop a warp. It might not warp either, but why bother second guessing the engineer's who designed it that way? It's not meant to improve performance, it's there to improve fuel economy, reduce pollution etc, mainly while cruising. They have put it there for a reason, so I think leave it connected.
At higher vacuum you want to advance the ignition timing because it takes longer for the detonation to complete in these situations. You can use an LED test light to check that it sends a signal to the ignition control unit, from memory I think it only sends a signal at part throttle. You could use a timing gun to check that the computer is altering the timing. I guess no one has done that yet on this forum. You could check the exhaust emissions at cruising, with the vacuum switch connected and compared with it plugged, but I'm guessing it would be a tiny difference.There might even be a drop in revs if it is plugged to remove the signal while cruising.
Post by: daveson on August 18, 2026, 05:24:47 PM
If we think about the carby days, we had a distributor with mechanical advance, vacuum advance, and points. These parts would wear out and become increasingly inaccurate over time and use, and often fail. The ideal gradual increase in advance required for increased RPM is not identical to the gradual change with the springs in the mechanical advance, but it was considered good enough at the time. So they weren't even accurate when new. Computers can use sensors with fewer parts that wear out, and can be programmed to provide the ideal amount of advance. A vacuum sensor was used for vacuum advance. Then it was realised that the computer can be programmed to provide the correct amount of vacuum advance without a vacuum sensor, due to the inputs it gets from the hall sensors.
BMW often made parts backwards compatible, in this example there would be no need to change the part number, anyone wanting to buy an ignition control unit would be supplied with the new version.
In later models with a motronic computer, it was realised that the fuel pressure regulator doesn't need vacuum assistance to reduce fuel consumption in high vacuum situations, again the computer already has enough inputs to predict the ideal amount of fuel required, and provides it according to its program.