1993 BMW K1100LT Cafe Racer Motoscope Pro Tach Project

[mjones866]

Admitted first time poster, long time reader.


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I am working on a 1993 BMW K1100LT cafe racer project (I know, blasphemy) and I was able to wire in the Motoscope Pro without headache, except for the tach input. I am using the BEP 3.0 to interface the OEM instrument cluster plug with outputs to the Motoscope Pro. Just to note, I am not using the RPM output on the BEP 3.0 as I was not getting a reading on the Motoscope. I'm now bypassing the BEP's RPM output in the below explanation.

I tried directly connecting the tach input to the negative (low tension side – black/blue wire) of the #1 ignition coil. I got a reading on the Motoscope Pro with an accurate and stable readout on initial start up. I began to rev the engine and at about 1200RPM the tach bottoms out and the there is no longer a reading what-so-ever. It doesn't even rebound at any point. All values show 0, even the numeric output as well as the sweep across the top arc. I even gave the #2 coil a go (black/red wire). I have tried to tap into the tach signal wire (black/blue wire) coming from the Motronic EMC unit that leads to what was the OEM brotbox instrument cluster trying to see if the reading would be different/stable/miracle fix when the Motronic brain sends the tach signal to the instrument cluster. Sadly, the same issue arose where I got a good strong reading, but the tach bottoms out at around 1200RPM.

I did a voltage reading when I tapped the black/blue tach signal wire from the Motronic. The voltage, at idle, just having started the engine, and where the reading on the Motoscope Pro was at the expected and accurate RPM value that I would expect was a strong ~12.5v. At this point I rev the engine over 1200RPM, the voltage reading now jumps up just over 13v (it sits steady at just over 13v there after). This is where the RPMs bottom out and no more RPM values are available. I have tried the various param settings and filtering options in the menu without success. The unit does stay powered on.

Now for my question. Thanks for reading and hanging in there. From what I have read, the K1100 engages the alternator and begins to charge the battery. Could this lead to the spike that's over 13v and the Motoscope Pro doesn't like this? Could it be sensitive to anything just over 13v? Any help/clarification/direction/tips/insights would be greatly appreciated.

Thanks

[Laitch]

Consider posting a photo of your BEP and its present wiring connections. Describe the colors and connections of the wires to the BEP when it didn't operate the tachometer.

[rbm]

You might have blown the Motoscope.  The output on the Black/Blue wire from the coil primary is like this:



This is for a 87 K75 but a 93 K1100 will be similar.  The spike can reach 80 Volts or more.  That signal needs to be conditioned to bring it inline with the gauge's needs, a good square wave of 7Vp-p - 12Vp-p.  The unconditioned signal would have overloaded the input section of the gauge.

Why did you bypass the BEP 3.0?  If it wasn't working properly, get support from Maru-labs or the store that sold it to you.  Guess you might have two things to RMA now.

BTW, taking a voltmeter to the coil primary will not indicate much at all.  Besides, which scale did you have the voltmeter set to?  The response time of the voltmeter's internal circuitry will cause a false reading.  It might only have indicated the signal's average plateau of 12V.  You need an oscilloscope to troubleshoot this problem with RPM indication.

[mjones866]

interesting and some really helpful info.

I bypassed the BEP when I didn’t get a reading due to the official instructions for the Motoscope is to actually connect the tach input (yellow wire) to the low voltage side of the ignition coils. I thought it wouldn’t hurt to see what the BEP would spit out.  I don’t believe it’s toast since I can still test the tach by tapping the yellow input wire to ground and it will return an output on the gauge.

Given the voltage spike and strange reading  drop out at 1200 RPM  I might go down the road of ordering the ignition signal senor. I was attempting to not add an additional device if I could help it and keep the install clean. But it seems to be the case here.

[mjones866]

The cafe in question is pictured in my opening post.

[rbm]

Try a simple test to see if the gauge is functioning.  Take a 22GA stranded insulated wire of suitable length to reach from the gauge to the coils.  Take one end of the wire and wrap it 7 times around the #1 spark plug high-tension lead, making sure the coils of the wire are adjacent to each other.  Then use some tape to hold it in place.  Like this:



Take the other end of the wire and attach it to the RPM input on the gauge.  Start the bike and observe the RPM indicator.  If the gauge is operational, the engine speed should show on the gauge.  If it doesn't show, there are two possible causes (1) the configuration of the gauge is incorrect or (2) the tach on the gauge is broken.  Review your gauge's configuration if this is the response you get.

[johnny]

greetings...

this cafe racer unique build needs a www.bskspeedworks.co.uk radiator...

j o


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[Laitch]

Take one end of the wire and wrap it 7 times around the #1 spark plug high-tension lead, making sure the coils of the wire are adjacent to each other.  Then use some tape to hold it in place.

7 is the number attributed to spiritual perfection, mjones866, so count your winding carefully.

[carrotbay]

mjones866 .. did you ever solve this issue? I have the same when trying to use a BEP3.0 with a Motoscope Pro .. no output, although when I connect a meter to the BEP output there is one.

[Laitch]

mjones866 .. did you ever solve this issue?

This thread is over a year old, carrotbay. Try sending him a personal message through this site. The message icon is under his information on the left side of his post.

[rbm]

mjones866 .. did you ever solve this issue? I have the same when trying to use a BEP3.0 with a Motoscope Pro .. no output, although when I connect a meter to the BEP output there is one.

If you are using the cheap Chinese gauge that is popular to retorfit, it requires a negative going signal on the tach input to work.  It's a simple fix.  Put a series ceramic capacitor of 0.22uF between the output of the BEP and the gauge tach input.  Then connect a 2.9KOhm 5% 1/4Watt resistor between the gauge tach input and ground.  Here's a link to a picture of one that someone else built using my instructions.
http://www.motobrick.com/index.php/topic,11148.msg97089.html#msg97089

[Laitch]

If you are using the cheap Chinese gauge that is popular to retorfit, it requires a negative going signal on the tach input to work. 

The poster, carrotbay, indicates full function of a Motoscope Pro is the challenge.

[rbm]

Thanks for pointing that out Laitch. I skimmed over that detail when I read the post and missed it