BMW K bikes (Bricks)

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roach374

roach374
Silver member
Silver member
I've actually pretty much torn out the wiring harness, and installed an m-Unit, motoscope pro, replaced the starter relay, new alternator. It would in fact be shorter to list the things that *haven't* been changed, upgraded, or replaced on this bike at this point. 

Among the many electrical changes, I dropped the charging light for a 5W 50 Ohm Cement Resistor  . This resistor is wired in series from the blue wire (negative terminal) of the alternator to the m-Unit AUX1 output (and AUX1 is configured to power up when the bike is unlocked).


Charging circuit is working fine, bike runs fine, everything seems to be working, but that resistor is getting HOT as soon as I turn on the bike (without the engine running). Like almost (but not quite) too hot to touch. It's also in the wiring "tray", right under the gas tank, and I'm not nuts about hot stuff next to those overflow tubes.


So my extremely ambiguous question is: how hot is this resistor "supposed" to get? Should there be current flowing through it, even when the bike (and alternator) is not running? Do I need a different resistor value?


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1993 K75 standard
    

Laitch

Laitch
Life time member
Life time member
@roach374 wrote:So my extremely ambiguous question is: how hot is this resistor "supposed" to get? Should there be current flowing through it, even when the bike (and alternator) is not running? Do I need a different resistor value?
Your question isn't ambiguous; it's multi-faceted.  cheers

Click this link at snowbum's site then scroll way down to the Nerdy section then to the subsection heading  Low ohm resistor information. I think you'll find some affirmation and some solutions. Of course, nothing beats the personal touch, but this is my limit. Smile


__________________________________________________
1995 K75 81,000 miles
['93 K75] Rewired the bike, replaced the charging light with a resistor, now the resistor is getting HOT? Usa-lo10
    

roach374

roach374
Silver member
Silver member
@Laitch wrote:
@roach374 wrote:So my extremely ambiguous question is: how hot is this resistor "supposed" to get? Should there be current flowing through it, even when the bike (and alternator) is not running? Do I need a different resistor value?
Your question isn't ambiguous; it's multi-faceted.  cheers

Click this link at snowbum's site then scroll way down to the Nerdy section then to the subsection heading  Low ohm resistor information. I think you'll find some affirmation and some solutions. Of course, nothing beats the personal touch, but this is my limit. Smile

This is exactly what I was looking for. Looks like some amount of heat is expected with this resistance. If I'm worried about it, I can mount it somewhere more ventilated / cooled, or switch to a higher resistance value. 

Thanks!


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1993 K75 standard
    

charlie99

charlie99
VIP
VIP
have you actually measured a 5 watt light globe before buying the resistor ?

you would find that the resistance is fair bit below 50 ohms , more likely around 30 odd ohms (but globes mesure differently  off than when lit with an increasing resistance as they illuminate

dont forget that the voltage drop on a standard alternator setup is high when the alternator is not charging , ie gets hot , and when charging - charged  should be low

i guess that the cement resistor you are using is about 1.5 inches long , which is or has what they call a 5 watt rating before it gets too hot to handle , likely better to use a 3inch long resistor (10 watt )  which will still get warm but not as warm along its length and has greater dissipation ,  for not much more money .

just a suggestion


hope it helps


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cheezy grin whilst riding, kinda bloke ....oh the joy !!!! ...... ( brick aviator )

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%O['93 K75] Rewired the bike, replaced the charging light with a resistor, now the resistor is getting HOT? Au-log10

'86 k100 rs.. #######..  "Fred " (f(rame) red ) ( Fredrick leichtundschnell ) - -
bits and pieces from many kind friends across the k100 world ...with many thanks ..
    

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