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1Back to top Go down   ['93 K75] Temp sensor relay wiring? Empty ['93 K75] Temp sensor relay wiring? Sun Feb 14, 2021 12:23 pm

roach374

roach374
Silver member
Silver member
I'm doing an m-Unit conversion on my bike, and I want to keep the temperature relay intact (to turn the fan on/off the way it's always done). 

From reading the wiring diagram, I know that:

  • "E" --> temp sensor
  • "31" --> Ground
  • "A2" --> Fan


What I can't seem to figure out are the "15", "32" "A3", and "9" terminals, which connect variously to the fuel injectors, load shed relay, the horn, the fuel injection relay, the ICU, warning lights, and seemingly just about everything else on the bike.

Could someone help me out with some pointers on these three mystery terminals? How do I power this relay?



Last edited by roach374 on Sun Feb 14, 2021 6:13 pm; edited 1 time in total

    

Laitch

Laitch
Life time member
Life time member
@roach374 wrote:. . . I want to keep the temperature relay intact (to turn the fan on/off the way it's always done). 

From reading the wiring diagram, I know that:

  • "E" --> temp sensor
  • "31" --> Ground
  • "A2" --> Fan


What I can't seem to figure out are the "15", "32", and "9" terminals, 
Where's the 32 on this K75 temperature sensor relay from MaxBMW? Can you post a photo of yours?
['93 K75] Temp sensor relay wiring? Scree126


__________________________________________________
1995 K75 81,000 miles
['93 K75] Temp sensor relay wiring? Usa-lo10
    

roach374

roach374
Silver member
Silver member
Sorry, that was a typo on my part. I meant the "A3" terminal. I edited the original post to avoid confusion...

    

robmack

robmack
Life time member
Life time member
15 = Switched power feed input for fan
A2 = power output to fan
31 = Ground
E = water temp sensor input
A3 = Temp indicator on dashboard output
9 = switched power input for module's electronics

There are two power sources required.  One goes to pin 9 and powers the electronics within the module.  The other is a power feed to pin 15 that ultimately causes the fan to spin and is connected to an internal relay that is controlled by the module's electronics.


__________________________________________________
Robert
1987 K75 @k75retro.blogspot.ca
http://k75retro.blogspot.ca/
    

roach374

roach374
Silver member
Silver member
@robmack wrote:15 = Switched power feed input for fan
A2 = power output to fan
31 = Ground
E = water temp sensor input
A3 = Temp indicator on dashboard output
9 = switched power input for module's electronics

There are two power sources required.  One goes to pin 9 and powers the electronics within the module.  The other is a power feed to pin 15 that ultimately causes the fan to spin and is connected to an internal relay that is controlled by the module's electronics.

Robmack, you're the real MVP. I'll buy you a beer next time I'm in T.O. Thank you!

    

roach374

roach374
Silver member
Silver member
Wait, so is A3 going to be high whenever E is high? (maybe just with different current draw?)


__________________________________________________
1993 K75 standard
    

robmack

robmack
Life time member
Life time member
@roach374 wrote:Wait, so is A3 going to be high whenever E is high? (maybe just with different current draw?)
No, A3 won't be high every time E is high.  The input to "E" is the variable resistance caused by the temperature of the water, which is an analog value over a wide range.  Remember that the temp sensor is a negative temperature coefficient (NTC) resistor (resistance varies inversely to the temperature) so in fact E decreases in resistance the more water temperature increases.   Inside the temp relay electronics, there are some thresholds set.  One threshold is to determine the value of resistance below which the fan will turn on (A2 turns on), and another threshold the value of resistance below which the high temp indicator will turn on (A3 turns on).  The first threshold is higher than the second, meaning the fan will turn on before the High temp indicator will turn on.


__________________________________________________
Robert
1987 K75 @k75retro.blogspot.ca
http://k75retro.blogspot.ca/
    

roach374

roach374
Silver member
Silver member
@robmack wrote:
@roach374 wrote:Wait, so is A3 going to be high whenever E is high? (maybe just with different current draw?)
No, A3 won't be high every time E is high.  The input to "E" is the variable resistance caused by the temperature of the water, which is an analog value over a wide range.  Remember that the temp sensor is a negative temperature coefficient (NTC) resistor (resistance varies inversely to the temperature) so in fact E decreases in resistance the more water temperature increases.   Inside the temp relay electronics, there are some thresholds set.  One threshold is to determine the value of resistance below which the fan will turn on (A2 turns on), and another threshold the value of resistance below which the high temp indicator will turn on (A3 turns on).  The first threshold is higher than the second, meaning the fan will turn on before the High temp indicator will turn on.

Super informative/helpful, Rob. Thanks!


__________________________________________________
1993 K75 standard
    

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