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thestza

thestza
active member
active member
howdy.

having an issue with a previously running k75s. the bike won't start – when i press the ingnition button, the engine isn't turning over, relay isn't clicking, no sound at all... (this happens when the bike is in neutral and / or when clutch is pulled in)

this came about after trying to start the bike with a low-juice battery. it fired up initially but died soon after. tried starting again... no luck.  

have been reading around on the forum and it seems the issue is potentially starter and / or starter relay related.


TROUBLESHOOTING SO FAR:


  • test battery voltage (fresh charge. multimeter reading 12.59V)
  • test voltage at starter relay (rear contact / red wire. multimeter reading 12.36V)
  • test voltage at starter relay (front contact / black wire. multimeter reading 12.36V)
  • test voltage at starter relay (front contact / black wire. button pressed. multimeter reading 0V)
  • test voltage at starter (nut where black cable attaches. button pressed. 0V)
  • tried 'jumping' the 2 contacts on the starter relay to see if the starter motor engages (it does!)


all this leads me to believe that starter relay is maybe trashed (i've read that trying to start with a dying battery can sometimes do this)...

is this sound logic?

s



Last edited by thestza on Mon Sep 13, 2021 7:28 pm; edited 1 time in total

    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Normally, a low battery will cause the start relay contacts to weld themselves together. This makes the starter run all the time. It is possible that they welded and were broken apart leaving the burned contacts unable to carry enough current to turn the starter.

The other possibility is that the coil in the start relay is damaged and not closing the main contacts. Can you hear the contacts close when you press the start button? If not, the coil may be bad or it may not be getting the signal from the start button.

Another test would be to check for 12 volts at the coil connection when the start button is pressed. If not, you could try jumping from the red wire to the coil terminal to see if it will close the main contacts to turn the starter motor.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

thestza

thestza
active member
active member
hi Point-Seven-five

thanks very much for your response. to address your suggesstions, specifically:

Can you hear the contacts close when you press the start button? If not, the coil may be bad or it may not be getting the signal from the start button.

  • there is no sound at all when i press the start button... no clicking, no nothing... total silence. all the lights + indicators still function, however


Another test would be to check for 12 volts at the coil connection when the start button is pressed. If not, you could try jumping from the red wire to the coil terminal to see if it will close the main contacts to turn the starter motor.

  • is the coil connector the contact that runs directly to the starter from the starter relay (black wire)? i have measured voltage there when the button is pressed but it comes up empty....
  • i have also tried jumping the red wire to the other contact on the starter relay (the red wire where the battery connects). it does indeed turn the starter motor...


thanks again for your reply

s

    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Try putting 12v on the starter relay terminal where the black/yellow wire is connected and connecting the other terminal to ground. Tht should make the starter motor run.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

thestza

thestza
active member
active member
starter + starter relay troubleshooting (k75s) (SOLVED) StxT4QAAAABJRU5ErkJggg==
i want to make sure i understand correctly. when you say "connecting the other terminal to ground", are you referring to the Red / Brown terminal?

    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Yes


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

robmack

robmack
Life time member
Life time member
You've verified power is available up to the start switch in your other thread.  To test beyond that:
- ignition on, kill switch normal, voltmeter between Black/Yellow wire on the starter relay and other lead to ground.  Press the start switch and you should read 12V.  If not, check the start switch.
- If still no start relay activity when you press the start switch even though you measure 12V, temporarily attach a ground to Brown/Red wire at the start relay.  If the starter turns over, then you have a defective ICU or troublesome Hall Sensors or maybe a problem with the FI relay.


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

thestza

thestza
active member
active member
following up again here. per robmack's suggestion in another thread, i was able to locate Bert's diagram of the starter / ignition circuit (pictured here just in case). lots of interconnectedness for sure... but it really helped to understand how the various parts sync together.

BERT'S STARTER PRINCIPLE DIAGRAM (available in downloads section)
starter + starter relay troubleshooting (k75s) (SOLVED) Starter-1

ADDITIONAL TESTS
i then tried the two tests suggested by Point-Seven-five + robmack
ignition on, kill switch normal, voltmeter between Black/Yellow wire on the starter relay and other lead to ground.  Press the start switch and you should read 12V.  If not, check the start switch.

  • this test produced 12V at the Black / Yellow wire on the starter relay but still no activity, so i moved to the next one


If still no start relay activity when you press the start switch even though you measure 12V, temporarily attach a ground to Brown/Red wire at the start relay.  If the starter turns over, then you have a defective ICU or troublesome Hall Sensors or maybe a problem with the FI relay.

  • the starter turns over with this test! (this might ultimately be bad news because of a defective ICU or other part... but it was at least nice to hear the some life in the bike)
  • Bert's diagram doesn't include the ICU, Hall Sensors, and FI relay, but i gather from this test it's related to current off the Red / Brown wire not reaching ground (via the components listed here)


NEXT STEPS

  • figure out how to test the ICU (not sure what dark magic happens inside of that thing...)
  • same for Hall Sensors and / or FI relay


thanks again for the help.

    


10Back to top Go down   starter + starter relay troubleshooting (k75s) (SOLVED) Empty No go Tue Aug 31, 2021 9:29 pm

daveyson

daveyson
Life time member
Life time member
deleted, wrong answer.



Last edited by daveyson on Thu Sep 02, 2021 4:16 am; edited 1 time in total


__________________________________________________
11/1985 bmw k100rt (late model)  Vin. 0090567
 ~120,000 km
    

thestza

thestza
active member
active member
thanks Point-Seven-five + daveyson

some quick follow-ups: 

  • Point-Seven-five, is it safe to conclude that the ICU is the culprit at this stage? (i'm definitely happy to see that the part is relatively affordable if so)
  • daveyson, i'll do some more reading on the downloads page Bert's guides tonight

    

robmack

robmack
Life time member
Life time member
What's you've now proven by this test is that power is able to get to the starter relay but there is no return ground circuit for the current to get back to the battery. There is a transistor inside the ICU that connects the Brown/Red wire off the starter relay to actual bike ground. That transistor is on when the ignition turns on and the engine at rest, but the transistor turns off when the engine ignites and the RPMs exceed 700. The ICU knows the engine RPM via the hall sensors. Apparently, the transistor in your ICU is open circuit and is failing to tuen on with the ignition. what you ended up doing with the last test is provide a temporary path for current to get to ground at which point the starter motor turned over. If you continued, it's possible that the bike would start.


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

13Back to top Go down   starter + starter relay troubleshooting (k75s) (SOLVED) Empty Starter troubleshooting Wed Sep 08, 2021 9:22 am

Kr4mo

Kr4mo
active member
active member
@thestza wrote:howdy.

having an issue with a previously running k75s. the bike won't start – when i press the ingnition button, the engine isn't turning over, relay isn't clicking, no sound at all... (this happens when the bike is in neutral and / or when clutch is pulled in)

this came about after trying to start the bike with a low-juice battery. it fired up initially but died soon after. tried starting again... no luck.  

have been reading around on the forum and it seems the issue is potentially starter and / or starter relay related.


TROUBLESHOOTING SO FAR:


  • test battery voltage (fresh charge. multimeter reading 12.59V)
  • test voltage at starter relay (rear contact / red wire. multimeter reading 12.36V)
  • test voltage at starter relay (front contact / black wire. multimeter reading 12.36V)
  • test voltage at starter relay (front contact / black wire. button pressed. multimeter reading 0V)
  • test voltage at starter (nut where black cable attaches. button pressed. 0V)
  • tried 'jumping' the 2 contacts on the starter relay to see if the starter motor engages (it does!)


all this leads me to believe that starter relay is maybe trashed (i've read that trying to start with a dying battery can sometimes do this)...

is this sound logic?

s
Sounds like the typical "Dirty Starter" problem to me.  The load shed relay grounds through the starter.  If the brushes are dirty with carbon and not making good connection the relay will not energize allowing you to crank the engine.  Quick and dirty test, put the bike in gear, with clutch released, pull the bike backwards sharply a couple of times.  This will try to spin the starter backwards, usually enough to clear the carbon and allow starting.  Either way, pull the starter out and clean it out and check brushes etc.  The brush plate is available from BMW

    

92KK 84WW Olaf

avatar
Life time member
Life time member
having an issue with a previously running k75s. the bike won't start – when i press the ingnition button, the engine isn't turning over, relay isn't clicking, no sound at all... (this happens when the bike is in neutral and / or when clutch is pulled in)


Do the lights go out if you press the start button like this? 


__________________________________________________
1992 K100LT 0193214 Bertha Blue 101,000 miles
1984 K100RT 0022575 Brutus Baja Red 578 bought 36,000 now 89,150 miles
1997 K1100LT 0188024 Wotan Mystic Red 689 58,645 now 106,950 miles Deceased.
1983 K100RS 0011171 Fricka 606 Alaska Blue 29,495 miles Damn K Pox Its a Bat outta Hell Now 37,190 miles. 
1996 K1100LT 0233004 Lohengrin Mystic Red 38,000 miles currently 42,640 miles.

Past:
1968 Yamaha 80 YG1
1971 Yamaha 125 YAS-1
1968 Honda 125 SS
1970 Honda CD 175
1973 Honda CB500-4
Honda CX 500
    

thestza

thestza
active member
active member
a quick follow here as i was able to work on the bike a bit over the holiday weekend:


  • after testing the harness side terminals of the ICU, i was able to determine that the ICU wasn't functioning as it should...
  • i ordered a used ICU off ebay, installed it, and the bike is running again!
  • of course, some new issues have popped up... 


thanks everyone (robmack, Point-Seven-five + daveyson in particular) for helping to troubleshoot and find an eventual solve

    

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