I was testing the voltage of the fuel pump leads and found they were dead. I then identified Fuse #6 was blown, and replaced it with another 7.5 amp fuse. When I hit the starter switch, the fuse blew again.

Later, I discovered that the Green/White wire leading to the Temperature Sensing Switch (box) #895092 was dead shorting to Ground (#9 wire from fuse #6 is shorting to brown #31 ground). I found two bad/blown diodes in the Temperature Sensing Switch Unit and replaced them with new ones and the dead short was gone... BUT when I hit the starter switch , I blew Fuse #6 and both diodes again.

I found that with the Temperature Sensing Switch (box) disconnected from the harness, I have 12v on Green/Brown (connector #15) AND Violet/Black (connector #A3). I have reviewed Bert's wonderful color-coded wiring diagrams but am wondering about the Violet and Black wire which seems to be coming from the temperature light. I do Not have the anti-theft option, and that connector is not used.

1. Where is the +12v coming from on the Violet/Black wire feeding back to the Temperature Sensing Switch Unit at the connector A3 ?
2. Is this 12+ v feed on the Violet/Black wire part of my problem leading to the dead short of the Temperature Sensing Switch Unit?

What could me causing my Temperature Sensing Switch Unit problem?

I am ready to buy a new Temperature Sensing Switch Unit but want to be sure I don't blow it too with the situation I am describing after plugging it in and hitting the starter switch. I want to be sure the Violet/Black wire is not a problem

Thanks for your review and any input you may have on my problem.

Steve

There definitely should NOT be 12V on that lead. Normally the relay should switch 12v to it if a too high temperature (low resistance from temp sensor) is detected.
Is the high temp light burning all the time?

The Temperature light does not light with the ignition turned on and the bad Temperature Sensing Switch Unit plugged-in or unplugged. I did not see it on when I put the repaired Temperature Sensing Switch Unit back into place before blowing it either. Thanks

I think you measure 12V through the warning lamp.
The warning lamp got a constant 12V feed, the temp. management relay just ground the circuit when the temp. gets to high.

That you got 12V+ on terminal #15 is quite normal, this is the feed to the cooling fan...managed by the relay.

Sorry, but I can`t pinpoint why your relay diodes blowing.

One further note is the Violet/Black wire 12 volt reading at the harness drops to 10 volts when I press the starter switch and the engine turns over... on a new BMW battery... with no Temperature Sensing Switch Unit attached.

The starter draws a lot of current, causing the average voltage drop.

I am not sure how I can get +12 voltage onto the Violet/Black wire if Bert's 2009 "Temperature" electrical schematic is correct... showing the ground running through choke switch... Violet/White should be a ground -12v. I have a 1985 K100RT.

Thanks everyone for any input you can provide.

I don't agree with Inge here. The warning light is connected to ground in the speedo and would get 12V switched from the relay.
But how did you measure the contacts? black lead on ground and the red probe on the contacts or was all relative to other contacts?
Regarding leads/contacts there are some colour code conventions here. Brown is ground, green/black is a switched +12V, red or red/yellow is an unswitched +12V. I'll see if I can find a link with more info. HERE

Have a look HERE for relays, contact ID and measuring them.

quantech wrote:I am not sure how I can get +12 voltage onto the Violet/Black wire

As earlier mentioned the bulb got a constant 12V+ feed with the ignition on, and you then also would read 12V+ on the other pole on the bulb.

Take a piece of wire, a 3w bulb, a battery and your instrument........

ReneZ wrote:
Have a look HERE for relays, contact ID and measuring them.

Thanks Rene. This site is great to learn basic electricity.

CF

Hi Rene!
Haven`t checked Bert`s diagrams, but on the ones I got the warning bulb has 12V+ switched in the instrument.

Have now checked Bert`s diagrams which correspond with the ones I got,
and as far as I understand it the warning bulb is feed with 12V+ via the
green/black wire from fuse #1.

I have a Fluke 88 Automotive tester and some reasonable diagnostic skills...and have lots of familiarity with different mechanical systems. I examined the wiring harness and electronics and cannot see any evidence of shorting or wear. The motorcycle was stored for a few years indoors and has low miles...
and ran when parked.. I know the bike and kept my eye on it, and it's owner, over the years. I have not seen evidence of mice... and the
Violet/Black wire seems snug in the harness protected from everything.

So, I pulled/removed fuse #1 and the +12v was gone from the Violet/Black wire (aka the wire leading to A3 on the Temperature Sensing Switch Unit... with the unit unplugged from the harness.

Do you think it's time to remove the fairing and dissect the instrument cluster to examine it's innards for anomalies?

Thanks

BMW`s own diagrams:

Inge, noted, you're right

it would apear to me that there is something suspicious in the output or circuitry of the temp relay

a question though ....check the fan itself ,,,does it rotate freely ?

even if the globe fed 12 volts was wrong the worst that should happen is that the globe illuminates ....there must be something else in the same circuit path- branch causeing this issue .

could the input from the temp sensor be shorted out to earth ? so many things to test .

try disconnecting the fuel pump connector as well and test that you have the correct voltages on there

diodes blow cause of hight current or reverse voltage

it does mystify me that it happens at press of the start button but not before ?

just food for thought

quantech wrote:So, I pulled/removed fuse #1 and the +12v was gone from the Violet/Black wire (aka the wire leading to A3 on the Temperature Sensing Switch Unit... with the unit unplugged from the harness.

Since you have removed the fuse it`s no power going to the warning lamp.
Put back the fuse and replace the bulb with a piece of wire it wouldn`t make much difference in your measurements, with a bulb with this resistance.

Thanks for the wiring diagram Inge...

Now, I will not worry about that presence of voltage being an issue with the Temp Switch Unit.
I agree that there is something suspicious in the output or circuitry of the Temp Switch Unit too... So, I am thinking about plugging a new one in it's place... That's what lead me to ask the questions about what everyone and anyone knows! With som much going on in that unit, power feeding from multiple sources, it could be a transistor or the Integrated Circuit has gone bad.

The fan powers-up fine, spins freely. The Fuel pump is new, as the old one seized from sitting. New pump hums.
Fuel pump connectors are straight forward.

I tested temp sensor it is Not shorted out to earth.

The fuse only zaps when I hit the starter button.

With regards to the measurements;

indeed there should be +12V on;
- contact A3 (violet/black) from the warning light in the speedo
- contact 9 (green/white) over fuse 6 from contact 87A of the fuel injection relay
- contact 15 (green/orange) over fuse 7 from contact 30 of the no-load relay

The others
- contact E (violet/green) is coming from the temperature sensor
- contact 31 (brown) is ground
- contact A2 (violet/yellow) is going to the fan to power it

the relay will switch 15 to A2 to power the fan. In addition it will switch the warning light, but the big power user (fan) is dealt with with a relay contact whilst the warning light is an electronic switch.

Hope that helps a bit.

quantech wrote:The fuse only zaps when I hit the starter button.

You probably have found out yourself, but suggest that you search along the path for the black/yellow wire from the starter button to the starter relay, which include a branch to the ignition ECU.

Thanks to everyone who provided feedback. The bike is now running, so let me give you the background and wrap-up this post.

The 1985 K100RT was ridden to its former home, where it was parked indoors for 7 years. The battery was dead and the tank was dry. I purchased it and immediately pulled the tank to check the fuel system as I could see white
sediment in the tank. I cleaned the tank, replaced the fuel filter, changed fluids throughout and noted the scary-looking fuel pump leads severely corroded... I was worried about power getting to the pump, but I put the new battery in and hit the button to see if it would spin and it did not. This, unknown to me at the time, I believe, blew the Temperature Sensing Switch Unit and then fuse 6.

I removed, examined (it was seized and rusted tight), then replaced the fuel pump. I did not connect the new fuel pump, immediately however. I put my tester on the pump leads and found no current flowing when pressing the starter button. This led me to discover blown fuse 6 and then later the dead short the Temperature Sensing Switch Unit leg #9 was making on the green/white wire on that circuit shared with the fuel pump.

My question posted originally was due to the schematic not showing adequate detail related to the +12v violet and black wire coming from the instrument cluster. I repaired the Temperature Sensing Switch Unit by replacing the bad diodes, but apparently, that did not “fix” the problem with the module. The diodes blew again when I pressed the start switch, even though neither the fuel pump, nor the fuel level sensors were attached.

I methodically looked at all wires in and out and around that module… and could see no problem with continuity or grounding… so, I went to Gina’s BMW in Iowa City (7 miles from me) and paid the $75 for a new module. She had not sold a module for 111 months, so I suspected the modules were not a big problem.

I put the module in place and did some more tests with the multi-meter with the ignition on until I finally said “do it” and pressed the button. The fuse did not blow. I attached the fuel pump and sensor, and tried again. No Problem. So, I reinstalled the tank and everything in the electrical box. I added some fuel diluted with a bit of gum-out injector cleaner.

I turned the cycle over several times and it fired up. It ran rough… but it ran! After running a while I put the seat on and rode it one lap in my driveway. It ran for an hour or so and it seems like the injectors are fine.

Thanks to this forum for helping me along on this project

Good one!! And now have fun

yay ! another k rides again ...

have fun !!