Electrical Gremlins- no horn?

[Brahma]

Hi all!

In 31 years of ownership, my early K100RT has thrown me my first puzzler- no horn.

* Starts and runs fine, everything operable but no sound from either horn.
* Sound comes from the relay when operating the horn button.
* Pulled the tank, open the electrical box, swapped the relay out- same symptom.
* Pulled the connection to the horns/parking light/turn indicators, measured the voltage when the horn button is depressed, and 13+ v measured
* Reconnected, measured voltage at horn with ZERO v measured.
* Applied battery power directly to horns- they work!
* I checked continuity of each wire and get weird results, like there is a short or broken wire.
* I checked the Max BMW fiche and my early RT harness is not available- it looks like BMW separated the indicators from the horns, leaving my main harness one connector short.

Recommendations from inmates appreciated...

[Elipten]

Sounds like you have your answer.  The BMW undersized wires are shot.  Just run new wires along the harness and eliminate the original factory wiring.

[rbm]

Sounds like a failed connector.  Two possible locations - the relay socket itself or the connector in the harness.  Remove the relay socket from its perch in the relay box and make a voltage measurement on the Green/Grey wire (the fact you hear the relay means Green/Red power and control cabling are probably OK).  Press the horn button and you should read 12V.

Locate the 5-pin connector for the horn, parking light & turn signals under the tank.  Clean it and test voltages at that point with the Green/Grey wire. Test both sides of the connector to isolate the problem.  Are you also experiencing any turn signal or parking light problems? That might also point to the same connector.

As for wire size, they are standard 1.5mm2 which should be OK for the current drawn by the horns.

[Brahma]

Thanks for replying!

12 v is getting to the 5 pin connector of the harness to where the outer harness that leads to the horns, parking light and turn indicators. 

I plug in that outer part of the harness, and 12v is getting to the horn leads.

I plug in the horn leads, and measure there with the horn button on, and it's only .25v!  WTF?  But, the parking light comes on, and the turn lights work!

I checked for continuity and all wires are good.  I tried to check resistance (don't quite know what I'm seeing) and the meter comes up 000.0 on all, which I think is good. 

The system has a load-shedding relay to kill power sources while the starter button is pressed, but I'm not doing that.  The horn always was live when the key is on.

Where did my other 11.75 volts go as measured at the horn?  Bad switch that can't handle the load?  I swapped relays and same results....

[kioolt]

You should leave your horns plugged in for all of the measurements that you are taking so that you are loading the circuit.  Do not unplug anything.  Stick you meter lead into the back of the plugs to take your measurements.  You should be able to find out where the voltage is dropping by doing this.

[Brahma]

Well, I took data points in all configurations, and the bottom line is there is .25v measured at the horn when plugged in at the horn, which I do not understand, when there is 12v before plugging the horn in.  I did test the horn with 12 v straight to it which it worked fine.

Something isn't taking the load.  I will order another relay first, because it's cheapest, the only thing left is the switch, but I would think if it triggers the relay, that is all that is needed.

[kioolt]

You should leave your horn plugged in.  Let's say that point A is the relay, point B is the next connection, point C is the one after that and so on.  If you've got 12v at point A and not point B, then your problem is between the two points.  If you've got 12v at point B and not point C, then your problem is between point B and point C.

I don't believe you ever said what the voltage is at the relay with the horn plugged in.  What is it?

[Laitch]

Why wouldn't the horn button contact within the multi-switch be considered the culprit? I realize that's easy for me to ask because almost every other possibility except the effects of heavy gravity has been listed.

[Chaos]

don't have the schematic handy but another bike I had the horn button completed the ground circuit, the positive side was unswitched.  Confusing until I figured it out

[Inge K.]

I will order another relay first, because it's cheapest,

First you can test by swapping the horn relay and load shedding relay, as those is identical.

[Brahma]

I swapped the relays, and the same symptoms exist.

How do I measure at the relay- take it apart?

I measured with one horn being plugged in, not both. .25 v at that horn.

The parking and turn lights work fine.

[kennybobby]

When you had the tank up did you inspect/clean the ground collection point at the frame bolt?
There appears to be another ground collection point near the horn harness connector pin 5 also--maybe inspect for corrosion. 

It seems that you are getting voltage to the horn when unloaded, but the return path back to the battery is blocked.

Where are you placing the negative test lead of the meter when measuring 0.25?

[Brahma]

I have the negative test lead at a bolt on the frame. 

The parking light and turn lights share a common ground in the sub harness, and they work. 

Do I need to have both horns hooked up?  I wouldn't think so....

The problem kind of points to the sub harness connector for the horns (both power leads come from one pin), but what I don't understand is that both read 12v when not connected to the horns, and .25 v when one horn is connected.  Am I missing something by not testing with both horns connected?  I ask, because I am at the beach right now.  :)

[kennybobby]

Pull fuse #7 and make sure it is good, then does your radiator fan turn freely by hand or is it locked up or stiff to turn?

The power for the horn is on the same circuit as powers the fan, and powers the flasher unit--disconnect the temperature switching relay unit under the tank and try the horn again.  Disconnect the flasher unit and try the horn again.

That's all i can see that could remotely affect the #7 fuse 12v line that becomes hot when the key is turned to the ON position, aka circuit 15.

[Brahma]

The fan turns freely.

Fuse #7 is fine. 

I tested with both horns connected- no change.

Again, I measured 12v at both sets of horn leads NOT plugged into the horns

I then plug the horns on, and measure .25 volts when the horn button is pressed. 

WTF is going on?  I'm getting "perturbed".

[kioolt]

Again, I measured 12v at both sets of horn leads NOT plugged into the horns

I then plug the horns on, and measure .25 volts when the horn button is pressed. 

WTF is going on?  I'm getting "perturbed".

You still haven't said what the voltage is with the horns connected at all of the other points in the horn circuit that you have checked.  This WILL tell you what connection is causing the voltage drop.

[kennybobby]

i just went out and checked by horn.

i disconnected the horn and measured 12v at the green/gray wire when the key was ON, kill switch at RUN, and the horn button pressed.

i measured the resistance of the horn at less than 5 Ohms.

With the horn once again connected i measured 0.25 V when the button was pressed, and the horn was sounding.

i measured the horn current at about 4.5 Amps with the button pressed and horn sounding loudly.

It seems that you are getting the voltage drop across the horn but not enough current flow to make it toot--definitely a gremlin of some sort...

[Brahma]

Thank you- I guess .25 v is normal then (?)

The lights share the same ground of the harness, so I really don't know.

[Brahma]

No wonder the sub-harness design was changed...

[Brahma]

I was reading your reply while you were making edits and I agree, weak current would have to be the relay.

I kept an old one and will try to resurrect it...  I will order one this week.

THANK YOU!

[kennybobby]

Fuse 7 and the horn relay are powered thru the load shed relay, so swapping the horn and load shedder doesn't rule out bad relay contacts as the root cause--the horn current has to travel thru both relay's contacts.  You can disassemble the relays to inspect the contacts, but i'll put $ on a bad relay as the culprit.

The return path for the load shed relay is thru the starter motor brushes, which is johnny's electrical whack number 1.  i don't believe in trying to back-drive the starter with the jenner thru the engine, but it may be that the carbon dust in the starter motor needs to be cleaned out to make a good ground path for the load shed relay.  A flaky ground lets the contacts bounce and then they arc and get frosty and high resistance junction, etc...

[Brahma]

So, today I bought some decent clip leads and performed a test of the relay.  I powered up poles 85 and 86 with a fully charged motorcycle battery, heard the contacts close, then performed a continuity test across poles 87 and 30.  Test was positive for continuity.

I then moved over to the bike and connected poles 87 and 30 to one horn, then powered up the relay. 

Nada.

Puzzled, I read the voltage between poles 87 and 30, and it was non-existent!

How can a relay have continuity powered up, but have little or no voltage coming out of it? 

Bad coil?

[kioolt]

I then moved over to the bike and connected poles 87 and 30 to one horn, then powered up the relay. 

Nada.

Pole 30 should be connected to the battery or hot fuse, not the horn.  Pole 87 should be connected to the horn.

Puzzled, I read the voltage between poles 87 and 30, and it was non-existent!



If you put one lead of your meter on pole 87 and the other on pole 30, zero would be the correct voltage.  You should have 12v on pole 30 to ground even before you press the horn button.  Pole 87 should have 12v to ground while pressing the horn button.

[Brahma]

So I tested it backwards?  I should put the 12v on 30, ground on 87, then read voltage from 85 and 86?

[kioolt]

Do not put 12v on 30 and ground on 87.  That IS NOT what I said.  I said you should measure the voltage on pin 87 to ground.  That means put one meter lead on pin 87 and the other meter lead to ground.

I'm assuming you are trying to test the relay out of the bike for the following instruction.

Put 12v + to pin 30 and 86.  Put ground to pin 85 and one side of the horn.  Connect the other side of the horn to pin 87.  The horn should blow.