While removing the tank to change the antifreeze I disconnected every connection I could find and sprayed WD40 in it before reconnecting. I have since bought DEOXIT and will redo this winter. I checked the connections on the right horn and they are solid.

Did I not reconnect the right horn at the source while under the tank?
According to my understanding of the electrical diagram they share they same source (Green-Gray). So if one works the other should work?.

Any advice on further troubleshooting? Just being lazy by asking. My next step would be to connect right-horn to left-horn wiring and vice versa...

You could try to connect the horn to the battery to test it.  That will isolate the failure to either the horn or to the wiring.

You're correct -- the horns share the same power and ground.  They are wired in parallel.  Swapping the connections will also prove or disprove the wiring.

Thanks! Hadn't thought to connect it direct to the battery. Will do and post results.

I tried using the battery tender lead and some lamp cord but I couldn't get the wire to spark. So instead of going straight to the battery terminals I switched the horns and have found that it is the "L", low horn that is not working. The "H", high horn works on either side. Without the L horn the H horn alone sounds like a clown horn, lol. Will start looking for a replacement or an upgrade to both, though from what I read searching the forum the OEM horns are hard to beat!

You could also try to adjust the screw in the center that dampens the diaphragm.  Maybe it's too tight.

Stealing your thread a bit here...

On my nekkid K100 (that'll be on the road soon), I fitted a pair of cheap 'n' nasty horns to replace the BMW ones. Fashion over form.

My daily ride (RS) has the original horns. Going home the other day, a car kept entering the roundabout that I was already on, so I gave him a blast of the horns. He was looking everywhere for the truck!

Keep them. I'm gonna refit mine.

The OEM horn I have is a FIAMM AM80:
http://www.amazon.com/AM-80-Trumpet-405Hz-frequency-72012/dp/B0002KL7GE/ref=sr_1_2?s=automotive&ie=UTF8&qid=1414069713&sr=1-2&keywords=fiamm+am80

The top review explains they've been discontinued since 2005.

I went with the AM80S to save a few bucks:
http://www.amazon.com/FIAMM-72012-Note-Universal-Bracket/dp/B000DN7T3A/ref=pd_sbs_auto_3?ie=UTF8&refRID=1SVWAJDSZREXN6DFT8BC

I'm a prime member so 2 day shipping is included. Hopefully I'll have installed this Saturday.

If you are thinking of a set you may want to look at these:
http://www.amazon.com/FIAMM-74100-El-Grande-Horns/dp/B000DLBIGG/ref=pd_sim_sbs_auto_5?ie=UTF8&refRID=0AYSEJ8R75VWNP60C3CE

blaKey wrote:Stealing your thread a bit here...

On my nekkid K100 (that'll be on the road soon), I fitted a pair of cheap 'n' nasty horns to replace the BMW ones. Fashion over form.

My daily ride (RS) has the original horns. Going home the other day, a car kept entering the roundabout that I was already on, so I gave him a blast of the horns. He was looking everywhere for the truck!

Keep them. I'm gonna refit mine.

A few years back I had a really nice '67 Jaguar S-type (3.4 MOD, chrome wires), the horns had ceased to function and were coated in some mysterious slippery fluid from under the car, never did figure that out.

I fitted a pair of K100RT horns from my stash.  They were a cheap fix and impressed anyone that heard them "They are motorbike horns?"

Had to sell it, the Jag and the sidecar would not fit into the garage together (amongst other reasons).

Al

The Fiamm AM80S arrived today. I bent the connection blades 90 degrees to better match oem. I used the supplied bracket as it is a little lighter ( every little bit helps right. ) I couldn't see using the old bracket that was stronger than the horn itself.

Some pics - https://imgur.com/a/vPnuX

Before video - Bmw k100 RT 1985 Hi Horn only: https://youtu.be/sB7GTZgDAXo

After video - BMW K100 RT 1985 both horns: https://youtu.be/H82Pa75hyjM