Horn Relay Ground Problem

[DennisV]

I have a 1977 BMW R100S equipped with dual Bosch horns and a relay. The original relay had 2 #87 posts but many years ago I had to replace the relay with one that had only one #87 post. The ground post was #85 on the original relay and I used #85 on the new relay and it seemed to work. The horns recently stopped working and I immediately thought of replacing the relay (again). When researching the subject I noted that Matthew Parkhouse in the January 2009 issue of his BMW Owners News "keep'emflying" column (3rd last paragraph) confirmed that post #85 is for the relay ground. However, I now have a dilemma as in the August 2009 BMW Owners News "benchwrenching" column by Paul Glaves, post #86 is stated as being the ground post. Unfortunately, I haven't found any reference to the proper horn relay connections in either the Clymer BMW guide or the shop manual for my bike. Can anyone out there help me by advising me the correct horn relay connections?
Dennis Varcoe
Member #22663

[mneblett]

I've seen them listed both ways -- 85 or 86 ground.

Here's how I would approach it:

The 85/86 terminals are across the solenoid coil that pulls the plunger in to make the 30/87 terminals pass the heavy current to the target device.

Basically, if you connect the terminals one way, current flows through the coil in one direction. If it's the right direction, the plunger will pull in. If the connections are backwards, the current flows the other way through the coil, so the plunger doesn't pull in. If hooked up backwards and hit it with 12V once, no damage will occur to the relay.

So, I'd just hook it up one way and see if it works. If not, swap the 85/86 connectors.

[20774]

This has generally been a good article on relays:

http://www.airheads.org/content/view/159/98/

In the Other Relays section, it mentions that "L" is the relay for horns up to 1978. It lists that 85 is the ground post for the trigger side of the relay.

[535is]

Basically, if you connect the terminals one way, current flows through the coil in one direction. If it's the right direction, the plunger will pull in. If the connections are backwards, the current flows the other way through the coil, so the plunger doesn't pull in. If hooked up backwards and hit it with 12V once, no damage will occur to the relay.
A solenoid coil is nonpolar. It should activate regardless of which way the current is flowing. All it requires is enough DC current.