Spent the weekend making some XJ-S repairs and I learned a whole lot more than I would ever hope to know about the horn. The car is a 1977 model.
I was racing the clock so I didn’t document with photos, but I hope this will help someone-
The horn was intermittently functional for a while then became totally inoperative.
The horn is simple- under the front bumper there are two different horns with different tones.- These are controlled by a black circular relay near the headlight relays at the front left corner of the engine compartment, near the bottom of the stack- you need to pull the fuse panel for the headlights to get there.
That relay is controlled by the horn pad on the steering wheel. The positive side of this is located directly in the center of the horn pad and is passed though the column by way of a brass rod that sits suspended in the center of the column by use of a funny rubber bumper. The problem is that the outside of the column is the Earth or Negative in this circuit so if your brass rod falls or touches, the horn will go off.
This positive signal is carried outside the column by way of a small hole in the side of the column that is held to a brass ring by way of a spring inside a rubber collar, this way, the signal can pass through with out grounding and setting off the horn.
Diagnosis- I started at the horn relay and used a combination of continuity tests with the multimeter and jumping connections to find where in the circuit was bad. I eventually traced it all the way back to the steering column itself checking all the connectors including CF2 (located under the hood release, INSIDE the cabin behind some trim on the left side of the driver footwell.)
At first I thought it was an issue with the copper glide connector to the brass ring, no such luck, though I did end up removing the hokey factory connector (A piece of copper bent into a circle upon which the wire sits.). This was replaced with a aluminum pop-rivet through the copper upon which a spade connector was attached.
Still no luck.
Finally it came down to an issue with the positive side of the horn circuit (Purple / Black wire)
It seems there was no connection INSIDE the column.
So, I removed the column for the first time since I’ve had the car.
Once I pulled EVERYTHING apart, I found the issue to be a spring that was used to maintain continuity from the internal brass shaft to the collar upon which the copper glide rests.
The spring was rusty and in bad shape. I spent about 10 minutes trying to figure out where I could get a spring that may fit late on a Sunday evening. Finally, I found just what I needed on another workbench in the basement- The solution- a firing pin spring for a Browning Hi-power (Though I bet many a firing pin spring will work)
To use this, I drilled out the hole in the rubber collar to a bit larger size and stuck the spring into the hole, it turns out the spring was a bit longer than needed so I cut it off about 3/8" longer than the channel through which it passes, so there will be tension (Oh, and the spring was coated in copper never-seize)
Then everything went back together.
In the end, I was able to get everything back together and drive the car again. The total elapsed time in fixing was about 4 hours not counting the time spent finding the fault. Now to fix the speedometer that stopped working on that drive, almost certainly my fault for touching the cable when removing the cluster.