Horn not working dims digital clock

[Michael bluey]

Good evening

I have a strange one with my horn. Went to MOT Saturday and Elan failed on no horn and unbalanced rear brakes. Going to change pressure proportion valves handbrake passed normal brake test passed only failed as 40% unbalanced on hard braking test. Horn issue is as follows 12 volts at button ring/ plunger both horns work if you connect battery straight onto terminals. Relay at position O under dash cowling checked and clicks off car when connected to battery. 15 amp fuse ok in fuse box in footwell. Have re earthed steering column to chassis. When I push horn button no click at relay but digital clock dims to nothing I’ve run out of options need help please.

[Simon_P]

See wiring diagrams

The horn button is a switched ground for the relay.
The relay and clock share a common permanent +12v 27 (that means sheet 27)

27 is battery services Fuse A12

[Michael bluey]

Thanks very much for information finally got round to looking at car today changed fuse it had blown and it all works again straight down to Mot for a retest and it passed.