Horn and Indicator Wiring: the Stator Tube and Its Lucas Switch
The horn and indicator switch on the Ace and Aceca is a small Lucas assembly at the top of the steering column, and its wiring runs down the centre of the column through what is called the stator tube, the fixed inner tube that stays still while the wheel turns. Rewiring or refitting it looks simple and often is not, because the wires are meant to stay attached to the switch, and the switch itself is a fragile, decades-old part that is easily ruined by taking it apart the wrong way. Here is how the wiring routes, the safe way to thread it, and the one mistake to avoid.
Confirm against your car
This is an old Lucas assembly, common to the Ace and Aceca, now six or seven decades old and not easily or well replaced. The single most important point: do not force the switch apart at the metal tags on its base, they break. And because the car is positive earth, respect its polarity when reconnecting anything. Confirm details against your wiring diagram, and where the original switch is at stake, err on the side of a specialist.
Where the wiring runs: the stator tube
The horn and indicator wires run down the centre of the steering column, inside the stator tube, the fixed central tube that carries the wiring while the steering shaft and wheel rotate around it. At the top, the wires attach to the horn and indicator switch and hub assembly; at the bottom they emerge near the steering box. On the Aceca this is the same as the Ace.
The safe way to thread the wiring
The classic, low-risk way to route new wiring through the stator tube is not to push the loom through directly, but to feed a length of plain wire, a pull-string, through the tube first, attach the loom to it, and draw the loom through. That avoids forcing a thick cable through and damaging it.
A word on direction. The switch's wires are meant to come attached to the switch at the top and feed down the tube. Threading a wire up from the steering-box end can make the routing easier, but if it leaves the wires detached from the switch and hub assembly you have created a problem, because reattaching them means dismantling the whole switch (below), which is exactly what you want to avoid. Where wires must be joined inside the tube, stagger the joins along its length so the cable does not bunch into a fat section that will not pass through.
The switch: don't prise it apart at the base
The Lucas horn and indicator switch is the fragile part, and the commonest way it is destroyed is by well-meaning owners trying to open it from the base, unbending the metal locking tags there. Those tags tend to snap.
The correct way in is from the other end: gently prise off the horn push, and beneath it you will find the concealed screws that actually hold the assembly together. Work from there, not from the base tags. Re-assembly is fiddly, the switch has sprung internal parts that take patience, and often several attempts, to seat.
Before refitting, it is worth testing the switch for continuity, especially the horn-ring contact, which is designed to work with the wheel at any angle.
Original versus replica switches
These switches are now six or seven decades old, and owners report mixed quality in the imported replica replacements on the market. That makes the original worth preserving where it can be, another reason not to wreck it by forcing it apart. If you do need to replace it, it is worth asking around the AC and Bristol communities about which current replicas have proved satisfactory.
Troubleshooting: test before you blame the switch
A dead horn, lazy indicators, or indicators that stay on rather than flash are not always the switch's fault. Before you open the column or touch the switch, work through the simpler causes first, many of them sit outside the stator tube.
- Horn does not sound. Check the horn itself, its earth connection, and the fuse or fusible wire first. A good horn should earth through the body; on a positive-earth car, a poor earth breaks the circuit just as surely as a broken live wire. Try earthing the horn directly to a clean body point with a temporary lead: if it now sounds, the problem is upstream in the wiring, fuse or switch, not the horn.
- Indicators glow but do not flash. The flasher unit (or the fuse and its holder) is the usual culprit, not the switch. A poor fuse contact can give the same symptoms as a tired flasher. Swap the flasher with a known-good unit before assuming the switch is at fault.
- One side only works. If left works but right does not, or vice versa, the fault is almost certainly in the wiring, bulb holders, or earth on that side, not inside the shared switch. Check the bulb, the holder contact, and the earth path through the bodywork.
- Indicators flash too fast or too slow. A failing bulb, a weak earth, or a flasher unit of the wrong rating can change the flash rate. Bulbs and earths first, then the flasher, then the switch.
Testing continuity and polarity safely
A simple continuity test across the switch's wires, with the battery disconnected, will tell you whether the switch is making contact in each position. Work from the wiring diagram to identify which wire is which: the horn, left indicator, right indicator and earth connections are separate, and on a positive-earth car the earth path is just as important as the live side.
Test in both directions. On a positive-earth system, the body is positive and the loom carries the negative feed. If you test the switch assuming a negative-earth layout, you may misread what you see. Connect your meter between the switch terminal and a known-good body earth point, or test the switch between its relevant terminals and the chassis, whichever your diagram shows.
For the horn-ring contact, turn the wheel slowly while you test continuity. It should make contact at every angle. If it drops out at certain positions, the contact ring or brush is worn before the switch body is the problem. And if the wiring between the switch and the fusebox shows low or no resistance, the wiring is intact and the fault is likely in the switch or the components it feeds.
In short
The horn and indicator wiring runs down the stator tube in the centre of the steering column, the same on Ace and Aceca. Thread new wiring with a pull-wire rather than forcing the loom through, and keep the wires attached to the switch. Above all, never open the delicate Lucas switch by unbending its base tags, go in from the horn push and the concealed screws, and respect the car's positive-earth polarity when you reconnect.
Sources. This article draws on the experience of owners in the AC Owners' Club community with the shared Lucas horn and indicator switch.
A general guide to the horn and indicator wiring and its Lucas switch, not a substitute for the wiring diagram or an auto-electrician. The switch is old and fragile. Do not force it apart at the base, and the car is positive earth, so respect polarity when reconnecting. If in doubt, a specialist is cheaper than a broken switch.
About the author
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca, writing the single-model reference he always wished existed. Read the full bio.