The wiring loom
Three things an owner needs before touching the wiring: whether a loom can be bought, what the colours mean, and where the diagram is. All three have answers, and the third is the one that surprises people.
You can buy a loom, and it is engine specific
Autosparks in Nottinghamshire manufacture replacement harness sets for these cars, handmade, and they are differentiated by engine and by where the control box sits. For the Aceca there are two five piece sets, one for the AC engine and one for the Bristol, both in braided cable with a braided outer covering, listed for 1954 to 1961. The Ace equivalents are six piece sets listed for 1953 to 1963.
There is a separate early set for the earliest Aces, chassis numbers up to 63, which has the control box on the bulkhead and a horn relay, and which is built as a single piece in mixed PVC and braided cable with a taped outer rather than braided. If your car is early, that distinction matters before you order.
Options that can be built into the loom at the time of making rather than added later include alternator wiring, radio feeds, an upgraded dynamo, an electric radiator fan and a dash accessory socket. That is worth knowing if any of those are on your list, because doing it at re-loom time is far tidier than splicing afterwards.
Two cautions. Autosparks state plainly that they supply no fitting instructions, on the grounds that the harnesses are like for like replicas of the original. So the loom does not arrive with a diagram, which makes the next two sections the important ones. And the American distributor's listing for the Ace harness describes PVC wire with PVC tape wrap rather than braided, under a near identical part number, so confirm the construction you are buying rather than assuming.
No loom is listed for the Ruddspeed cars. With eight Acecas so fitted that is unsurprising, but it means an RS owner should speak to the maker directly rather than ordering from a page.
The Lucas colour code
The Aceca uses the standard Lucas code that ran across the British motor industry rather than anything peculiar to AC. That is confirmed for the Ace on the owners club forum rather than merely assumed, which matters, because it means the code below can be relied on as a framework.
| Colour | What it carries |
|---|---|
| Brown | Main battery feed, unfused and permanently live |
| White | Ignition circuits, fed from the ignition switch, unfused |
| Green | Accessories fused through the ignition switch: wipers, flashers, gauges, stop lamps |
| Blue | Headlamps, lighting switch to dipper |
| Red | Side and tail lamp feed |
| Purple | Accessories fused direct from the battery rather than through the ignition: horn, interior light |
| Black | Every earth connection |
Tracers are read as base colour then trace, so a white wire with a green trace is WG. The ones worth knowing on this car: blue with white for headlamp main beam and blue with red for dip; brown with green from the dynamo F terminal to the control box F, brown with yellow for D to D, and brown with white from the ammeter to the control box; white with black from the coil CB terminal to the distributor; green with red for the left hand flashers and green with white for the right; green with purple for the stop lamps; green with black from the fuel gauge to the tank unit; purple with black from the horn push; and red with white from the panel light switch.
One caution that matters. No source the archive has found gives a circuit by circuit colour list for the Aceca itself. The code above is authoritative as a code, and the principle that these cars follow it is confirmed, but that is not the same as a verified list for your chassis. Check against the diagram before cutting anything, and be especially careful with the horn circuit, which demonstrably changed during production.
Where the wiring diagram actually is
This is the part people do not expect. There is no electrical section in the workshop manual. The only AC Ace and Aceca workshop manual in circulation covers the Bristol engine and gearboxes and says so explicitly. Autosparks supply no diagram with the loom. So the factory Lucas diagrams held by the owners club are effectively the only route.
Four Lucas diagram numbers are recorded on the club forum: W29340 of April 1955, the earliest found and showing a single speed wiper; W29340A of March 1957, which supersedes it and covers 1955 to 1957; W31520 of April 1956; and W31520A of February 1957, described as covering 1956 and 1957 Home and Export models and superseding W31520.
They are obtainable through the club archives and library, and through the members' area under the model directory. The practical advice from members is to ask on the forum quoting your chassis number, because the diagrams are period specific, and to get an A3 copy rather than A4, since the diagrams do not reproduce legibly at the smaller size.
One point the archive cannot settle: the club forum describes W31520A as an Ace diagram while a copy offered for sale described the same number as an Aceca diagram. Since the Aceca is a bodied Ace, one diagram may legitimately cover both, but nothing here asserts that.
Earthing, and why there are so many earth wires
Most cars of this period earth through a welded steel bodyshell. This one cannot. The body is aluminium panelling over a wood and steel tube frame on a tubular chassis, and wood does not conduct.
The AC two litre community, whose cars are built the same way, put it simply: being a wood framed car, there are a lot of earth wires. Earths are run individually back rather than assumed through the structure, with connections made to the aluminium bulkhead and to the rear frame. The fuel gauge sender is called out as a known weak earth point, worth an additional lead of its own to the aluminium frame.
Two details from the same source that are easy to get wrong. Use brass screws rather than steel for sender units. And protect bullet connectors with petroleum jelly or a dielectric grease, because on a car with this many separate earth paths, a high resistance connection anywhere produces symptoms that look like almost anything else.
Original insulation was rubber under a cotton braid. Modern replacements use PVC under cotton braid, which looks right and lasts far better.
This is AC two litre practice applied to the Aceca by construction analogy rather than Aceca factory specification, and the archive would rather say so than imply it has seen an AC instruction to this effect.
Charging, and the case for leaving it alone
The dynamo is a Lucas C39 or C40 with an RB106 control box, and on a production car it sits at the front of the engine driven by the fan belt. If yours is mounted elsewhere, that is a previous owner's work rather than the factory's.
Output is modest and is described in the AC two litre community as marginal when several accessories run at once. That is the honest root of most charging complaints on these cars, rather than any fault.
There are three routes from there. An electronic control box keeps the dynamo and improves the charging. An alternator can be fitted inside a dynamo shaped body, with a dummy control box, which is what most owners who want reliable charging and original appearance do. Or the loom can be built for an alternator from the outset when it is renewed.
One consequence worth knowing before committing: with such a unit fitted, the original ammeter no longer functions and a voltmeter is needed if you want a charging reading. And note that positive earth alternator units exist, so none of this requires converting the car to negative earth.
What the archive could not establish
The Aceca's exact fuse count, fuse ratings, fusebox type and location. What is known is that RB106 cars use a separate fusebox rather than the fuses integrated into the earlier control box. There is a thread on the club forum about fuses which the archive cannot read and which is the likely answer. Also not established: the Lucas starter motor model for these cars, ignition and lighting switch part numbers, the instrument panel wiring, and any club or specialist position for or against negative earth conversion, as distinct from the practice, which is documented.
Related reading
- Positive earthWhat the polarity means for battery, coil, dynamo and accessories.
- The Lucas control boxWhat the RB106 does and its commonest fault.
- Fitting a solid-state regulatorKeeping the dynamo while improving the charging.
- Horn and indicator wiringThe stator tube, the Lucas switch and how to thread it.
- Parts and specialistsWho supplies parts and who works on these cars.
- Clubs, registers and recordsThe club archives where the Lucas diagrams live.
This guide is not a substitute for the factory wiring diagram for your own chassis. If anything here is wrong or can be improved, please get in touch.
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.