I am recommissioning an 1969 CT90 k1, imported to the UK from California. One of the many jobs needed to get the bike running is to replace the damaged wiring harness, so I've colourised the wiring diagram to make it easier to read. You can get a copy below.

Full diagrams

When the CT90 k1 was released in the US, Honda fitted it with the wiring for turn signals - presumably in anticipation of their usage becoming a mandatory requirement in the US (which they did later in the 1970s) - however, the lamps and relay were not included as standard. The diagram on the left shows the wiring as fitted to the standard bike, and the diagram on the right is the wiring with the turn signal equipment added.

Back in the day, the winkers and relay were available as an optional extras from US dealers and I'll be retro fitting them. As you can see from the diagrams above, this is a straightforward job since the bike came with (redundant) L/R switchgear, all the necessary wiring and even a turn-signal indicator light in the speedo.


The CT90 K1, which succeeded the K0 model, came with several design improvements, and to mark the new design Honda restarted the serial numbers stamped on the engine and frame at zero, hence the references to Frame No. 000001A on the parts and wiring diagrams for the K1 models. In practice, from this point forwards, Honda prefixed the serial number with a single digit number indicating the year of manufacture and resetting the numbering for each annual revision to the range, starting with the 200000 range for the K1, 300000 for K2 and 400000 for the K3. At some point in 1972 the serial number was extended to 7 digits and restarted at 1400001, this then rolled over to 1500001 for the K5 in 1974 and so on. The very late K0 frame numbers went up to around 198000.

The K0 and K1 models used a very similar wiring arrangement, the main difference being the K1 included a switch to activate the rear stop light when the front brake is used. There were other modifications made to later models, including the addition of a kill switch and a change to 'always on' lights, but much of the K1 wiring will be familiar on later bikes.

Subsystem diagrams

It is easier to follow the wiring diagram when each of the subsystems are drawn separately. See below for the separate diagrams for the charging, ignition and lighting system:

If you'd like to know more about how these subsystems work then these articles might be of interest.

There are lots of similarities between this bike and the C90 I've already written about, but I'll include some information on the special features added to the CTs and details of the repairs I do. One of the early jobs is to make a replacement wiring harness, of which more later