I have just finished recommissioning a 1969 CT90 K1. The bike was originally sold in the US and was imported to the UK from California a few years ago.

One of the many repairs that needed doing was to replace the damaged wiring harness, so I colourised the wiring diagram to help with the job. You can get a copy below.

K1-K4 (1969-73) 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.

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.

Note that turn signals became a standard fixture in 1973, following a change to USA federal regulations. Since this was an optional extra previously the connections in the harness needed for the lamps and the winker relay, plus the L/R equipped switch gear did not need to be changed.

Wiring diagrams for other models

Although much of the K1 wiring will be familiar on other models, there were some small changes over the years

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NOTE
As a result of the introduction of a kill switch in 1974, there are two versions of the wiring harness:
* 1969-73 (K1-K4)
* 1974 onwards (K5 and later)

The two version are easily distinguishable by the number of connections in the part of the harness terminating in the headlight bucket (the earlier version has 10 connections and the later has 12). This is explained in more detail below.

Before looking at these changes in more detail, a reminder on the CT90 model numbers used by Honda (this table is based on information from the Honda model identification book and various parts lists):

Model numberFrom frame numberYears
CT90 K01000011966-68
CT90 K12000011969
CT90 K23000011970
CT90 K34000011971
CT90 K414000011972-73
CT90 K515000031974
CT90 K616000031975
CT90 K717000041976
CT90 K818000081977
CT90 '7819000101978
CT90 '7950000111979

notes

Clear as mud!

K5 (1974)

There were a couple of changes with the K5

The addition of a kill-switch involved making some small changes to the wiring harness since the power from the main switch - in addition to supplying lighting switch, speedometer, horn and so on - now also had to send power to the new kill switch. As a result there are two additional power connections in the section of the harness that terminates in the headlight bucket: an additional black wire - connecting the battery to the kill switch - and a black/white wire that connects the other side of the kill switch to the ignition coil.

Other than this change, the wiring is the same as the earlier models.

The other minor change (possibly made before 1974?) was the introduction of a connector blocks to attach the rectifier and alternator instead of the individual wired connections used previously. A good idea since, if you wire up these components incorrectly, they can be damaged or can damage other electrical components on the bike.

K6 (1975)

The K6 introduced new switch gear adding a main lighting on/off switch to the right hand side and the moving the dimmer function to the left hand switch:

I can't find a clear picture of the K6 wiring diagram to confirm, but presumably Honda were able to make this change by altering the wiring in the switches, rather than modifying the harness.

Its not obvious why Honda thought this change to the switch gear was a good idea, perhaps it was needed to meet some regulatory requirement?

K7 (1976 onwards)

The final major revision was to rearrange things so the headlight and taillight were permanently on, which became a federal requirement in the US around this time.

This relegated the left hand switch gear to a single-function (kill switch).

The alterations are highlighted in the diagram for the 1977/78 model below:

1977/78 wiring diagram. Note the main harness is the same as the K5 version, bar the adoption of all black wires for switched power, compared to the earlier versions which used a mix of black and red wires (for no apparent reason)

Note that as part of the move to 'always on' head and tail lights there was also a small change to the harness connections in the headlight bucket. In previous versions of this bike the headlight switch - as well as turning on the lights - also connected the yellow and white wires in the harness together so that the output from the 'booster' alternator coils would be added into the charging circuit to compensate for the extra power used by the lights. Because the lights were now on all the time this switching operation was no longer required and the wires in the harness were joined together in the headlight bucket. This connection is shown in the magnified section circled in the diagram above.

You can read more about the charging system on this - and other small Hondas of the same era - below.

Honda C90 - charging system
The charging system used on the 6v C90 super cubs is pretty straightforward and consists of a battery (to store electricity), an alternator (to generate electricity) and a rectifier (which converts the alternating current created by the alternator into the direct current needed by the battery). Here is the wiring

Early models

Notable differences for the K0 are

The closest diagrams I can find for the K0 models is the Honda diagram for the earlier Honda Trial 90 (CT200 - 1964), and the Clymer version for the K0 (buyer beware!).

Future articles

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 in future articles. One of the early jobs is to make a replacement wiring harness, of which more here...