You have a TS controller, 1800C has a single indicator.
Are both right turn lamps flashing?
There is a 9 pin connector on the unit, separate the cable from
the TS controller and spray some Wd40 or something similar on the contacts.
Also in cold weather sometimes TS stops working when grease is old and dirty.
I suggest a clean and lube of the TS switch.
A PDF for cleaning/lube early 1800 with 3 wire flasher and 1300 with 2 wire flasher, both had dual filament bulbs in front for Run/TS. 2nd Gen 1800 has an Electronic flasher with a single filament bulb with dual mode Run/TS in the front bulbs. If you examine the T/S switch and the rear TS bulbs...
The left and right turn signals work properly.
The entire switch assembly on the handle bar has been replaced with a brand new assembly.
All connectors have been cleaned and meticulously lubed with dielectric grease.
The flasher unit has been replaced.
The fuse boxes have been cleaned and lubed.
Do you have the correct TS controller???
This Honda 38301-MEC-671 TURN SIGNAL RELAY fits the following models and components:
1800C, several Valkyrie Rune. Single TS indicator.
This Honda 38301-MCV-N11 TURN SIGNAL RELAY fits the following models and components:
This one shows to fit all 2nd gen 1800. Problem is all the other 1800 have two TS indicators.
I would check to see if one of the pins on your TS controller has a Right Turn indicator and no wire attached.
If that is the case then I would make a dual diode, have both TS indicator wires connect to the diodes
on the anode side, then the cathodes connected together go to the TS indicator.
Thank you for the suggestions. I will check to see what relay it has.
In case it may be important,
This 2006 1800c is a trike conversion with a Champion Trikes kit. (So it now has Two tailights) rather than the Single taillight
I have also installed upgraded LED bulbs in the front turn signals, as well as a LED compatible flasher.
If you have LEDs in the turn signals, don't you also have the required two-diode fix for the indicator light in the instrument cluster?
I'd wager an educated guess that one of the diodes is blown. Or its wire came loose.