An indicator relay works by having a bimetallic strip that acts as a closed switch when cold. The circuit is completed when the indicator bulbs are connected to the circuit. This is done by the handlebar switch. Electrical current then flows througth the bulbs illuminating them and also through a wire wound wrapped around the strip heating it up. Because the bimetallic strip is made of two different metals, one metal expands more than the other causing the strip to bend until the switch opens.. Now electrical current no longer flows and the indicator bulbs go out. As the strip is not being heated, it cools down until the switch closes and the cycle is repeated.
Ok, the Vmax inidator unit is a bit smarter in that it takes a signal from the speedo so that the inidcators turn off after a specific time or distance travelled - this is to prevent leaving the indicators flashing when people have forgotten to turn them off.
Anyway, none of that is relevant other than the heating up of the bimetallic strip. The amount of power available to heat it depends on the resistive load of the indicators.
Filament bulbs have a much lower resistance than LED bulbs which means that fitting LED bulbs, less current flows so there is less power to heat the element so it no longer will open or the bulb flash rate changes.
To get around this, a resistor is fitted across the each indicator bulb, as in diagram below. It simiulates a filmament bulb being fitted so needs to be of a reasonably high wattage (23 W is what the manual says but 12 W may work). You would need 4 resistors, one per bulb, or 2resistors, one for each pair of bulbs on the same side.
Trying to find the above diagram, I came across these:
They look like they fit a resistor across the bulbs; separate the connector going to the bulb, connect it to the unit below and the then connect same colour wire goes to the bulb. Do that for both red (power) and black (ground) wires.
You mention there is one wire going to the indicators you have - that would be the positive, the negatvie is grounded to the chassis via the metal indicator stalks.. Unless I've misunderstood. With these, units, you would need to find a suitable earth to connect the black wires to, just find a bolt on the frame, connect the black wires to a ring terminal and connect it with the bolt - you would need to scrape paint off the frame for good electrical contact...... but that should not be necessary as I am sure the Vmax loom has four wires for the indicators; two positves and two earths.
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