I've just replaced the old tail lights on my 1993 Triton ute with LED ones. Now I have the issue where the indicators flash at such a high speed it may as well not be flashing at all. If I put the hazards on, they all flash, although at a faster speed than usual.
I've come to the conclusion that the flasher circuit in the old truck must somehow use the resistance of all the lights to determine flash speed and as there is still some of the old bulbs at the front, when they're all on, it works... but when it's just one side, the resistance isn't enough.
Does anyone know if I am correct in thinking that and if so. How would I go about fixing it?
You're exactly correct in that thinking (I replaced the bulbs in my old Volvo once and it caused it to think they were out because the bulb was slightly different (these weren't even LED's; just a different filament or connector or something, I forget; no tolerance at all on that circuit).
I don't know if there's a standard way to fix this (other than use LED's that were designed for it), but it should be easy enough to measure the resistance across the original bulbs with a multimeter and then go out and get some resistors of the correct size. If you're not using LED's designed for this, or aren't putting a resistor between the LED's and your power source anyways you're probably just going to burn the LED's out rather quickly.
I believe it can be looked at like this: When you have a turn signal bulb go out it causes the other to blink almost twice as fast. Since LED's use much less power, then it stands to reason that by switching to LED's you are not using the expected amount of amps to light the LED's hence, the system see's this as a burnt out bulb.