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Turn signal speed varies with headlight usage

My car is a 1998 Honda Civic. The left turn signal blinks at double speed when the headlights are off. When the headlights are on, it blinks at half speed. Should I replace the left-sided bulbs? Or the turn signal relay? Or something else? None of the bulbs are out in the front or rear.

One other fact: If the hazards are on with headlight OFF everything blinks in synchrony. If the hazards are on with the headlight ON the front blinkers flash alternating (left-right-left etc).

It’s only a minor annoyance, but if it’s an easy fix I’d be interested. Thanks!

It would be interesting to know if the voltage going to the blinker relay changed when the headlights were on vs off. I guess that’s the first measurement I’d make.

Sounds like some sort of ground problem.

I like @NYBo’s suggestion. Just to expand on his suggestion, A lot of turn signal bulbs are dual filament style, with a bright filament used for the actual turn signal, and a dimmer filament used for the parking light. A bad ground at a dual filament bulb will allow a turn signal voltage to seek another ground through the other filament, changing the load on the relay and changing the blink rate. With the headlights on, the other filament is also activated and the load changes again, also affecting the blink rate.

I am not a mechanic, but I had what might be the same problem in my 98 Civic Si Coupe. One of the filaments was burnt out in one of the turn signal bulbs. Left turn signal blinked fast.
Replaced front and back bulb for a couple of bucks and that solved the problem.

From what you say about the problem I suggest you look for a bad ground connection on the front left side. It looks like there is a resistance in the ground for that side. You could prove by running a ground jumper to that side.

I would check all the bulbs carefully. I don’t know what style of bulbs your car uses, but the older style bulbs have either a single point of contact on the bottom or 2 points, like little round buttons. These are not interchangeable with each other. If you recently replaced a burnt out bulb, is it possible the wrong bulb was used? The ones with a single button on the bottom when used in a socket for the other style will sometimes bridge both connections in the socket and give you all sorts of weird effects depending on how the circuit is wired.

What you say there @oblivion makes a lot of sense. You may have called it.