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I have a '76 F250 that's driving me a bit nuts. My blinkers only work when no other electrical accessory is on. When nothing is on they work great. As soon as I turn on any other electrical load, i.e., brake lights, parking lights, heater, etc, the blinking rate slows until they don't flash at all. When I turn off the other accessories they return to normal function. I've recently replaced the alternator, voltage regulator and ICM. What's wrong? Thanks!
Check your battery voltage and the voltage at the input of the flasher. It sounds like the voltage drops when you turn other stuff on. If the voltage is too low, the flasher won't work.
As it works in a 1971, the flasher supplies (+) to the turn signal switch for the lamps. The headlamp switch contains two relays, one for the headlamps, one for the other lamps. I would tend to think that the flasher(s) is bad.