Filament in one of the bulbs has broken and fallen onto the other filament. This makes the blinking cross over to the other side via the parking light circuit.
Either replace the front turn signal/parking light bulbs and the rear turn/brake/parking bulbs or remove and inspect them all till you find the bad one.
Bad ground do cause weird things to happen, verify all grounds.
Charles
Why do both rear turn signals blink when the right side turn signal switch is turned on?
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On my 90 ford ranger, both rear turn signals will blink when the right side turn signal switch is turned on, while the right front turn signal blinker blinks, ok, as it should..
The left front and left rear turn signal blinkers work fine when the left side turn signal switch is turned on?
I don't know what's causing both rear turn signals to flash when I use the right turn signal switch blinker, as the left side works fine without any problems..
Another thing I noticed is that if I put on the right side turn signal switch blinker, without the key in the ignition, and press the brake pedal..
The right front, driver's side, turn signal light will light up and the turn signal flasher will start clicking, but the right front turn signal doesn't flash, while the brake pedal is being depressed and the rear brake lights will stay on..
While the turn signal flasher is clicking, as explained.. When I turn the heater blower motor's switch on, without the key in the on position.. Each time the turn signal flasher clicks, the heater blower motor will start up on each click the flasher makes and then stop between each click..
Another thing I also noticed is if I put on the emergency flashers they will work fine, until I depress the brake pedal..
Then the emergecncy flasher lights will stay on and stop flashing, as long as the brake pedal is depressed..
I've never had a problem like this, before..
What is the solution to this problem, when the right side turn signal switch blinker is turned on and both rear blinkers will start blinking?
Do you think the turn signal switch is faulty and is somehow crossing power over to the rear left side blinker, when the right side blinker is turned on?
Has anyone else experienced this kind of problem, before, and know what needs to be done to fix it?
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Nothing I've done, so far, has stopped both rear turn signals from flashing when I use the right turn signal..
I tried replacing all the front and rear turn signal bulbs..
I tried a different turn signal flasher..
I tried a different emergency signal flasher..
I took the turn signal switch apart and cleaned it..
I tried using my other 89 Ranger's good working turn signal switch..
I've cleaned all the ground connections that I could find..
Every electrical connector that's under the the hood I've taken apart and cleaned and sprayed with WD/40..
I took the brake light switch off, and sprayed it with WD/40..
I don't know what's causing both rear turn signals to flash when I use the right blinker, as the left side works fine without any problems..
Another thing I noticed is that if I put on the right side turn signal switch blinker, without the key in the ignition, and press the brake pedal..
The right front, driver's side, turn signal light will come on and the turn signal flasher will start clicking, but the right front turn signal won't flash, while the brake pedal is depressed and the brake lights are on..
Another thing I also noticed is if I put on the emergency flashers they will work fine, until I depress the brake pedal..
Then the emergecncy flasher lights will stay on and stop flashing, as long as the brake pedal is depressed..
I've never had a problem like this, before..
If anyone else has had a problem like this, before, what did you find was causing it..
I still haven't been able to figure this out, yet..
If anyone reading this is smart enough to be able to figure this problem out, and know what has to be done to fix it..
I'd like to find out the solution, so I can get this very frustrating problem fixed..