1990 Ford f250 brake lights and turn signals won't work - checked all fuses, flasher and bulbs and all are good, what could problem be??

any truck that old is super hard to find schematics
to simple voltmeter checks find this cause.
fuses, switch, wiring, and lampsl
fuse = haz/stop
goes to stop switch
then stop sw output pin red/black
goes to DIRECTIONAL switch
if wiring is good and any trailer hitch wiring mess is repaired, then this switch below is bad.
brake wires is GREEN I think. see it?

Do you have brake and parkng lights? The turnsignal switch on the column is just designed to relay a path to the flasher and then it is designed to interupt the brake and some of the parking lights lights, which cause the flashing on and off. Double check fuses double check bulb socks and bulbs for good contacts. Does the 4 ways work? If the lights all flash then locate the turn signal flasher and switch with emergency flasher if possible to rule out relay. If those are all good and the emergency flasher lights work and bulbs, then you would have a problem with the turn signal switch. Hope this all helps

Check your fuses for the turn signal - also the relay unit could be bad - these are located drivers side just below the steering column (there should be a removable panel there) - I have an 89, and that is where it is on mine.- google aerostar fuse dia and you should be able to find the fuse box layout, so you know the ones to check.

I would have said to check a bulb, but since the flashers work, you know it's A) not a bad bulb, and B) not anything with the wiring...

Good Luck

Yes, it does have a flasher, although it may be a combined unit with the hazard flashers relay. So you hear the clicking of the flasher relay? That seems to indicate the relay is working properly. Have you checked the bulbs and the turn signal fuse? Any turn signal bulbs light up anywhere, front or back?
How about your brake lights, do they work? Your car may have separate, stand alone turn signals in the rear, but many American cars use the same bulb and same filament on the bulb for the brake lights and turn signals. If your merc is like this and the brake lights work, then the bulb must be good.
Turn on the hazard flashers and see if all bulbs work and flash at each corner of the car. This would be the same bulb as for the turn signals. If they work, it could be a bad turn signal switch, may even be the flasher relay somehow malfunctioning, and in a few cases, a bad hazard switch can affect the turn signal switch. Check fuses and bulbs first. If you have any type of body control module that controls lights and other accessories, you may have to investigate that. I know I'm not being very helpful, just throwing out a few suggestions. Good luck.

You may have a bad signal switch..Your brake lights run through the signal switch..Try hitting the brakes and move the switch around and see if your brake lights come on intermittently..

In the fuse panel with the fuses inside the car there are two big round units plugged into the fuse panel, one is your turn signal flasher one is your hazard flasher. I would start with those and move to other things if that doesn't fix it. Hope this Helps.

there is a flasher for both 4 way & regular turn signals ( check to make sure the bulbs are good
the backs are easy >>> Just step on the brake, If the lights light up the bulbs are good you my have to remove the bulbs one at a time in the front check your ground wires to the lights if everything is good replace the flasher unit if that fails replace the turn signal switch

did you check light bulbs if so and ok
check fuses

First check your fuses. All I believe are separate. If the lights come "on" when your signal is switched on then could be a bulb is bad. This is true when it blinks at 179 mph also. Check all your bulbs..most likely part number 1157's or 2057's. If one bulb isn't working could cause all not to. Being that all bulbs must function to complete the circuit. Change your flasher just for the fun of it too. Cheap fix. If not any of this then you need to start checking your sockets for conductivity. Chow!

Many cars have the same fuse for turn signal and brake lights. Check that fuse for a start.