After towing RV with bad batteries, truck blinkers...

When I had a similar problem several years ago it was a flasher relay gone bad. Easy to replace and problem solved. O'Reilly's has been great help to me on several occasions and saved me big bucks at auto repair shop.

Sometimes the clutch in my brain slips before engaging. Maybe the delay mentioned in the fault is due to a bad relay.
Often there are several relays in the box with the same part number. Swap the headlight relay with another one of the same part number and see what happens.

groundhogy wrote:
Brake controller removal did not change anything.

CLUE: So I pulled the tail light and had the bulb out to observe.

It is a dual filament bulb. Top filament for blinkers and stop. Bottom filament for nighttime tail lights.

When you turn on the turn signal the blinker speed is normal, you correctly see the top filament blinking.
But then you see the bottom filament start to light up . And the blink speed goes up.

I would suggest there is a very small contact between the positive side of the brake/signal bulb and running light. Just barely enough power getting to the bottom filament to cause havoc. Corrosion may play a part in this too, something on the back of where the bulbs are inserted.

Way back on my 94 GM, the brass buss ground bar(neg)on the backside of the rear lamp assembly was so corroded I had to solder a wire across the terminals.

Im thinking melted something at this point, due to the fact that completely unrelated circuits are interacting.

But that is a great suggestion about the fuses.

Try this to isolate the affected circuit. Start pulling fuses one by one for the lights involved. Put the fuse back you just pulled before pulling the next one. Hopefully that will give you a glue.
Also since you are in PA where rust is a "thing" a frame harness may have been compromised.
That generation Ford had corrosion issues at the top left corner of the windshield allowing leakage down onto the components behind the dash on that left side. That's where the headlight switch is. Ohm your switch or try a new one. Maybe you have a pal that would let you try his switch. You don't have to pull yours, just pull the connector down to plug into the test unit.
Do the 4 ways work correctly? Do you have a trailer connecter in the bed?
Something is back feeding either due to a bad or missing ground or a bad switch somewhere but the delay puzzles me. I hate these type of problems and a lot of techs just throw parts at them and hope they get lucky.

Brake controller removal did not change anything.

CLUE: So I pulled the tail light and had the bulb out to observe.

It is a dual filament bulb. Top filament for blinkers and stop. Bottom filament for nighttime tail lights.

When you turn on the turn signal the blinker speed is normal, you correctly see the top filament blinking.
But then you see the bottom filament start to light up . And the blink speed goes up.

Unplug the brake controller and see what you have then. I've had that issue. I forget the brand but they sent me a new one overnight at no charge.

updated more accurate symptoms
-Blinkers.. both L/R blink
-Breaks... seem ok unless maybe dim?
-Running tail lights.. none in back
-Reverse lights .. none
-License light... ok but flickers with turns

-Blinker speed... Starts almost normal speed and speeds up like when a bulb is out.


I am capable of fixing this, but I need drawings and I am having a hell of a time finding Schematics or Assembly drawings showing component locations.
Still plodding away at this.
F-250 2003 7.3L

This seems to be a single point type of failure. Like one point that is common to each of these unrelated circuits has failed causing multiple failures.

Grounds seem to be distributed at each bulb, so the chance of a bunch of grounds lifting at once is small it seems to me.

Maybe a supply voltage wire might be common to all of these circuits?

Or yes, maybe a wire got hot?

Or a wire was already corroded from age and opened up with high current?

So, I have discovered that the truck batteries and the trailer batteries are connected together directly it seems when you plug in. So the truck was probably heavily charging the trailer batteries during the towing.

Like.. the back up lights and also the night time tail lights are both out.
They should have NOTHING to do with each other. SHould be totally different wires and circuits...

Front lights all good. Look fine.

Sometimes you have to bite the bullet like I recently did with truck electrical and start problems and take it to a AAA auto garage or Napa garage, etc. They kept the truck for two days before looking at it then fixed my problem in only several hours. I have a spare dodge ram so not too much hassle for me. I have time but not knowledge or patience to troubleshoot electrical problems even though i tried for a week. Cost was 170.00 dollars with senior, veteran and AAA discount. Unlike others Sierra Car Care center at Plumb lane and Lakeside Drive was polite and courteous about taking my money.

Yes. The problem remains after RV is disconnected.

Update on symptoms. My first post was relaying what my wife said as she was behind the vehicle.

So last night I went behind the vehicle and looked myself.

When you turn on full night lights, nothing. So when you are driving at night the tail is completely black.

When you put on blinkers, you get a faint fast blinking. The license plate lights will dim and bright together with the turn signal.

When you put on brakes, both back lights blink just like it's a hazard except dim.

Front lights seem unaffected except they blink more rapidly.