I am nearing the end of my year's long journey to bring a 2002 E350 Super Duty RV back to life. Have overcome about 1,000 issues and obstacles. One of the last is turn signal Hyperflash. I updated the (one) flasher relay under the dash with an LED compatible relay. Still experiencing hyperflash on both sides. Question: Is there a second Flasher Relay that I didn't find? Could not locate a diagram for where the flasher relay(s) live. Found one flasher relay dangling from behind the fuse box under the driver's side dash and updated it. And yes I know that installing resistors in all four corners will definitely fix it, but I'd sure rather update a relay and move on.

 

Open circuit makes them flash fast (your electronic flasher does not know bulbs are present)
You could
Check each bulb socket for continuity (futile as they do work right)
Verify you have the correct bulbs in each socket (means get rid of the LED's)
The LED's are your problem if all the sockets check out
NO LED's is a good policy going forward (unless you like trouble)
The LED's in the brake lamp circuit will affect the transmission
Bad news those LED's in a vehicle not designed for them

 

There is only one factory installed flasher module. Your motorhome may have modifications that added another. Here is the wiring diagram:
https://charm.li/Ford/2002/Econoline...urn%20Signals/

Note there are wiring pigtails for cutaway versions...like a motorhome chassis.

I don't understand why a flasher module designed to work with LEDs is hyper flashing. Can you return it and try another product? I am generally against LED exterior light "upgrades" on older vehicles. They can be a pain to get the right color, intensity, beam pattern etc. But I confess I installed LED rear tail lights on my E350 extended van. The problem was the old wiring, sockets and connectors were not providing the turn signals with 12v for flashing. The flashing voltage reached about 10v (when the system was providing 13-14v) then turned off for the the next flash. Different flasher modules gave the same results. This created a very weak turn signal light. I bought the brightest red LED combination bulbs and used resistors and the results are good. This works because the LED reaches full intensity very quickly at a lower voltage (voltage depends on specific bulb design). But I created a problem where the brake lights could flicker if the brake pedal was depressed just enough to quickly trigger the brake light switch on/off. My solution was complicated and not worth a discussion here.

The LED lights and transmission problem affected older vehicles than yours (and my '97). But it illustrates some pitfalls of LED retrofits.

 

If the vehicle was not designed around LED lamps, using them always causes problems
This problem is just the beginning
On that 2002, the ABS won't like it nor will the transmission, cruise might not even work