86 930 - left turn signal flashing fast - all bulbs working
 

Normally if you have a signal light out, the signal lights in that side flash fast.

Mine are flashing fast. All bulbs are ok. I removed the bulbs and cleaned them up and it made no difference.

Any ideas?

I just had a similar issue on my ‘87. Turned out the bulb contacts were making intermittent contact, so one bump would send it from working to not working. Also, with key off, turn signal will turn on (non-flashing) lights on that side… easier to see intermittent contact when you wiggle the bulb if it’s not in flashing mode. Tougher to diagnose when they are flashing.

If lights are working, but you’re getting the rapid flash, your bulb may not be generating enough resistance. LEDs will cause that type of issue if they don’t have proper resistance added.

I changed nothing from when they were normal.

Maybe my flasher relay?

Yes pretty much one side flashes fast when a bulb circuit is out. And both dash green indicator and the exterior bulb is flashing rapidly.

You say you never did any exterior bulb replacement and you checked bulbs on that side, Was that a continuity test or visual - and/or did you swap them out to the right side to isolate if it the bulb or the socket?
Original bulbs usually are brass bases and new replacements are nickel plated just in case someone replaced them beforehand.
Visually a filament could look attached but under load when signal is activated it will detach hanging on to one side, causing the circuit fault failure.

Whats the status when 4 Way flasher is On? No harm in checking flasher, clean the terminals.

You could check the Brown ground wire on the housings are still intact and clean them.
And maybe checking/cleaning bulb sockets and adjusting contacts.
Anyone could have put an incorrect bulb (posts are staggered) in there long ago messing up the connection points.

I will check with the 4 way. I did a stationary visual test. All lights on the left flash… fast. The right flashes normally.

I will second the comment to clean the flasher terminals . . also add clean the fuse ends and contacts . . had the same issue with my 72 and this solved the problem.

The fast blink is the car’s way of telling you it thinks a bulb is out, even if the filaments are fine.

I would Replace the dual filament bulbs. When one of the filaments start to fail, you start getting weird things.