Have had my ’74 1.8L since new and this problem just started – the single turn signal indicator light does a steady flash, once every 5-6 seconds, for no (?) apparent reason, all by itself. The relay clicks and sounds as expected, but the turn signals aren’t on (they all work just fine) and the parking brake switch seems OK. Changed out the relay (3 times – the URO replacement), cleared up all turn signal connectors and replaced all signal bulbs. All fuses look good. This had been an intermittent situation for a few weeks, but now it’s all the time. Went away for 20 minutes when replaced the relay the second time, but came back. Seems like a square wave signal is getting sent to the flasher relay? Any clues as to what’s going on?
Thanks!

I think I have a spare original relay if you want to play with it. Might help narrow down problem.

That's part of the problem. When we don't have quality parts, it makes it harder to maintain and diagnose our problems.

There are a bunch of threads on this issue with some explanations of that circuit. Search is your best friend with this one.

I recently solved this one but my car is a 73.

So you've been through 3 relays and all do this slow flash?

When you say 5-6 seconds, is this 3 seconds on, 3 seconds off or different?

If all 3 relays do the same thing, it seems you may have some sort of leak either through the brake switches (ebrake and master cylinder brake).

You can check the brake switch by removing the brake warning bulb (temporarily) and see if the slow flash stops.

BTW, the turnsignals will only come on if the stalk is pushed for L or R turnsignals or the 4-way flasher switch is engaged.

It goes on briefly for 1 second (not too slowly nor too quickly) then off for 4-5 seconds, then back on. Sort of like a short "square wave" signal coming from somewhere (or being generated by the flasher itself). Yes, quite unusual for all 3 to exhibit the same characteristic.

Thanks, Mark. I've been able to get my hands on a "nicely" used original so will try that, along with checking out those two break switches. The red brake warning works just fine and goes on/off when I press the parking brake button manually. So, maybe (?) either something with the master brake cylinder switch or indeed errant relay design for '74+ models.

As impossible as it might seem, it looks like it was the flasher relay, despite using/installing 3 other new ones (all URO). Removed/cleaned the parking brake switch just in case, but to no solution (but now nice and clean!). Installed a new Wehrle flasher that arrived yesterday and it's back working fine again. Will keep monitoring and take it out for a spin this weekend. Of course, the parking switch didn't look ANYTHING like the replacement parts though somewhat close to a Karmann or 412 door switch, but that's another thread...