All 4 external turn signals stay constantly illuminated. Same for the 2 indicators in the instrument cluster.

Indeed. The FSM "Electrical Troubleshooting Manual" has detailed schematics which proved useful as I worked this matter. Though quite predictably, the relay is referenced only as "SOLID STATE".

So far, the hazard switch does not appear to be the source of the problem.

Thanks for the replies.

Current status:
The relay's polarized electrolytic capacitor has failed. What a surprise after 37+ years! I'm seeking a suitable replacement.

Hmmm, good to know that there is a cap in there to go bad. Looking at the picture, looks like on on the tach cluster I just replaced.

When my hazards stop buzzing and stop working, I'll know where to look now!

I am an info magnet and I could not stand not having a decent manual for my Mercedes bedause I know much money can be saved if you have a good manual.

I have long had the CD manual and later bought the paper manuals. Unfortunately, I cannot remember the exact details of my own turn signal failures. I took a chance on the hazard switch being the issue because it was cheaper compared to, they turn signal relay. I believe I bought a Hyco switch.

Never done much replacing or testing of circuits.

__________________
84 300D, 82 Volvo 244Gl Diesel