You have to remove the Klima relay and jumper the compressor on to test the voltage generated by the speed sensor. It's expected to have 0v when the compressor is not turning.

There are a bunch of causes of premature compressor shutoff in the 124 cars. I don't recall seeing failed speed sensor as very common. Have you checked the air gap on the compressor clutch? It increases over time as the clutch material wears. The specification is 0.50mm, plus or minus a tolerance that I cannot recall at the moment. Suffice it to say that once the gap hits 0.80mm or so compressor engagement becomes a dicey proposition.

Sometimes simply cleaning the compressor clutch air gap to remove grease & grime is sufficient to restore normal operation. Use brake cleaner or just simple green and water.

Have you used the search function? There are a ton of old threads on troubleshooting compressor cutout. Grab your beverage of choice and settle down for an evening of reading...

- JimY