There are things like this:
https://www.amazon.com/dp/B009H4AYIII use specifically this one:
https://www.picotech.com/oscilloscope/2 ... 0-overviewwhich is quite a bit more, but since I really don't know what I'm doing most of the time, I wanted to have good software and quality documentation to fall back on.. Pico often assumes the user is an idiot like me, and they have howtos on how to test a variety of common automotive sensors.
In the intervening years, things like this have started to appear:
https://www.amazon.com/dp/B08DG1C129/and I've been tempted to try one. For what I need, high accuracy isn't *super* important - with automotive sensors you're generally looking for patterns so I think something like this would work great. And not having to drag my laptop into the garage would be a plus! But until the darn CRD stops bleeding me dry, I can't even think about it.
I would *think* most modern multimeters can measure duty cycle, so assuming my theory on operation is correct (and I'm *reasonably* sure it is) you should be able to throw a multimeter in % mode and read that wire. I don't know whether the control is + or -, but that would only take a moment to determine and swap the probes around. On some (most?) multimeters polarity won't even matter.
The gotcha is that without specs, I'd think it tough to correlate what you read with anything truly useful. The control module just says "gimme more" or "gimme less" based on its algorithm. Without knowing that algorithm, you'd never know if what you saw is correct or not for current conditions. Also, it'd be impossible to determine the functional max value ... like, is 50% duty cycle enough to get full performance from the alternator, or is 100% duty cycle required?
Just a WAG here, but probably the even though the dimensions are unique to the CRD, the alternator itself shares operational attributes with a bazillion other alternators. If the "family" is known, probably how it works is documented somewhere. That's true of Bosch, GM, and Ford... I'm sure it's true in Mopar-land too.
Here's the flavor...
https://www.youtube.com/watch?v=-zC24W7dEH8