Firstfives.org

I checked our parts interchange FAQ, but don't see this one.

I believe the early E12-based E24 has the same coolant reservoir, except the E24 part has an extra port
designed for a level sensor. Can anybody with both cars confirm?

I consider that extra port an upgrade - extra gauge, extra sensor, plug it if not used. In this case, I'd like to stick
a sacrificial anode in there to help protect aluminum components in the cooling system, like my all aluminum radiator.

Yes, Robert this is true. I had found one at the junkyard from E12 and turned out it did not have the fluid level sensor, but otherwise was the same. If I recall, I am still using that reservoir and just tucked the fluid level wire out of view in my '80 635 (type code 5331 in Real OEM).

Awesome! Thanks Tom. I thought about digging up some old pictures, but a confirmed swap is certainly priceless
knowledge before dropping $$, especially on a new one. It also helps explain the giant, unused, raised circular region
on the E12 part mold - it's a threaded hole on the E24 version.

My reservoir cracked and I bought an e24 reservoir as they are easier to find, and it came with the hole so I then bought the sensor and plugged it up. It's held just fine for a few years now.

The sensor circuit is wired back to front to what I would expect (designed to work with the "check" light) - if the coolant is okay the circuit is closed and the indicator light would light up to say "coolant all good". A little weird because if your coolant was low, the light wouldn't come on and you might assume the light is broken.

So, you would need to invert the signal if you wanted a light or buzzer to indicate when coolant was low, I planned to do that but didn't get around to it.

Sure thing! At the time I was too cheap to buy a new one when the junkyard one was uncracked and almost translucent still.

Lock is exactly right as this operation is opposite of the E23/E24/E28/E30 operation. They must have realized that a warning light is more intuitive than a light being unlit.