Yes, it works exactly like you suspect. The nominal frequency is 100Hz and pin 23 is just an inversion of pin 10, with duty cycle and inertia of the valve's armature determining the air allowed through.

And my sincere apologies for the brain fart, it is indeed a -510 I was thinking of - perhaps jumping to the conclusion LH2.0 was used in 84 on 700's as it is on the 200's. I hope Bob saved you from snatching any ebay 503.

You have to be careful interpreting those meters' frequency function and so forth because you don't know what level it is getting to count crossings. It could be just stray noise on an open line if the transistor is opened up. A test light should show you if the transistor is open by connecting it between +12 and the two leads alternately without the IAC motor in the circuit. The open side won't light.

Sorry again for the -510/-511 mixup.