I've got an AMM that's suspect. The car is a '84 B23F with LH2.0

Symptoms: Rough idle (despite new vacuum hoses, propane checks for leaks at gaskets and seals, etc.), but when the AMM's plug is disconnected, the idle smooths out (although also speeds up). Normally I'd jump to the conclusion (just on this basis) that it's bad. But given the price of AMMs (there's no p-n-p around here with any 240s left), and not having a known good one to swap as a test (my other 240s are both LH2.4 with an incompatible AMM), I've used Bentley to make some additional electrical tests.

First, the pot readily adjusts the whole resistance range, and I leave it at 300-320 ohms because it gives good state emissions test results over many years (a tip, long ago, from this forum). Power and ground checks are also good.

However -- and here's the really strange thing -- I also checked the AMM using an old dwell meter (per Bentley: 4 cyl. setting, testing at the pink wire side of the test plug near the coil) and the meter doesn't show any dwell! It could be a bad meter, but a frequency meter (the only other thing I have) at the same pink wire test point shows a fluctuating 25 to 30 Hz (and turning the pot can change this only slightly).

So, on one hand it seems good (the pot seems good, along with power and ground), but on the other hand the idle is bad with it plugged in (contrasting with it unplugged), and there's no measurable dwell.

Is this enough consistent evidence to justify buying an AMM?

Thanks, all