I have a 2001 Ford Focus that has some funky problems at idle under load. A nice high-load scenario is by abusing the windows motors (rolling them up while windows are up), which causes the RPM to tank and the IAC kicks in after 200ms or so to keep the engine from stalling. 
What's weird is on another vehicle (2001 Saab) I see the voltage go down, but the engine RPM doesn't change at all! I've tried keeping my foot on the gas slightly to account for the difference in idle RPM, but still the RPM tanks.
I recently replaced the battery to see if this would help, but no avail.
Why wouldn't these intermittent electrical loads go through the battery?
I will look into the harmonic balancer. It's a mazda engine: Rough idle only under load
Other issues with car:
- Rear oxygen sensor doesn't look so hot.
- Intermittent bad ground related to fuel relay.
- Fixed a bad motor mount, nearby the harmonic balancer. So that might've abused it.
Update January 25 2018
The titles for the chart are:
- RPM = RPM
- ENGLOAD = Engine Load
- IAC = Idle Air Controller %
- VPWR = I think voltage at battery, will confirm
- GENVDSD = Generator voltage desired. This tanks too, which doesn't make sense...
- S11 = Upstream O2 sensor
- S12 = Downstream O2 sensor
Tested resistance between battery negative and far end of engine block (< 0.1 ohm) and negative of fuel pump relay (3 ohms), both of which are less than 5 ohms. However I will check voltage next and look around for the ECM.
Update January 27 2018
Not sure what genmon means exactly, but it definitely moves. Also the gencmd (commanded duty cycle of generator from pcm) looks like it takes off too. VPWR is the voltage at the PCM, so I think that's ok.
