For awhile now, I've been having occasional problems with my '99 Ford Escort where the (automatic) transmission would have a nasty habit of hunting gears at low speed. Computer never turns up any codes, but if I plot the car's speedometer reading in "real time" (via my laptop & OBDII cables/program), whenever the problem exists, the vehicle speed reading shows spikes while "real speed" is in the 5-12 MPH range (occasionally spiking to an interpreted 20-25 MPH reading @ 10-12 MPH real speed). Get the car over 13 MPH and there's no problem, and I find if I accelerate moderately (and not gingerly) from a stop, I can usually avoid the goofy shifting issue altogether.
Unfortunately, the VSS seems to have seized itself into the transmission from age (shame, since I have a new VSS laying around), so whenever the problem would become a pain, I'd jack the car up, take the connector off the VSS and throughly clean the inside with electronic spray cleaner, which usually seemed to work for awhile (at least it seemed like it) as the car could go awhile (weeks-months) without the problem turning up again. From the sound of it, the transmission would have to come out to get at a seized VSS, so obviously that was a solution I'd tried to avoid, if at all possible.
Which brings us to last week...after a long trip, I've found that the car is now making a ghostly "moaning" sound while idling/low speed as well as whining while driving. Tracing the source of the noise leads me to the front of the engine, right near (if not at) the alternator. I'd occasionally have some groaning noise from that area when the weather was cold (winter) which usually went away when it warmed up (ambient) which I attributed to just the weather being cold, but now it doesn't shut up. Serpentine belt was changed awhile back and the tensioner and idler pulleys seems to be OK (no wobble and the tensioner holds) so I'm assuming it's not either of those.
For the time being, the battery seems like it's still being charged (12.5x V off, 13.7-14 v at idle, around 13-13.2 v at 2500 RPM w/everything on), but I'm wondering--could the alternator be screwing up the ECM if the output is less than perfect? I know the VSS circuit does have a "filter" to clean up it's signal but doesn't even that have limitations?