Can a failing alternator cause transmission issues?

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?
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On Thu, 6 Jul 2006 08:16:12 -0400, "Stephan Mynarkiewicz"

The alternator may be failing but, the part you are after for the trans shifting problem is the VSS. As far as a bit of a moan, that is sometimes normal for an alternator working at full load which may be the case on a very cold start when the battery voltage is most likely to be the lowest. If it is most or all of the time, it is likely a bad bearing. Are you sure it is not a belt tensioner bearing? They are the most common failures in the area. I have also seen a couple of cam belt idler bearings fail and get noisy. If you can get at any of these things, get yourself a piece of something like an old broom handle. Hold one end against your ear while you carefully place the other end one suspected areas to isolate the noise. Br carefull not to get yourself engaged with any of the moving parts.
Lugnut
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Stephan Mynarkiewicz wrote:

I'm no computer or electronics trouble shooter but I did go through a similar problem with an Isuzu Rodeo and found out via the Isuzu n/g that it is common with Isuzu's. Occasionally the trans would just go into a kind of free wheeling (like being in neutral) after coming to a stop. A quick restart and it would be fine. New alternator and battery fixed the problem.
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How many miles are on the alternator?
And instead of trying to fix the transmission (potential symptom) try concentrating on the known problem (alternator) since it's much easier to fix.
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Too much ripple in alternator output can cause any number of concerns with electronic systems.... You can measure ripple using your DMM set to a low AC volt scale -one probe on the B+ terminal and the other on a good engine ground (not the B- terminal). Volt reading should be less than 0.1 VAC with no spikes.
We have also seen secondary ignition concerns cause false VSS readings - bad enough to shut fuel off momentarily (PCM sees overspeed condition).
For the VSS itself, you can try cleaning the area thoroughly, remove the attaching capscrew and rotate the sensor while bathing it in a good penetrant.... Often, the area between the O-ring and sensor top can collect grit or corrode or the O-ring can get a "death grip" on the housing.

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I did some testing on mine today--set my meter on the 200mv AC scale and checked. On cold startup I'm getting around 100mv (AC) from B+ at the alternator to the alternator case ground. Reading seemed to drop back off to a couple millivolts after a few minutes so I went down for the mail and drove the car around for a couple of miles around the neighborhood then came back home and measured it again...B+ alternator terminal reading (AC) was now driving my meter nuts (overrange--"BEEP BEEP BEEP"). All the connection points I checked yesterday seemed to indicate that the basic wiring circuit is OK (battery 12.82v with the engine off), so definitely seems like I've got a leaker.
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Update on my situation--new alternator fixed the ripple issue (new one has <.05VAC at idle, and in the .02-.03VAC range at speed), but unfortunately, the screwy transmission issues persist. After spending part of the day yesterday testing out the VSS circuit, cleaning the VSS harness connector (with a small needle and some contact cleaner), as well as rigging up a rudimentary heat shield for it out of some sheet aluminum, I tried it out today. Not any sooner than I get out of the driveway, it starts acting batty again. Usually, I wouldn't have any problems for a couple of miles (at least) until it got hot, but today I didn't even make it 200 feet before having problems. Transmission acts like it doesn't know what gear to be in as well as the speedometer jerking/stopping in "mid air". Something tells me that unless the speed sensor itself suddenly "died", the real problem may be in the VSS connector/wiring.
Being that I was "lucky" enough to have bought another VSS (gee, would be nice if I could get the old one out to replace it), I'm thinking that I'll plug it into the harness (in place of what's installed), attach a drill to the other end, and spin it while observing (on my laptop) what the computer is seeing. I'm figuring I can also jiggle/bend/tap/pull on the wiring to see if some intermittent glitch turns up. Only problem, of course is that I'll have to "add" some miles to the odometer from the drill trick, but if it helps me to condemn what the real problem is, I guess it's a small price to pay.
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