Hi All, To start off, I've read up on several threads here concerning alternator issues w\ these S197 Mustangs. On the last 2 maintenance services (1-19, 6-19) done by the Ford dealership on my Stang, the battery check came out in 1-19 as bad (590cca batt-421cca test) then as marginal in 6-19 (590cca batt, 525cca test) but all seems to be fine w\ the car but since reading all these threads I went & ran a series of checks as follows: 1. Tested batt using DVOM (Klein MM300 DVOM w\ DC blocking diodes) set for VDC w\ Key Off Engine Off @ 12.3v. With Key On Engine On at hot idle (750 RPM) test @ 14.4v says alternator is charging OK. 2. Tested alternator using DVOM (same as above) set for VAC @ 200v (lowest scale). Disconnected alternator B+ main cable from batt + terminal & isolated then started engine & test for AC ripple....got 1.2v so this result says that my alternator is potentially starting to break down as this is greater than the .5v AC ripple amplitude threshold that I've read.....this correct? Shut engine down, reattached alternator B+ main cable to batt + terminal, restart engine then test for VAC between batt + & batt - terminals at hot idle (750 RPM) & got 30.5v. Is this the amount of total AC voltage leaking from my alternator over the alternator's B+ main cable due to the amount of measured AC ripple voltage & swimming around in my car's electrical system since the DC battery won't accept/absorb any AC current? Is this excessive? Sure looks to be but there seems to be no ill effects observed anywhere from any system on my car at the moment (for now anyway). So since this battery has tested to be marginal\bad on 2 prior load tests, the date on the batt was 10-1-15 (assuming the date the batt was installed on the car for warranty purposes which makes it 3yrs, 11 mos in service) & was on the car when I bought it on 10-17 & have nothing to validate any warranty claim on it, I went ahead & bought a new replacement battery (DuraLast 96G series 590cca w\ 95 min reserve from AZ w\ 5 yr warranty, 3yr full replacement w\ last 2 yrs prorated replacement) & installed it on my car so that I can give myself some cushion for the time being (planning to take her on a 2,000 mi round trip very soon) but from all this it appears that I also need to look at replacing the alternator soon as if what I have measured is correct then this much AC voltage residing in my car's electrical system has got to be affecting any/all sensors that create their own voltage/current (CKP sensors, both CMP sensors, all O2 sensors, VSS, etc) sent to the PCM potentially skewing the numbers as well as any engine controls being controlled from the PCM (TPS, ETC, MAF, IAT, VCT solenoids, injectors, FPDM, FRP sensor, ign coils, spark plug resistors, etc). After replacing the batt & restarting the engine then measured VAC across the terminals of the new batt at hot idle (750 RPM) the VAC was now 37.5v instead of 30.5v. Switched to VDC & measured across same points & got 14.78v instead of 14.4v so the new batt did raise system voltage & alternator output. My Stang is a '09 model so it should be equipped w\ at least a 135A Denso unit, correct? If so then I might look at getting the 160A Denso unit to replace it...….. What is the max allowable AC voltage threshold on these S197's electrical systems before component interference starts to manifest itself in system operations? Have looked in the Ford Workshop Manual but found no info concerning AC ripple voltage threshold. I plan on running tests on all input sensors for signal clarity due to these findings before\after alternator changeout to record\validate any interference found. Would appreciate any info given.