Potential Alternator Issue

Discussion in '2005+ Mustang GT 4.6L Tech' started by GlassTop09, Sep 8, 2019.

  1. GlassTop09

    GlassTop09 Member

    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.
    Last edited: Sep 9, 2019
  2. 07gts197

    07gts197 forum member

    If you had a denso on order that is known to be good or had one in hand I would say run it and keep the spare with you on your trip but since thats not the case Id have to say that youre over thinking it. Im not sure what the allowable amount of ac ripple is but if its not throwing any codes and you dont have any symptoms I would leave it.

    Sent from my iPhone using Tapatalk
  3. Dino Dino Bambino

    Dino Dino Bambino I have a red car

    Agree with above.
    Since your alternator is maintaining a 2.1v higher battery voltage with the engine at a 750rpm idle compared to engine off, I'd say your alternator is charging very nicely. You definitely don't need an upgrade beyond 135A.
    Though your old battery wasn't fully charged, it wasn't on its last legs and still had plenty of life left in it.
    I think you're using the wrong method to measure the AC ripple voltage. If your numbers were true AC ripple, the cam sensors would have been playing havoc and your battery would have been fried. Since you have no codes and the engine runs perfectly, I'd say all is well. It might not be a bad idea to carry a spare alternator and some tools with you for the road trip, but I have a feeling that you won't need to use them.
    07gts197 likes this.
  4. GlassTop09

    GlassTop09 Member


    I followed the alternator AC ripple test procedure as described in thread above using a Klein MM300 DVOM that has the DC voltage blocking diodes internal on the test leads when switched to the VAC side so only AC voltage can pass thru (I also have an Autel AL539B DMM that doesn't have these DC voltage blocking diodes & it read the same DC volts reading as read on the VDC side thru the VAC side verifying this & the Klein also shows the DC blocking diode diagram at both V, Ohm+ & -Com test lead plug sockets) while in the process of changing out the battery (the 2 prior load tests verify that the prior battery is starting to go, the date says that the battery has passed the 3yr mark where, thru my experience, most batteries w\ a 5yr warranty will start showing some signs of break down if not outright failure depending upon charging system condition, engine compartment temps & total power usage of vehicle). I changed out the battery ahead of our trip as a precaution (and to obtain verified warranty for future) but the rest of what I saw is subjective as the results I got according to what is stated in the various threads in here says the alternator is in the process of failing even though it shows to be charging fine (which can happen, tis why it's a good habit to occasionally check for AC ripple voltage output) but I'm also not seeing any ill effects from this much AC ripple voltage residing in my car's charging\electrical system. This is not to say that there's no ill effects at all occurring, just not enough to trigger any DTC fault codes, immediately fry any electrical components or cause very noticeable operational issues....yet.

    So as far as the testing procedure I used to test for alternator AC ripple voltage being wrong or not you'll need to discuss that w\ others in here as well...… Besides it's the only way to access the alternator's B+out post w\o disassembly of the car....as long as the fusible link in the cable doesn't affect the output, which it shouldn't.

    This is why I posted here to see if anyone knows the official Ford design threshold for AC ripple voltage in charging system for these S197's as I certainly know that when Ford designed these cars they also designed for this as well. Next thing to determine is if the AC ripple designed voltage threshold has been changed (meaning raised) starting from '09 forward since Ford started using a better designed alternator, meaning that the '09-'10 cars can tolerate more than .5v AC ripple voltage in the system w\o ill effects vs '05-'08 cars. Maybe the '09-'10 cars can handle as much as 10% AC ripple voltage in the system (mine comes out at 8.3% FYI) but until some official Ford documentation is found that lays all this out then yes, this is all speculative on my part but the data I have gained from the testing isn't.

    These results are also shown on my car w/ a Steeda Underdrive Pulley system installed for almost 2 yrs (installed on 11-17) which is under driving this alternator 25% slower across the full RPM range (added this for relevance of discussion). I don't think this has any relevance in the results given either as I believe that Steeda also designed this system w\ all this in mind as well, but I also didn't get all the test data prior installation to verify my statement so it can be viewed also as somewhat subjective. Just thought I'd mention that.

    In the end I don't see it as over thinking the results as the results are what they are, just that I try to cover all the bases to allow the data itself to determine the results w\o any subjection mixed in (which being human can be a TALL order in itself).

    If I were truly overthinking these results I would over react & not drive the car at all for fear of the unknown...…...NOT!
    Also, if I'm going to go thru all the effort of replacing something I'm certainly going to try to replace it w\ a better designed, higher output part as this will reduce the possibility of replacing this part again in the future by reducing the operational load on it, thus the heat output on the internal components. Note I didn't mention any part that outputs >160A as this would be gross overkill for the current system's needs & wouldn't justify the hit on my wallet...……..

    Last edited: Sep 9, 2019
  5. 07gts197

    07gts197 forum member

    On the technical side you have things squared away for sure but if anyone knows that value is unlikely unless they worked R&D at Ford or similar.

    What were getting at is that you’re doing more work than whats needed. What you want to do is like replacing a stock tire with good tread with another tire. It has good tread, not giving any issues however maybe sometime in the future you’ll run over a nail and have to replace it. Not the best analogy but its what I came up with. The point is if it ain’t broke don’t fix it.

    Sent from my iPhone using Tapatalk
  6. DiMora

    DiMora More Is Better

    I’m at 13.2-14.0 at idle
    AC .8 mV

    I would add a Denso alternator and be done with it.

    DOB or a 2010 will both fo the trick.

    DOB is the best.
    AndrewNagle likes this.
  7. GlassTop09

    GlassTop09 Member

    No problem here, your point is well taken & is why I’m continuing to drive the car. I ran all these tests due to me replacing the battery thus was simply addition to existing process so IMHO is good practice (there’s a reason why batt CCA is dropping off so tests are warranted to ensure that degradation isn’t due to mechanics). My “issue” if any is the results aren’t reflective/indicative of the “established failure parameters” even though the test results implies failure & my car is a ‘09 which is equipped OEM w/ the better Denso unit so either the ‘09 up cars have different specs for AC ripple tolerance or the “established failure parameters” are very conservative, to allow a margin of error, thus the importance of actual OEM design parameters. Would think this info was available in the Ford Workshop manual (I have a copy of it for S197’s) but it doesn’t even mention anything on this so this is a very subjective subject indeed. So the only way to know for sure is to periodically gather test data so when the failure actually happens I’ll have data to support it....for my car at least.
  8. GlassTop09

    GlassTop09 Member


    Just got back home from our trip.....put a total of 1,752 mi on the car & no issues to report. Car ran very well (especially in the denser air in E. Dallas, TX area)....my 4.6L was LOVING that 14.3 atm air, didn't matter about the air temp vs the 12.2 atm air she was tuned in back home (Farmington, NM @ 5,380' alt......could swear she gained about another 8-10 lb\ft TQ while busting moves thru traffic around I-635). Before shutting the car off after backing her into the driveway I hooked up my Foxwell NT301 scan tool to get a looksee at the vitals before shutdown & noted the system voltage at hot idle @ 750 RPM's was between 13.7v-13.9v......didn't think too much on this as I would expect the PCM to cut back on the alternator's charging output due to batt getting fully recharged over the course of a 852 mi run (approx. 13 1\2 hrs).

    I'll give her a good run over in the AM to gather a full set of data for my records but so far, all is well w\ my Glass Top...………...

  9. Pentalab

    Pentalab forum member

    There appears to be a dozen ways to conduct a ....'cold cranking test'. How low does the battery voltage drop to....under a massive load? What's the temp of the battery to begin with ? Did they toss the battery into the deep freezer over night, before the test ? What is the viscosity of the eng oil used ? What is the start up current vs the running current of the start motor ? How many times was the test conducted ?

    I never had any issues with starting my 2011 Fusion in cold WX...... until a battery test was done.
    IMO, it's a scam to degrade the start battery, placing a massive load on it..... esp conducting a 'test' every 6 months.

    BTW, Mobil-1 now makes 100% synthetic eng oil in 0W-20, 0W-30..and also 0W-40. Ideal for winter.

    Diodes in a DVM won't block AC. A rvs biased diode will block DC.... but allow 1/2 of an AC sine wave to easily pass through. A forward biased diode will allow the DC to sail right on through, and also pass the other half of an AC sinewave. Only a 5-20 uf capacitor in each leg of the DVM will block the DC component.... while allowing the AC component to pass.
  10. GlassTop09

    GlassTop09 Member

    Got around to making some checks as follows using my Klein MM300 DVOM:
    Batt voltage key off engine off cold @ 12.6vDC (up from the 12.3v w\ old batt)
    Batt voltage key on engine on hot idle @ 750 RPM @ 14.57vDC (up from the 14.4v w\ old batt but lower than initial test 14.78v w\ new batt so PCM reset alternator charging rate to reflect new batt full recharge)
    Alternator AC ripple test hot idle @ 750 RPM (used same procedure) @ 1.2vAC (same result as before so no change...good sign)
    VAC at batt terminals hot idle @ 750 RPM (used same procedure) @ 34.7vAC (up from 30.5v w\ old batt but lower than initial test 37.5v w\ new batt....don't know what to make of this outside of PCM most likely backing off alternator charging rate due to new batt reaching full recharge).

    From these tests it shows all is operating fine. Hooked up my Foxwell NT301 scan tool & checked batt voltage as measured at PCM @ 14.0vDC-14.2vDC vs last reading @ 13.7vDC-13.9vDC (right after parking from 852 mi run home) so all is looking just fine w\ IM readiness check green (all good). Unhooked this scan tool & hooked up my SCT X4 tuner to run datalogger (which showed all good) so for giggles ran DTC code check & guess what came up....DTC P0626 Generator Field/F Terminal Circuit High. This was odd as my Foxwell didn't show any DTC's (no MIL on either) so I pulled the X4, hooked the Foxwell back up & ran DTC's again & got all clear w\ no MIL, no DTC's. So I then got my Autel MD808Pro full system scan tool & ran the OBDII scan & it came back the same as the Foxwell, no DTC's\MIL. So I set the Autel up for Auto FORD scan (full systems scan), ran it & now it picked up the P0626 DTC code (but no MIL) so this code isn't an emissions related DTC (explains why my Foxwell/Autel tools didn't pick it up thru OBDII) but I had never saw this DTC before today (have ran the full system scan several times prior this time) so I did a little research on this DTC as follows:

    DTC P0626 is set when the PCM reads alternator/regulator load input frequency that is higher than what the expected load frequency should be under normal operations. This frequency is usually generated in the alternator field windings section (the coated wire windings that the armature spins inside of) & is regulated by the PCM thru the alternator voltage regulator (or generator control module). Since nothing on the car has been changed except the battery (which did cause higher input voltage/frequency to be seen by the PCM over what it had recorded prior) I'm pretty sure that the battery swapout is the most likely reason why this DTC set. Since this issue has to repeat itself more than 1 time before the PCM will set the MIL (read after 6 consecutive instances) & I know the PCM has recorded the new load input frequency readings now I went & cleared this code to see if it will reappear. If it does then I will start looking for a short somewhere in the system to rule it out (I highly doubt that 1 exists due to the 12.6vDC cold voltage batt test result) as the DTC readout indicates that a load input that went high is indicative of a battery short to ground, but is most likely the alternator or alternator voltage regulator failing which we'll then get a '10 Denso 165A unit to replace if proves out.

    If it doesn't then I'll know for certain that it was just due to the new battery install & all else is fine going forward.

    This is why IMHO it's a good idea to run periodic system checks to gather operational data over time......can come in handy. Also shows that you need to have a scan tool on hand that can perform a full systems scan as well as an OBDII scan to check for any other hidden DTC codes.

  11. GlassTop09

    GlassTop09 Member


    I was messing around this morning when it dawned on me that my new Autel AL539B scan tool\DMM has a testing mechanism built in to test start systems (starter load test) & charging systems (high voltage, low voltage & AC ripple) so I used it to run both these tests today.

    For the start test the results came back as normal (max system load voltage drop during cranking was 21% of starting voltage--79% of 100%--so starter is good as well as battery.....already knew this but was interesting & have some more data on this now).

    For the charging system test the results came back as normal also (test system w\ no load @ 2,000 RPM (14.21v) then w\ max load at same RPM (14.35v) w\ no detectable AC ripple voltage present (have more data on this now as well).

    In the end nothing new to see here just reporting on the results from a different test mechanism concerning the same issue.

    Hope this helps.