Finally found the cause of my low voltage - Beware the all important grounds.

Discussion in '2005+ Mustang GT 4.6L Tech' started by DieHarder, Sep 17, 2021.

  1. DieHarder

    DieHarder Member

    A little background... Since installing a DOB supercharger kit (2 years ago) I've been experiencing a low voltage issue. At first it seemed like I had a bad alternator. I thought that was weird since I bought DOB's best (read very expensive) alternator. Still, I could not get more than about 11.5 - 12v volts out of it and the idiot lights on my dash came on indicating a non-charging condition. I couldn't find anything wrong electrically at the time so I took the car to several places for "expert" opinions who all said I had a bad alternator. I also removed the alternator and took it into a couple of auto stores for testing. All said it was bad. Again, I thought that odd so I found a place that rebuilds alternators and had them test it. They said it was fine. Still, I couldn't get more than 12v out of the system.

    Frustrated, I called Jason at DOB, got a return authorization and sent it back to the manufacturer for testing. That alternator shop too said the alternator was perfect and sent it back. Now, I'm really ticked off and thought time to start looking for other causes. Did I damage something when I installed the new one? Were my cables bad? Had I reversed something when I extended the cables to relocate the alternator? Did I have a bad PCM? Given I have some electrical background... used to be an Electronics Tech in the military I decided to go back to basics. First rule of troubleshooting; understand the circuit you're working on. Broke out the manuals and read up on how Ford alternators work... interesting to say the least.

    My other mantra for troubleshooting electrical issues is if it worked before 99% of the time the problem will be something you've touched, changed, damaged, forgotten or missed reconnecting after you've worked on it. After verifying that everything I touched on the car was correct I was still left with this low voltage issue.

    Then I started reading up on grounds.... yup, simple, everyday grounds. When I looked at the system all of the cabling seemed good but I read several stories of issues with grounds in these cars and started looking closely at them one by one. I ended up removing several; cleaning and reconnecting them to no avail. Eventually, I ended up stumbling on the idea of dissimilar metals (and anodizing which I opted for my manifold) and thought maybe adding a ground from the body of the alternator to one of the main battery grounds would be a worthwhile troubleshooting step. To ensure a good ground I bought a 5 foot (4 gauge) heavy duty stranded copper cable at my local hardware store, a couple of cable ends with bolt eyes and using a torch soldered the ends of the cable and connected it from a mounting bolt on the alternator back to the main ground on the passenger strut tower as shown.

    IMG-2303.jpg IMG-2297.jpg
    Main ground on passenger strut tower. Connected ground cable to alt mounting bolt.
    Note additional 4 gauge ground cable.

    Started her up and low and behold the idiot lights went out and my alternator was working. At this point I was getting about 13.2 - 13.3v out of the system. At the time I was extremely happy and just glad I could drive the car without issues.

    So, fast forward to last week still unhappy with overall system performance I figured the wiring in our cars is 15 years old and likely seen better days so figured I'd try replacing the main battery/starter cable and ordered an OEM Motorcraft unit from a dealer and installed it. Again, pleasantly surprised my voltage output went from 13.3v to 14.0v.

    New main Battery/Starter Cable Harness

    Now, I figured I was on to something and started looking into the manuals again and noticed there was a section on troubleshooting the charging system that I had seen but didn't necessarily delve into that deep at the time when I was working on the original problem. That said, in re-reading it I noticed the discussion about measuring voltage drops for grounds and the B+ circuit and figured what the heck I'd take another look at it. In measuring the grounds all seemed to be working but when I measured from the body of the alternator to the ground cable that ran back to the strut tower I noticed I had a 1v voltage drop or almost a 10% drop.

    In looking around I figured I'd try adding another ground from a body ground on the alternator to the main ground I'd added that ran back to the passenger strut tower. The new wire only needed to be about 4-5" long so shouldn't pose that much of a current issue since the primary load was still being handled by the 4 gauge cable. I found a piece of 10 gauge wire; soldered both ends and added eyes. I connected one end to a ground on the back of the alternator and the other to the upper mounting bolt on the alternator which is the same point where the 4 gauge cable connects. So now, the charging circuit ground is at the same potential end-to-end.

    Started up the car. New readings 14.5v. Eureka! Finally!! YKYMF!!!

    IMG-2291.jpg IMG-2300.jpg
    Ground added from back of alternator to a mounting bolt for the alternator.

    That, of course means I still have at least one or more bad engine/body ground/s somewhere. I don't like having to jump around bad grounds but compared to what I was dealing with I'll take the results. I figure the only way to truly "fix" all of them would be a new harness. Alas, that's probably not in my future. I suppose I now need to chase down that engine ground in the back of the engine bay that disappears down towards the transmission. Anyone have any experience with that one? Easy/difficult to get to?

    IMG-2289.jpg IMG-2288.jpg
    Original running voltage. Final after all grounds fixed/added.

    So, I suppose the moral of the story is bad/poor grounds will cause a number of headaches from burning out fan motors and wiring to wasting hundreds replacing charging system components that don't need to be replaced. All I can say in the end is adding a ground wire to two to verify/validate whether a ground issue exists is a legitimate troubleshooting step and can help you solve a host of electrical issues that you'll otherwise end up wasting scores of hours over when the issue is poor grounding. And, by all means follow the troubleshooting steps in the manuals. They're there for a reason. To help you narrow down the real problem and help you feel less foolish when you finally!!! figure out "WTF" the car was trying to tell you all along. Welcome to the matrix...

    Attached Files:

    Dumpy, svanur, AboStangster and 8 others like this.
  2. crjackson2134

    crjackson2134 Member

    Excellent post. Thanks for sharing.
    DieHarder likes this.
  3. byblazed

    byblazed Junior Member

    Great write up! I work on fire engines and I can tell you that ground issues are some of the worst to trace. Unless you know where every ground is, and even then it can be very difficult. Glad you got it sorted!
    DieHarder likes this.
  4. JimC

    JimC Senior Member

    The alternator reminds me of something that happened to a friend. He was having similar issues. He has his alternator powder coated as part of his engine prep for shows. After going through all kinds of things and not having any luck, he took another look at the alternator. The powder coating was stopping him from getting any ground so had to scrape some away to make the connection.
    DieHarder and crjackson2134 like this.
  5. jewc75

    jewc75 S197 Junkie S197 Team Member

    Read up on the Big 3, its been done for ages. You can never have too many grounds.
    VGMStudios and DieHarder like this.
  6. FrancoGT

    FrancoGT Junior Member

    DieHarder likes this.
  7. FrancoGT

    FrancoGT Junior Member

    Great info. What is the brand volt gauge installed on your dash?
    DieHarder likes this.
  8. TexasBlownV8

    TexasBlownV8 Formerly TexasBlownV6

    +1 on the great writeup! Tracing ground issues can be confusing or hard to find.
    In regards to your question about engine grounds, there's the 2 'factory' grounds on the engine; the one big one on an engine mount bolt on lower-passenger side, and then the smaller strap attached to the back of the driver-side head, secured to the body with a small bolt a little left of center at the rear of the hood opening. I have seen this strap get corroded and/or frayed and cause issues, as well as have a dirty connection to the head.
    The larger big ground to the engine mount stud, check that one, too; it may be causing some of your issues (I suppose if there's no voltage drop across the block to the battery-negative, there would be no issue there).
    DieHarder likes this.
  9. AboStangster

    AboStangster Junior Member

    This is a fantastic write-up. So many 05-09's have power gremlins. This will aid many. Thank you.
    DieHarder likes this.
  10. DieHarder

    DieHarder Member

    Roger, thanks for the info. I fixed the larger ground to the engine mount stud by replacing the entire battery harness. Found an OEM unit from a Ford dealer (must have been new/old stock) for $50. Wasn't too bad replacing it except for the engine mount stud bolt...what a bastard to get to and replace.... After I installed it I noticed a jump in alternator output of about a volt. Totally righteous result for the money.
  11. DieHarder

    DieHarder Member

    eBay.... comes in different colors: Put this in your search criteria: 5V 4.2A Dual 2USB Charger Socket Adapter Power Outlet 12V 24V Car Motorcycle Runs around $5 - 10 depending on whom you get it from. Simply replaced my cigarette lighter with it since I there's another cig lighter adapter in the center console. Note of caution - You'll want to put this on a switch as it will eventually drain the battery after a couple of weeks if you don't. I put my switch in the center console. Out of the way but easy to get to. You might want to go with red/green vs blue as the blue seems to be the hardest to see during the day time. Up to you of course....

    For anyone interested in replacing their battery cable here's the part number: FORD OEM 05-06 Mustang-Battery Cable 4R3Z14300AA for a V8 3v. V6 and GT500 are different. Prices range from $50 to over $100. If you're experiencing low alternator output might be a worthwhile upgrade. Was for me.
    crjackson2134 likes this.
  12. DieHarder

    DieHarder Member

    Agreed, for an aging system it's a worthwhile upgrade; especially if you have a high power stereo or running additional systems that will stress the charging system (elec water pumps, add'l fans, return style fuel pumps, etc). The issue I have with them is the Big 3 or 4 upgrade still does not address the alternator case-to-frame ground issue (which is vital to ensure the alternator back to the body/frame/negative battery terminal ground is the same end-to-end). If you don't you may not see maximum output from the alternator like in my situation.

    In my case I found that I needed to replace my battery/starter harness (which in effect addressed one of the frame grounds) and add two additional grounds; one from the passenger strut tower to a mounting bolt on the alternator (Thought that should ensure true ground for the alternator; it didn't, still had a 1v drop). So I had to add an additional wire from the alternator case ground to the ground at the alternator mounting bolt which now ensures the alternator is at true ground potential all the way back to the strut tower/frame/negative terminal ground.

    What I figure is going on (with my DOB setup; manifold/supercharger/alternator mounting changes, anodizing and age of wiring) is the grounds are seeing different resistances at some of the physical connection points (i.e. where metal surfaces bolt together) and those potential resistance differences create voltage drops at various points. To ensure maximum alternator output you must address the end-to-end ground issue. The easiest way I can see to solve for it is add a ground from the alternator case back to the main ground on the strut tower which ensures the ground at the alternator is now at the same potential as the frame/body/negative terminal ground. Or, you can also replace parts of the wiring harness (the battery to starter harness is still available) like I did or do Big 3 upgrades (like you suggest) as long as you pay attention to the alternator-to-frame ground you shouldn't have any problems.
    Dumpy and crjackson2134 like this.
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