Low Voltage

[SmokeOps]

2006 mustang GT - 110k miles - garaged kept 3 owners well kept. Went thorough 2 alternators and a battery, cars electronics started turning off and the car just started sputtering and died, bought a new battery and alternator and the car started at 14.2 volts and will stay there for like 2-5 mins then slowly drops to 13.05 volts sometimes will drop to 12.5 - 12.6 then shoot back up to 13 Volts. Thought maybe I just got a bad Alternator which was NEW not refurbished so I bought a different brand(Duralast) the car was idling and driving at 14.4 - 14.1 for like 10 mins then dropped to 13.6 - 13.5 volts even with everything on. Next day I drive a little longer and notice the car go down to 13.3 - 13 volts, not sure what it is I've checked the grounds even add a ground from the alternator to the battery ground, all the fuses look good non blown. The only thing I haven't checked is the ECM it doesn't smell like burnt plastic and looks clean. The car runs fine just runs low voltage.

 

[Pentalab]

2006 mustang GT - 110k miles - garaged kept 3 owners well kept. Went thorough 2 alternators and a battery, cars electronics started turning off and the car just started sputtering and died, bought a new battery and alternator and the car started at 14.2 volts and will stay there for like 2-5 mins then slowly drops to 13.05 volts sometimes will drop to 12.5 - 12.6 then shoot back up to 13 Volts. Thought maybe I just got a bad Alternator which was NEW not refurbished so I bought a different brand(Duralast) the car was idling and driving at 14.4 - 14.1 for like 10 mins then dropped to 13.6 - 13.5 volts even with everything on. Next day I drive a little longer and notice the car go down to 13.3 - 13 volts, not sure what it is I've checked the grounds even add a ground from the alternator to the battery ground, all the fuses look good non blown. The only thing I haven't checked is the ECM it doesn't smell like burnt plastic and looks clean. The car runs fine just runs low voltage.

Are you measuring directly at the battery terminals ? What are you using to measure, like a fluke DVM etc.

 

[86GT351]

Measuring device matters and where you are measuring from.

 

[DieHarder]

Had a similar issue (though my voltage didn't vary). Fixed it by replacing battery cables and adding a ground like you did. Over time the grounds and battery cables can increase their internal resistance due to oxidation. Pay attention to the main ground on the passenger strut tower and the engine ground underneath the passenger side motor mount. There is one more ground (braided cable) on the back firewall. A new set of battery cables runs ~$100 - 150.

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[GriffX]

You can measure the resistance of the ground cables by switching on all lights and defogger at idle rpm and measure the voltage between the negative battery pole and the engine itself. It should be below 40 mV.

 

[SmokeOps]

I am using a multimeter at the positive and negative posts of the battery to ge the voltage and I checked the continuity of the ground from the back of the engine block to the fire wall and it said zero so it should be good but I haven’t tested other ground cables continuity but I did test the voltage of each ground by having everything turned on in the car and put the one side to the negative post of the battery and the other to the ground and all of them read 0.05 volts which I’ve heard means it’s a good ground but I’ll try buying new battery cables that diehard sent.
Thanks for the support so far

 

[GriffX]

I have a voltage drop of 20 mV or 0.02 V under load.

 

[DieHarder]

Had a similar issue (though my voltage didn't vary). Fixed it by replacing battery cables and adding a ground like you did. Over time the grounds and battery cables can increase their internal resistance due to oxidation and galvanic corrosion. Pay attention to the main ground on the passenger strut tower and the engine ground underneath the passenger side motor mount. There is one more ground (braided cable) on the back firewall. A new set of battery cables runs ~$100 - 150.

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slightly better price: $125

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[DieHarder]

I am using a multimeter at the positive and negative posts of the battery to ge the voltage and I checked the continuity of the ground from the back of the engine block to the fire wall and it said zero so it should be good but I haven’t tested other ground cables continuity but I did test the voltage of each ground by having everything turned on in the car and put the one side to the negative post of the battery and the other to the ground and all of them read 0.05 volts which I’ve heard means it’s a good ground but I’ll try buying new battery cables that diehard sent.
Thanks for the support so far

You can determine if your grounds are good by testing various points of the engine with one end of the multimeter (on Volts DC) and the other end back to the main ground on the passenger strut tower with the engine running. If you read any voltage; let's say from the body of the alternator to the main ground or from one point on the engine to another you have ground issues. Old troubleshooting technique. It's how I found that my grounds were dropping more than 2vdc and the reason I had to add two grounds (a 4 gauge wire from a mounting bolt on the alternator to the main strut tower) and another short one (10 gauge) from the body of the alternator (on the back) to the same mounting bolt for the alternator which is connected to the main strut tower ground. This ensures the alternator ground is at the same ground potential as the main body ground for the electrical system. You can also try testing continuity from point to point but I've found the voltage drop method works better to identify ground issues.

 

[Pentalab]

Any voltage drop, with a load applied, is the result of current flowing through some resistance. The higher the resistance, the greater the V drop. The higher the current, the V drop will increase.
Vdrop = IR

In the op's case, the V drop is varying, and jumping about. That's indictive of either varying loads, AND / OR INT resistance.

To eliminate the alternator from the equation, measure the Voltage right at the output of the alternator. A 2nd DVM would help in this case. Put the 1st dvm across the alternator....and use the 2nd dvm to investigate V drops.

 

[SmokeOps]

I checked the mV it said 36 mV that GriffX said to check and when I checked the volts on the ground I puck one needle of my multimeter to the negative side of the battery and the other to the ground, the alternator case says 0.05 the chassis ground says 0.01 and the engine block says 0.04 volts. But I did notice that when I roll my windows down or up the cabin lights(the two lights inside the car on the roof) dim a little bit but it's super little though like I had to really focus on the light to notice it dim.

 

[DieHarder]

Pick a point to put the negative lead (say on the block of the engine)

I checked the mV it said 36 mV that GriffX said to check and when I checked the volts on the ground I puck one needle of my multimeter to the negative side of the battery and the other to the ground, the alternator case says 0.05 the chassis ground says 0.01 and the engine block says 0.04 volts. But I did notice that when I roll my windows down or up the cabin lights(the two lights inside the car on the roof) dim a little bit but it's super little though like I had to really focus on the light to notice it dim.

This might be worth a read: https://iihs.net/fsm/?d=361&f=Charging System.pdf and this: https://iihs.net/fsm/?d=361&f=Charging System.pdf&p=12

and this: https://www.s197forum.com/threads/f...tage-beware-the-all-important-grounds.139544/

Voltage drops can vary depending on where you place the test leads. The voltage drops at the points you tested are fine. So, that leaves something along the lines of a heat related issue, cables, grounds, alternator or PCM. Alternatively, the system is operating nominally (see the first link for system operation). Since alternator output drops after ~15 mins to see if it's operating normally turn the headlights on for 20 mins and then start it. The alternator should charge for a longer length of time. If it does and you see no ill effects I'd continue to drive it. In my case I did find that replacing my battery cables and adding the extra grounds solved my low voltage problem. See the links above for details.

 

[GriffX]

According to the Ford documents, we have a charging system with variable voltage, so a drop after some minutes could be normal?

 

[Pentalab]

According to the Ford documents, we have a charging system with variable voltage, so a drop after some minutes could be normal?

On my 2010 GT, with eng on, it's 14.75 vdc at idle....or driving for 2 hrs. U are correct though, it will vary a tiny bit depending on temps. But not the drops you are seeing. The oem alternator is rated for 135 amps.

 

[DieHarder]

Tend to agree w/Pentalab (your running voltage should be higher) so if you're willing to throw some money at it I'd start with replacing the battery cables. It's an easy job. Be sure to get the Motorcraft replacement. Besides the age of the cabling; it'll eliminate oxidation and fusible link issues. Would also be a good time to remove, clean and reconnect major grounds in the engine bay. To troubleshoot, follow the Pinpoint tests...

 

[Edqe]

I had the same issue. Bolted a ground cable on the back of the alternator body to the ground strap on the firewall ledge behind the engine. Also installed a new positive cable from alternator to battery. I get 13.8-14.2v now. Before it was around 12.8-13.6v

 

[DieHarder]

One troubleshooting step to help isolate the issue would be to run the car until you notice the voltage reduce and then repeat the Pinpoint tests. That should help you narrow it down to cabling, alternator or PCM.

Ford Workshop Service & Repair Manuals - fordrepair.info - 2005 Mustang Charging System - General Information Charging system

Ford Workshop Service and Repair Manuals, 2005 Mustang Workshop Manuals Electrical Battery and Charging System Charging System General Information DIAGNOSIS AND TESTING Charging System

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