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Old 04-07-2010, 03:45 PM   #1
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Serious Voltage Drop

Hey everyone. I searched the forums with no luck. My battery gauge drops severely if any of the accessories are turned on. When I am idling, the guage read 13-14 volts. Normal. If I turn on the headlights, it drops was down to around 9 until I give it some gas, then it returns to normal as long as I am accelerating. They same goes for pressing the brake pedal. The brake lights coming on causes the same voltage drop. I replaced the battery, alternator, battery cables, and ignition coil. I have not changed the cable that goes from the starter solenoid to the starter. I am an electrician by trade and it is apparent to me that there is a wire(s) with high impedance/resistance. The ones I figured could cause this problem have all been changed out with the exception of the starter wire. Could that be the issue? My previous 5.0 had this exact same issue, but I never got around to trying to fix it. Any ideas? Thanks - Mike.
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Old 04-07-2010, 03:47 PM   #2
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Are you going by the gauge on the dash or have you verified this with a multimeter?
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Old 04-07-2010, 04:05 PM   #3
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Quote:
Originally Posted by fogged306 View Post
Are you going by the gauge on the dash or have you verified this with a multimeter?
It's not the gauge because if this all goes on at night, you can literally see the headlights dim/brighten against the garage door in the driveway when it's idling or accelerating. I am starting to lean towards the alternator wiring being corroded.
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Old 04-07-2010, 04:11 PM   #4
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Did you install under drive pullies ?
Are you still running the stock alternator ?

Problem is the amps at idle the alternator puts out.
a higher amp alternator will put out more amps at idle and help a lot.

Ditching u/d pullies helps spin it faster at idle as well.

Upgrading all your grounds and adding more grounds to the battery, engine block
and frame also helps.
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Old 04-07-2010, 04:20 PM   #5
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Quote:
Originally Posted by r.barn View Post
Did you install under drive pullies ?
Are you still running the stock alternator ?

Problem is the amps at idle the alternator puts out.
a higher amp alternator will put out more amps at idle and help a lot.

Ditching u/d pullies helps spin it faster at idle as well.

Upgrading all your grounds and adding more grounds to the battery, engine block
and frame also helps.
+1 adding grounds never hurts, and underdrive pulleys ruin idle output of stock alternators in general.

I would also use a multimeter and see what your voltages are in different spots on the car. Check the alternator output and change where you ground the meter i.e. block, chassis, battery, and see if you have any differences there. Do the same for the battery.
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Old 04-07-2010, 04:24 PM   #6
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Quote:
Originally Posted by r.barn View Post
Did you install under drive pullies ?
Are you still running the stock alternator ?

Problem is the amps at idle the alternator puts out.
a higher amp alternator will put out more amps at idle and help a lot.

Ditching u/d pullies helps spin it faster at idle as well.

Upgrading all your grounds and adding more grounds to the battery, engine block
and frame also helps.
I have not installed under drive pulleys. The alternator is brand new, but is a direct factory replacement to the original, which leads me to assume that the factory one output sufficient amps at idle to not cause this problem. I am almost 100% sure it's a wiring issue but I was hoping someone would have ran into this problem before as I know it is common, in my experience anyway. On another note, is there supposed to be a vacuum line hooked up to the MAP sensor, because there isn't one on mine. Just the connector.
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Old 04-07-2010, 04:49 PM   #7
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I would check all the motor grounds I know the 94-95 alternator get grounded through the motor
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Old 04-07-2010, 05:07 PM   #8
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You did not say what year the car is, it does make some difference. The older fox body cars have a three wire connector on the altenator, that is known to have high resistance. The '94 and '95 cars are known for having ground problems. I would gess from your systems you either have high resistance in the wiring from the alternator to the battery, which could be the alternator plug if the car is a fox body. Or you have a ground problem, too much resitance in the grounds going to the battery. The '94 and up cars have also had problems with the main power connection on the power distribution box, if it is not tight, it will build up corrosion on the connector. If the connector is corroded, it will have a dull gray look and or have a light coating of white powder on it.
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Old 04-07-2010, 05:21 PM   #9
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+1 adding grounds never hurts, and underdrive pulleys ruin idle output of stock alternators in general.

I would also use a multimeter and see what your voltages are in different spots on the car. Check the alternator output and change where you ground the meter i.e. block, chassis, battery, and see if you have any differences there. Do the same for the battery.
+1 on this suggestion.There is no need to spend more money replacing parts.I can not think of any reason the starter cable can cause this problem.If you purchased this car used,I would check and make sure there are not under drive pulleys installed.Other than that get out your meter and do a little trouble shooting.Make sure your voltage reading between the + and - terminals is the same as your voltage between the + terminal and the chassis ground.
My car does the exact same thing,because of the pulleys.
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Old 04-07-2010, 06:26 PM   #10
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Well, it's a '91 5.0 LX vert. I am only the second owner. The original owner was an old woman who bought the car brand new as a retirement gift to herself. I bought it from her son who garaged it for a number of years. EVERYTHING on the car was stock when I bought it about 6 months ago. There are no under drive pulleys installed. They are all original. I have 14.24vdc between the + terminal and chassis ground (measured from various different chassis points). I have 14.24vdc from + terminal to the - terminal when the car is running. There are three wires to the alternator. Pins 2 and 3 are 0.02 ohms to ground and 12.32vdc on pin 1 with the car NOT running. Neither ground wire shows high resistance.
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Old 04-07-2010, 06:29 PM   #11
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What is your voltage between the positive terminal on the battery and the engine block when it's running?
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Old 04-07-2010, 06:55 PM   #12
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What is your voltage between the positive terminal on the battery and the engine block when it's running?
14.24vdc. When I engage the headlights, it drops to a steady 12.31vdc. If I press the buttons to raise the windows with them already up, the voltage drops to 10.93vdc.
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Old 04-07-2010, 07:13 PM   #13
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It could be the battery, the symtoms sound a lot like a week battery, that is building a surface charge. Just to be sure the alternator plug is not bad, check the two black with orange wires coming from the alternator with the engine running with the head lights and blower motor on high. If you have an amp meter with a lead that will clip over those wires check to see how many amps the alternator is putting out. If not touch the wires after letting the car run a few minutes with the head lights and blower on and see if those wires get hot, they will normaly be a little warm, but if they are hot, you have high resitance. The wires will usualy be hotest where the resistance is the highest. If those wires are hot, replace the large three wire plug to the alternator, you can get them from most parts stores, a lot of the time when you buy a new alternator that pig tail is in the box with the alternator.
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Old 04-07-2010, 07:23 PM   #14
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Originally Posted by GT/CS Guy View Post
14.24vdc. When I engage the headlights, it drops to a steady 12.31vdc. If I press the buttons to raise the windows with them already up, the voltage drops to 10.93vdc.
Does it do that when you test between the alt output post and the block too? If it doesn't then you have a bad ground or bad cable from the alternator to the battery. If it does then I'm going to guess the alternator or the battery is defective. A brand new alternator and a brand new battery shouldnt drop that low at idle.

Personally I would just start upgrading cables because sometimes thats easier than chasing down a ghost problem. New block to chassis, new chassis to battery, new alt to battery, ect.. but thats because I have access to good wire for next to nothing cost.... Being an electrician, if you can get some good 4ga copper strand wire for cheap, I would do it that way. That way you can eliminate wiring as an issue.
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Old 04-07-2010, 08:23 PM   #15
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Does it do that when you test between the alt output post and the block too? If it doesn't then you have a bad ground or bad cable from the alternator to the battery. If it does then I'm going to guess the alternator or the battery is defective. A brand new alternator and a brand new battery shouldnt drop that low at idle.

Personally I would just start upgrading cables because sometimes thats easier than chasing down a ghost problem. New block to chassis, new chassis to battery, new alt to battery, ect.. but thats because I have access to good wire for next to nothing cost.... Being an electrician, if you can get some good 4ga copper strand wire for cheap, I would do it that way. That way you can eliminate wiring as an issue.
Replacing the wiring IS the next step. I have already replaced the block to battery cable, but where is the block to chassis cable? I'll go ahead and order a new alternator pigtail as well.
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Old 04-07-2010, 08:25 PM   #16
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i have the same problem
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Old 04-07-2010, 08:43 PM   #17
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I'm not 100% sure where the block to chassis ground is on these cars, maybe someone else can answer that. I know there's one on the firewall in the middle, a little closer to the drivers side, but that's not the main ground. On my car it was going to the same point the battery was grounded at, but someone has definitely been under the hood of my car before I bought it so I can't be sure that is the same ground as factory. In any event, I would just run another ground right from the point where you grounded the battery straight to a solid place on the block. Leaving the ones already in there isn't going to hurt anything.

I don't know if I would order a new wire for the alternator just yet. You definitely don't want to throw money at it because that rarely works out in your favor. If you can't get 4ga in bulk for cheap, then pull the alternator wire off and check the connections, see if there are any breaks in the insulation, and look for corrosion. If it looks suspicious, then I would say replace it.

Last edited by fogged306; 04-07-2010 at 08:48 PM.
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Old 04-07-2010, 09:54 PM   #18
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Well, installed a new 4ga wire from the block to the chassis with no luck. Still drops at idle when more than normal current is drawn. I'll just order a new alternator harness and go from there.
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Old 04-07-2010, 10:03 PM   #19
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Do a 130-150 amp 3G alternator conversion.

Those stock alternator dont put out spit for amps at idle.
Dimming headlights and such on stock alternator is the norm.

It's only like a 75amp alternator if I remember right
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Old 04-08-2010, 01:12 AM   #20
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Originally Posted by r.barn View Post
Do a 130-150 amp 3G alternator conversion.

Those stock alternator dont put out spit for amps at idle.
Dimming headlights and such on stock alternator is the norm.

It's only like a 75amp alternator if I remember right
That is definitely a tiny little alternator, but even so, with a brand new alternator and a brand new battery, I think those voltage drops he's getting just when he turns the lights on alone, is a bit excessive.. I have yet to see a factory charging system fail to keep up with just headlights. But I could be wrong, I don't work on cars that are usually as old as these foxes and this is still 80's technology we're talking about here.
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Old 04-08-2010, 01:30 AM   #21
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Where did you get the alt?

Parts store reman ones are suspect. BTW the way the stock alt suffers from two issues. Fhe first is low max amps and the second is the RPM at which the max amps is devolped is too low. My guess is the voltage regulator in the new alt is suspect. Sometimes they even resue them if they test good. If you can increase the RPM's and the voltage increases it is most likely the alt. IMHO don't waste your money on new stock one.

The 3g 130 amp upgrade I did was the best mod I ever did to the car. Not only is it higher amps but more amp at a lower RPM. I bougth it here from s guy who sells brand new small case alt's that include the wiring needed to upgrade. Since it is a small case one so no grinding was needed.

3G Alternator Upgrade Complete Kit Small case

This guy is a site sponsor.
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Old 04-08-2010, 10:18 AM   #22
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This is not going to be an alternator problem(but a bigger alt may make the symptom go away).Even if you take the alternator out of the equation,the voltage should not drop down to 9 volts with only the lights on.You should not have more than a 1 volt drop with the lights.Do all of the same voltage test"lights on,lights off" with the car off.
With the car running,disconnect the + cable from the battery and turn the lights on.Then measure the voltage between the + cable(not the battery) and the chassis ground.See if you still have the voltage drop with the battery disconnected.
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Old 04-08-2010, 09:52 PM   #23
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I have a similar problem with my 97 gt. I was driving and my voltage dropped and then the battery light came on. The next day the voltage steadily went down while I was driving. It didn't go up at any time. It finally killed my car while I was driving. Took the battery and alternator to autozone to get tested and they both were no good. Well today I installed a new alternator and battery but it still has low voltage and battery light is still on.
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Old 04-09-2010, 08:05 PM   #24
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I solved my problem. The ALT fuse was blown.
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Old 04-09-2010, 08:08 PM   #25
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Quote:
Originally Posted by r.barn View Post
Do a 130-150 amp 3G alternator conversion.

Those stock alternator dont put out spit for amps at idle.
Dimming headlights and such on stock alternator is the norm.

It's only like a 75amp alternator if I remember right
This is "Normal" for all Fords of the era.
3G, 130A ALTERNATOR UPGRADE

The upgrade would have been less expensive than all you have thrown at it, so far.
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Old 04-09-2010, 10:54 PM   #26
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I have spoken to a couple guys in my local Mustang club and they are 100% sure that I have a bad voltage regulator on the alternator. One of them has the same car as me with a stock alternator, no under drive pulleys and his voltage is normal in all instances. No voltage drop or dimming of anything at idle like mine. Before I drop money on a new 130 amp alternator, I'll remove the one I have currently and get it tested. It's only 5 months old.
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Old 04-10-2010, 08:57 AM   #27
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Have you checked the voltage with the car off?You have other problems if you are getting a 5 volt drop just by turning on your headlights.Checking with a meter my car only has 11.9 volts and when I turn my lights on it only drops to 11.6 volts.This is with the car off.If you check your voltage with the car off,and you get a huge voltage drop it has nothing to do with the alternator.
You say you have over 12 volts(when off),so i will assume the battery is charged.If so the battery alone should be able to carry the small amount of current needed to run the headlights,without a voltage drop.
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Old 04-10-2010, 05:45 PM   #28
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Ok, so I replaced the voltage regulator ($27). No change. Here is exactly what is happening. When the car is idling, I have 14.24vdc at the battery. If I turn on the headlights and the blower to full power, voltage at the battery is 12.81vdc, which is like the alternator is not putting anything out. It shouldn't be dropping that much. If I press the gas pedal in the slightest, the voltage goes back up to 14.24vdc. I cut the battery wire that goes from the alternator connector to the battery and ran in a new one. No change. I plan on doing the grounds next, but I am not sure which wires are what. Anyone have a schematic? What's the other connector that's on the bottom side of the alternator? If I don't see any change after running new wires, I'll just do the alternator upgrade.

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Old 04-10-2010, 07:32 PM   #29
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Here this will help.

This is from a 1993 EVTM but others years are similar. If you are serious about testing this thing get a an EVTM and the Ford shop manual (Not Chiltons or Haynes). It has more than you ever wanted to know about testing and troubleshooting. It is a PITA for me to scan them in so you get one page. there are 15 pages of testing in eth Ford shop manual.




A few thoughts. Some of these have already been said. Remember there is a lot of my opinion in this. Not all agree.

This assumes all of your battery cables, and all you connections at the starter relay are clean, tight, and free of paint or corrosion.

Grounds.

-The ALT Grounds via the case. (Probably good)
-So the case must make good contact with the bracket. (Probably good)
-The Bracket must make good contact with the block. (Probably good)
-The block must be grounded to the car and battery. (Normally iffy even brand new from Ford.)
-The block has a ground cable on the driver's side on the timing cover that goes to the battery. Check to make sure this is clean and tight. (Not a good location for a ground IMHO, check for corrosion)
-The block goes to the frame in two places. A ground strap from the Trans tunnel to the pass side head and where the cable I talked about in the previous step goes to the inner fender near the battery. IMHO these grounds are the absolute minimum from Ford. Any deterioration at all of any kind and they no longer can do the job 100%.

Onto the ALT wiring.

Since I don't have the spec for the wire the amp ratings are a guess.

First the BK/O wires.

There are two of these wires. They start from the starter relay as a Green fusible link. Then then split off and travel to the ALT. IMHO these wires are once again marginal to do the job when new. They see a lot of heat from the engine, and the load they carry, and deteriorate over time. They are prone to catching fire especially near the alt when the movement of the engine cause the most strain. It is not unusual to find a hot spot in these wires.

-If you replace these wires you must provide a fusible link or fuse. Other wise any shorts in the alt will result in a fire. The battery will discharge at full amps into the alt via these wires until it is discharged or it melts something. You want it to melt a fusible link or the fuse.

Next the Y/W wire.

This is the most important wire IMHO. A lot of people inadvertently cut this wire or hook this wire up improperly. This wire's purpose is remote voltage sensing for the regulator. Wherever this wire is hooked up is where what the ALT measures and provides voltage for. Since Ford uses the starter relay as the main junction point this is where this wire should be hooked to. So stated another way IMHO this wire is what allowed Ford to use the bare minimum wire size to carry the load since it will increase alt voltage to overcome any voltage drop. Obviously there is a limit to the voltage increase.

My example is if voltage falls to 12 volts at the starter relay then the ALT increases it output to compensate. Having the voltage sensed away from the ALT also allows some dampening effect to allow for a more stable output. It does make the ALT slower to respond to changes but the more stable voltage is worth it. Electronic devices like stable voltage.

Don't cheat and hook up this wire anywhere but the starter relay.

BTW the wire also provides the field.


Onto the LG/R wire.


This wire "turns on" the voltage regulator with the key. It also goes through the cluster. Normally people who remove their gauges cause an open in this circuit. If power is not applied here the ALT does not charge.


The W/BK wires

This is feed back from the Generator portion of the ALT to the voltage regulator.
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Old 05-01-2010, 11:12 AM   #30
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Originally Posted by GT/CS Guy View Post
Ok, so I replaced the voltage regulator ($27). No change. Here is exactly what is happening. When the car is idling, I have 14.24vdc at the battery. If I turn on the headlights and the blower to full power, voltage at the battery is 12.81vdc, which is like the alternator is not putting anything out. It shouldn't be dropping that much. If I press the gas pedal in the slightest, the voltage goes back up to 14.24vdc. I cut the battery wire that goes from the alternator connector to the battery and ran in a new one. No change. I plan on doing the grounds next, but I am not sure which wires are what. Anyone have a schematic? What's the other connector that's on the bottom side of the alternator? If I don't see any change after running new wires, I'll just do the alternator upgrade.
Not to make light of your concern, but that sounds about normal for a fox with 2G alternator.
The stock alternator was notoriously underpowered. Although rated at 70~75a, they only put out about 22 amps at idle. If you add up your draw with head lights and heater fan blowing max, then include draws like the fuel pump, engine control ect, you'll see that you're topping the 20ish amps the 2G does at idle.
Ford knew about the problem, and even attempted to overdrive the alternator as a bandaid.
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Old 04-30-2013, 02:59 PM   #31
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I have a 1990 mustang gt convertible that I have put a new starter on and battery checked the alternator and it is good but when I drive the car and it gets hot it acts like the battery is almost dead and the wires at the battery get hot and one even melted I am at the end of my rope I am ready to sale the thing but wife will not let me but it is not fun any moor it is all stock the only thing that is changed is the mufflers but they have been on the car four most of its life I am the 2ed owner of the car
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Old 04-30-2013, 03:51 PM   #32
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Holy run-on sentence!
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Old 04-30-2013, 08:42 PM   #33
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get the alternater from a police and taxi package vehicle, it is 100 amp and fits with all the stock wiring but do the voltage drop things without the car running first to check for corroded wires and other problems that no alternater can fix
mine would dim at idle with the stock one before I made this change
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