Another voltage drop problem - Need an electical guru | Ford Explorer Forums - Serious Explorations

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Another voltage drop problem - Need an electical guru


Explorer Addict
March 16, 2007
Reaction score
City, State
Bloomington, Indiana
Year, Model & Trim Level
04 Ranger Edge
So, I've been having this problem with voltage drop at idle for some time now. The starter, battery, alternator, and battery cables are all new. There is also an extra 4 guage wire running from the alternator to the solenoid on the fender where everything is else is connected.

This problem only happens with the at least two things running that take a lot of power. For instance, headlights and heater fan on high, the voltage drops. Heater fan and rear defroster on, voltage drops. If just one of these things is on, everything is okay. The truck runs like a top, the idle is where it should be.

The voltage when normal is around 14.5 volts with it running at idle. When I have the most load I can put on it, it will drop to 12.5. There is nothing extra pulling power added to the truck, with the exception of an aftermarket HU, which makes no difference whether it is on or not.

So what is the problem here? I'm ruling the voltage regulator on the alternator out, since it should be fine. The alternator is a little over a year old and works fine otherwise. Truck always starts just fine. One thing I would like to know is what the other wires besides the big one are going to the alternator and what they do. That could be a source to my problem since I believe they play a part in controlling the alternator.

Thanks in advance for any help, this issue has me stumped.

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I think you have merely exceeded the output of your alternator at idle. You can either:
Not idle for too long with heavy loads, or
Change to a higher output alternator (although you will need to look at the specs to see if a high-output alternator puts out any more at your idle speed) or,
Not really worry about it. It's totally normal; gas-powered cars are not expected to idle long times (unlike diesels were) and as soon as the revs come up a bit, the system generates more than it consumes.



That is a good theory, but I do not believe it to be true here. With a stock vehicle, it should be able to support stock accessories, which is exactly what I have. With the headlights on and the heater going, there should not be enough draw to make the voltage drop to the point where all the lights dim and the heater fan noticeably slows down. It think there actually is a problem.

It also doesn't have to idle for a long time to get to that point. Almost instantly as soon as I stop at a stop light the voltage drops when the engine hits idle. It didn't used to do this, something isn't right still.

If your vehicle was new, you'd have a point. But alternators wear, connections corrode increasing resistance, idles drop lower as engines age, etc. It 's all enough to make what was a (barely) adequate new charging system not keep up. Sorry.

The fact that it drops from alternator contributing (14+ volts) to battery voltage means that at least in the opinion of the regulator, the alternator is not keeping up. It also means the regulator is working, the alternator is working, your heater is working, your lights are working, etc.

If you want it to behave like new, then clean all your wiring connections first. Corrosion-X is great. Test the resistance of all your wiring. Check that the resistance of the heater resistor isn't over spec. Test the alternator for output at idle, and use a quality tach to ensure that your idle is actually where it should be. Get all the corrosion out of your wiring, and you'll probably be happy. But it's a lot of work...

I see what you are saying, but all my major wiring is new, the battery is new, the alternator is new. I've gone over connections multiple times to no avail. I think something just isn't functioning correctly, or there is a smaller connection going bad I don't know about. I will assume the idle is right, being as how it's the computers job to keep it at a set RPM.

What I need is an idea of what else I need to look at. I don't know what the wires going to the alternator do, all I know is they play a part in controlling it. I think something involving these is where my problem resides.

Stupid question:
Did you have the voltage drop before you added the wire from alternator to solenoid as well? Did it get worse after you added the wire or just stay the same?

Did you add an extra grounding wire from the body of the Explorer to the negative side of the battery? That ended up where my problem was.

To aldive, I have measured the voltage from both the alternator, the battery, and the solenoid on the fender using the battery, body, and motor as different ground points on each. The only place where the voltage is very different was at the solenoid on the fender, so I thought perhaps the wire running between that and the alternator was bad.

Josh, I had the voltage drop with my old alternator, cables, and battery. The old alternator died and so I figured that was my problem. However the battery went out at the same time. Somewhere in replacing all of this the battery cables got replaced too. I didn't add the extra wire until recently, mostly just to see if that was where my problem resides. Apparently that wasn't it. I have measured voltages from all the grounding points, and even cleaned up the body ground connection anyways. I may throw an extra wire in there and see what happens.

When adding the extra wire, the voltage drop stayed the same.

Thanks for the suggestions guys, I'll keep looking. I might sort through the wiring diagrams and see if anything turns up from all of that as well.

When I was having my charging/voltage problems, it was the wires I had ADDED to the alternator to battery, whether it was power/ground wires, that were causing my issues. I removed them, but left the extra body to battery grounding wire and it's working perfectly now.

IIRC there's a wire that goes from the + side of battery to solenoid, and from solenoid to fusebox. I wonder if perhaps the wire that gives power to the fusebox is corroded on the inside? Perhaps bottlenecking the juice?

Is the belt old/new? Old belt might be not gripping properly... just something random. Never know.

FWIW i consider my '93 ex to be well maintained (by myself, meticulously) and as long as i've owned it, when the heater is blasting, and the headlights are on for instance (the two most voltage happy things) the voltage does infact drop a little at idle, but as soon as i touch the gas it pops right back up to full charge. i consider this normal.

you're not running underdrive pulleys by any chance, are you? on that note, you may consider running a slightly smaller pulley on the alternator, it's possible that the new alternator you bought came with a larger pulley, (possibly set up to fit a variety of vehicles, some with higher RPM's and/or larger crank pulleys, giving higher alternator rpms)...

those would be my next two steps.

btw, the batter isn't discharging at 12.5 volts. battery voltages work like this:

14.4 volts - this is the maximum charge voltage of the battery, more than this will damage the battery, and this voltage should only be applied to the battery for a short period of time (for instance, after starting to re-charge battery from running the starter)

13.8 volts - this is the "float voltage" of the battery, the battery can charge at this voltage, or have this voltage maintained to it indefinitely without every overcharging. this is what you're voltage should read after the car has been running for a wile, and the battery is fully charged.

below 12.4 volts (on a fully charged battery, less on a low battery) and the battery will begin to discharge.

here's some info, too

what i'm trying to say is, i don't think there's a problem, you're just over thinking it. (nevertheless, put on a smaller alternator pulley if you want more charge at idle)