Alternator Voltage and Voltage Drop at high RPM - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 05-08-2014
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Alternator Voltage and Voltage Drop at high RPM

2004 Ford Rangler 4x4 XLT 4L SOHC @ 93,000 miles:

Back in November I noticed when passing people that my battery light would flash and the lights would dim at high RPM. It only happened if either the Air, Headlights, or both are on and the RPM lingers above 5000RPM+ for some time like it does when accelerating from 50-80mph. In these conditions the battery light will also not be steadily lit, it flickers like a neon light switching on as if it's teetering and unable to decide if it should fully activate or not.

Because I only reach 5000RPM+ once in a great while, I haven't done anything about it since. I have a BlueTooth OBD2 ScanTool that has graphed the issue, The screenshot below was taken on December 1st. Volts(ecu) is the voltage reported by the ECU and Volts(OBD Port) is the voltage measured by the ScanTool at the OBDII port.



Anyway, I bought the OBD2 BT Scanner in the winter and today it was a 96 degrees outside, so I was curious what the coolant temp and all that jazz would be since I've never seen it before at this temp. The first thing I did notice was that Volts(ecu) seemed to be reading lower than I remembered it has in the past.

On December 1st, 2013, Volts(ecu) was reading 13.3v @ ~2000RPM.
Today May 8th, 2014, Volts(ecu) was reading 12.8v @ ~2000RPM as seen below:

For reference I checked historic data on December 1st 2013 at my location and sunset was at 5:00PM and it was 36 deg F at 6PM. So my lights would have been on and my heater would very likely also have been on.

For today's Voltage the AC was running at full blast.


In the garage, Volts(ecu) was reading 12.40V at Idle RPM with AC on fully.


In the garage, Volts(ecu) was reading 12.70V at Idle RPM with nothing on.


Now, I know for a fact that OBDII BT ScanTool reads low, so I also tested with a multimeter connected directly to the battery with vehicle at idle speed.
Quote:
Nothing On: 13.36v
AC: 13.24v
AC + Brake: 13.20v
AC + Hi Beams: 13.14v
AC + Hi Beams + Brake: 13.10v
In Drive + AC + Brake + Hi Beams: 13.02v
In Reverse + AC + Brake + Hi Beams: 12.70v
Batt. Voltage with Engine off for 5 minutes: 12.59v

So, I'm not sure if the voltage is low enough to call it a bad alternator for certain or what not, it only seems borderline, what do you guys think? Would just a new Voltage Regulator and brushes fix this? Need a whole new Alternator? Belt Slipping?(I'd hear this one wouldn't I?) idk..

EDIT: The battery is the stock MotorCraft battery model the vehicle came with. The original MotorCraft Battery lasted 8+ years and still started the vehicle fine, but on cold days it cranked slow so I replaced it before the Winter Season. I wanted that same reliability so I replaced it with the same model, so the battery is less than a year old.

Last edited by Derek_Z10; 05-08-2014 at 11:45 AM.
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Old 05-08-2014
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You have at least 1 failed "field" but probably 2.

And alternator has 3 separate "fields" that generate AC voltage, each "field" has 2 diodes that change the AC power to DC power.
The "fields" are in the alternator case.

The alternator's rotor is supplied 5 to 9 volts by the voltage regulator, as the rotor spins it creates AC voltage in the 3 "fields", the higher the voltage in the rotor the higher the AC voltage generated, or the faster the speed of the rotor the higher the AC voltage generated.

It is quite common to lose 1 field or diode set as an alternator gets older, you usually notice this as a slight dimming of the head lights at idle(no, this is not normal, lol), since engine is above idle speeds 99% of the time this usually doesn't effect anything long term.

When a second field fails alternator barely has enough output to run the vehicle, let alone recharge the battery.

Standard at rest battery voltage is 12.3 to 12.8volts, 12.8v is when battery is new, 12.2v would be when battery is about 5/6 years old and time to shop for a new battery.

Voltage regulator is set to recharge battery with +2 volts, so if battery is 12.6v then right after starting you would see 14.6v at the battery for a few minutes.
Maintenance charge is set at + 1 volt , so after recharging battery you should see 13.6v at the battery, +1 volt is enough to keep battery charged up but low enough not to "boil off" or damage the plates in the battery.

Battery light is the ON/OFF switch for the alternator, if left on an alternator will drain the battery.
It is a simple circuit.
A light bulb "glows" if power passes thru it, so if I hook up one side of a light bulb to 12volts and the other side to 0volts(ground) then bulb will glow.

Battery light is hooked up to ignition switch and to alternator.
When key is turned on the bulb gets 12volts from ignition switch and since alternator is not producing power it is 0volts so bulb "glows".
When engine is started, alternator is now adding power to the system, 13.6 to 14.6volts.
At this time the voltage on both sides of the bulb is equal, say both sides are 13.6volts, so bulb does not "glow".
If alternator produces less the 12.6 volts(battery voltage) then the one side of the bulb hooked to the alternator has less voltage than the ignition switch side, so bulb will start to "glow"(flicker).

Your alternator is failing, and it will take the battery with it if you don't replace or repair it.

Last edited by RonD; 05-08-2014 at 10:04 AM.
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Old 05-08-2014
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Thank you for the detailed response!!

The battery is the MotorCraft battery designed for the Ranger. The first one lasted 8+ years. The replacement battery is less than a year old, and it was expensive.

I'll be going to get the alternator replaced soon.
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At 1 year old and down to 12.6v at rest, that battery is suffering the effects of a low output alternator.

Yes, 8 years is doable, car batteries are rated by number of starts and length and depth of discharge.
10 starts a week for 50 weeks over 6 years would be 3,000 starts, that's about average

But if battery is overcharged or discharge too much(lights left on), the plates get damaged which shortens their life.

Car batteries are made with thin plates, this allows them to discharge high amps quickly, but makes them poor for long low amp discharges, it damages the thin plates.

Deep Cycle batteries have thicker plates, these can't discharge high amps quickly, but don't mind long slow low amp power draws, these are used in camper, trailers and boats where 12v lights and other power draws occur for long periods between recharging.
You could use a Deep Cycle to start an engine, many do, they just have less "starts" in them than a car battery.
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Old 05-08-2014
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Today I noticed that temperature seems to have an influence.

Here I was going to class earlier before it really heated up outside and the voltage was back to 13.3 like it was in December


Note that in the above screenshot the TripTime was also about 8 minutes in, not 2:24. The OBDII BT adapter wouldn't connect until I unplugged it and plugged it back in while at a stop light.

Later in the day when it warmed up, the voltage dropped back to 12.8v


You mentioned diodes earlier. Could it just be the diodes in the voltage regulator are burnt up and causing large Vdroop due to the increased resistance accentuated by the high heat? If so, isn't the alternator fine and only the voltage regulator need replacing? I ask because the cost of a regulator is only 20% of an alternator and I can replace it myself with the few tools I have.

Or is the higher voltage I observed just the a result of the increased charge rate of +2volts caused by the overnight drain on the battery from not unplugging the OBDII BT adapter overnight. If so, would this prove that the alternator is capable of generating more power than it typically is? Also that whatever resistance in the regulator that provides feedback to determine the Alternators Output is malfunctioning and causing it to be calibrated for a lower than normal voltage?

I'm just really hoping it's the regulator...
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Old 05-09-2014
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13.3v is still very low, if battery has 12.6v

First I would check all the wires on the alternator, look for frayed wires at the connector.

Disconnect battery's Negative cable, remove the larger B+ cable(s) on the back of the alternator and make sure it's contact is clean, then reconnect.
Reconnect battery.

If you have a Volt Meter you can test if the B+ cable has 12.6v when key is off, this means the alternator fuse is not blown, if there are two B+ wires carefully remove(major Short issue with battery hooked up) and separate them and test each one separately for 12.6v, each wire has it's own fuse.

Yes, you can TRY a new regulator, but if money is tight I would just get a used alternator from a wrecking yard, they usually test them, just ask.
Or if you have a second vehicle take your alternator to a parts store and have it tested, call first not all parts stores can test alternators.
In some larger cities you may also find a shop that rebuilds alternators/electric motors for about 1/2 the price of new, and it is usually an exchange so you don't have to wait a few days while yours is being rebuilt.

Last edited by RonD; 05-09-2014 at 10:40 AM.
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Old 05-09-2014
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For the voltage of 13.3, that's just what the slightly inaccurate OBDII reader was reporting. 13.3v read by the OBDII would be almost 14volts at the battery measured by an actual multimeter.

I can buy a new alternator, just no way of installing it. The shops I've been to don't let you provide your own parts, so they charge you an arm and leg plus labor.

Or, I guess, maybe I could do it myself. I just figured it would be way harder than if it was just the regulator and I'd probably need special tools or something.

Last edited by Derek_Z10; 05-09-2014 at 12:21 PM.
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I am a little confused.
If you have 2 different voltages from OBD II and Multi-meter then I would test the multi-meter on a few other voltages, i.e. AC and a battery(9v, 1.5v, ect..) and see how accurate it is.
If it shows accurate then I would IGNORE OBD II and test battery/alternator using the multi-meter ONLY.
At rest voltage
Voltage at idle, no electrics on
Voltage at 2,000rpm(approx.)
Voltage at idle, all electrics on, head lights, blower fan, door open, radio, cigarette lighter, ect......

Write it all down, your alternator may be fine
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Old 05-09-2014
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From my first post:

Also I will modify it a bit. The first voltage is the Multimeter voltage connected directly to the battery, the 2nd is the voltage reported by the OBDII BT scan tool.
Quote:
Now, I know for a fact that OBDII BT ScanTool reads low, so I also tested with a multimeter connected directly to the battery with vehicle at idle speed.

Multimeter: Nothing On: 13.36v OBDII: 12.7 to 12.8v
Multimeter: AC: 13.24v OBDII: 12.5v
Multimeter: AC + Brake: 13.20v OBDII: 12.5v
Multimeter: AC + Hi Beams: 13.14v OBDII: 12.5v ish
Multimeter: AC + Hi Beams + Brake: 13.10v
Multimeter: In Drive + AC + Brake + Hi Beams: 13.02v OBDII: 12.4v
Multimeter: In Reverse + AC + Brake + Hi Beams: 12.70v OBDII: 11.90v
Multimeter: Batt. Voltage with Engine off for 5 minutes: 12.59v

Revving to 2000RPM increases the voltage across the board by +0.2 ~0.3V, Revving to 3000RPM drops the voltage by -0.2V. Theres an RPM limiter at 3100RPM when not in gear.

EDIT: After I posted I realize that is not at all readable. I can't find a way for the forum to stop deleting my spaces and such.
Here's an excel spreadsheet


So from the Cold Morning, Driving 6-8 mins after starting, if the OBDII BT Scantool is reporting 13.3V, you could infer that the voltage at the battery must be just under 14V.
Once Everything is warmed up, the OBDII BT Scantool starts reporting values 12.8v ish and less while driving with AC on.

EDIT2:
You will notice That putting it in reverse drops the voltage by .4-.5V. I assume that switching the reverse lights on is the straw that breaks the camels back. Or is basically what increases the power draw to the point at which the alternator can no longer provide enough power at idle to support all the devices and therefor relies on the reserve power of the battery. Since this scenario is somewhat uncommon, I'm inclined to think that the alternator does provide enough power to be suitable for the majority of cases. I believe the alternator is still good and charges just fine. However, I also think it is somehow miscalibrated. Miscalibration, I would think, would be caused by the power regulator interpreting the voltage wrongly and then not supplying enough power. I believe, at the battery terminals, the voltage is supposed to be at 14.4Volt and at the very least 13.8V.

14.4V would be optimal and 13.8V would be the lower limit voltage. Anything less would be the result of grueling and uncommon conditions that don't necessarily need to be compensated for. The voltage of my Ranger is less than the 13.8v lower limit in all conditions except when cold and freshly started. The alternator is supposed to maintain a higher voltage in that condition which would explain the acceptable voltage level in that condition. After the motor has warmed up the voltage drops to values below the lower limit, no matter the RPM.

Last edited by Derek_Z10; 05-10-2014 at 05:06 AM.
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Yes you have some "fields" out.

1 Field would show under 13.5v, idling or at higher RPM.
And from the readings that's the max. out put of the alternator, voltage just goes down as power usage goes up.
With a working alternator voltage would drop then come back up to 13.6v, that's the voltage regulator reacting to the power draw.

You could TRY a new regulator, but I am doubtful that would help.
The Field Diodes are not in the regulator, just FYI.

In 2004 Ford started using the PCM(computer) to monitor and control charging, and here's the issue, it wasn't used on all Rangers or all F-150s, or all Taurus's, even up to 2010, some Ford vehicles used the PCM some didn't.
It was called "Smart Charging".

You probably don't have this because "Check Battery" light would be on all the time.

I found a diagram of a 2008 Ranger charging system here: http://www.justanswer.com/ford/75f0m...ery-light.html

Scroll down to see it, wire colors should be the same, check voltages and fuses
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