98 4.0l overcharging battery?

[Bigstick1973]

i have a 98 xlt 4x4 that cooked my battery. i replaced the battery and the voltage regulator and im still getting 14.5 to 14.8 volts. when i drive to work with my lights and heat running ill come to a stop light and apply my brakes and my lights will go dim and the volt gauge will drop this will be around 600 to 800 rpms. once i go above 1000 rpms the lights will get bright and the gauge will go back up. i checked the voltage with everything running at around 600 rpms and got 13.2 volts and with everything running at 1000 rpms its 14.5 to 14.8. like i said i replaced the voltage regulator and the battery. it was doing the same thing with the old regulator also.
Thanks

 

[ThreePhase]

Have you checked alternator?

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[Bigstick1973]

no i havent replaced the alternator i kinda thought id try the voltage regulator first . i thought it was charging just over charging but at low rpms it doesnt seem to be.

 

[RonD]

Also check the wiring.

Key off, measure battery voltage
B+, on back of alternator, should read the same "battery voltage"
Unplug voltage regulator wires, 3 wire connector.
Yellow wire should read "battery voltage", this is the "Sensing" wire telling voltage regulator what to do.

Make sure Yellow wire is not frayed or its connector corroded.


Yes, voltage regulator is most likely the problem.

System is pretty simple really.
Voltage regulator varies the voltage at the Brushes on the Rotor to set B+ voltage output, which it monitors via the Yellow wire.
Normal range is 13.5v to 14.2v, if battery is in good condition

Just after startup battery is drained so it is normal to see up to 14.9v for a few minutes.
Then it should slowly drop to "it's" normal range, say 13.8v
As you turn on lights or heater fan to HIGH the voltage will drop and then come back up to 13.8v, voltage regulator responding to "Sensing" wire's voltage change, and increasing voltage to Rotor's brushes.
At idle, 600rpm, you may see a slight drop, 13.6v it it was 13.8v, if all electrics are on, because voltage regulator has reached its maximum rotor voltage.
Then as RPM increase voltage will go back up and at 2,000rpms it should be back to stable 13.8v.
It should not vary by RPM more than .3v, if it does it means voltage regulator is not adjusting rotor voltage, it is stuck at a Fixed voltage, so varying RPM varies voltage.

In the alternator's Case there are 3 Fields that are energized by the spinning Rotor, these Fields output AC Voltage.
Each Field has a set of Diodes that change the AC volts to DC Volts.
It is not uncommon for one Field to fail as an alternator gets older, sign of this is dimming head lights at idle, reason being is the alternator has lost 1/3 of it generating capacity so even if voltage regulator put rotor voltage at Full, it still can't generate enough AMPs for the vehicle.
As RPMs increase then voltage regulator can decrease rotor voltage to set the 13.8v again

Voltage regulators come in two types,
Type A sends one brush full Positive voltage all the time and varies the Ground to adjust Output Voltage(B+), Fords mostly use Type A
Type B Grounds one brush then varies the Positive voltage to the other brush


Green wire is "startup" voltage, an alternator can not create voltage out of thin air just by spinning it.
The Rotor must have voltage first, then when you spin it it will multiple that voltage.
But if you Always send power to rotor, then battery would slowly drain.
So there is the Green wire, it comes from the Ignition switch Via the Battery Light bulb.
When you turn on the key 12v travels from ignition switch, thru the bulb and to the Voltage regulator.
Alternator is OFF(not spinning) so it is at this time a Ground, Battery Light bulb comes ON
Voltage regulator uses that 12volts when you start the engine, once alternator starts to create voltage that end of the Green wire is now 13.8v, and the ignition switch voltage is also 13.8v.
With 13.8v on one wire and 13.8v on the other wire the Battery Light bulb goes OFF

If alternator stops working then one wire(at alternator) is a Ground again, so Battery Light comes on.
If alternator voltage drops below Battery Voltage(12.8v) then there is a voltage difference and Battery Light will start to flicker depending on how much of a difference


One Wire Alternators:
These just have the B+ wire hook up
They have a voltage regulator inside
And they have an added RPM switch/sensor, this takes the place of the Green wire
When alternator starts to spin, engine starts, the RPM switch will close and send B+ voltage to Voltage Regulator, so alternator can start to create voltage.
When you shut off the engine RPM switch opens and cuts voltage to voltage regulator so battery doesn't drain
Very simple setup, but can't use battery light warning system, however you can use an accurate Volt Meter instead

 

[cp2295]

If im not mistaken isnt the voltage regulator built into the alternator? I was under the impression they are inseparable..


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[RonD]

Yes, newer alternators, around 1985 and up, have "built-in" voltage regulators.

But alternators can be rebuilt, and sold , so voltage regulator is a replaceable part, and some choose to just replace a failed voltage regulator, if that has failed.

Most just replace the whole alternator, depending on age, because bearings are a common failure point in alternators, so putting a new voltage regulator into an old alternator may only get you a few more miles out of the old alternator.
More of a "users choice"

And if you decide to change voltage regulator you need to TEST IT FIRST!
After you remove it test it, and the alternator, before assuming voltage regulator is the problem or the ONLY problem.
No charge or overcharge symptoms can be caused by other internal or external(wiring) issues.