Car batteries store energy using chemical reactions, eventually these chemicals are used up, 5 to 7 years is expected life of a car battery.

A new "12volt" battery will read close to 13volts, 12.8v is average
This voltage will drop over the years of use as chemicals are used up.
When it gets down to 12.3v you should shop for a battery sale.
At 12.2v or lower you can get the dreaded "click, click, click" on a cold morning startup.
Cold temperatures reduce the chemical reaction speed to release energy for starting engine, so marginal battery will be a "dead" battery on a cold day.

Batteries can get internal shorts so are self draining even when vehicle is off.
Remove 1 battery cable and put volt meter on battery terminals.
If voltage is slowly dropping then battery has internal short, replace battery.

Battery cables need to supply high amps to starter motor, so must be clean and making good contact at both ends.
And it is using DC volts so + and - BOTH need to be making good contact.
A starter motor on a 4cyl low compression engine, can draw as little as 60amps, V6 80-100amps, high compression engine 175amps.
If Ground cable can only pass 50amps then positive cable can only send 50amps, it is a circuit where the slowest part decides the whole for the circuit.


Alternators
Vehicle wiring part
Test with engine and key off, battery connected
B+ terminal is the larger terminal on the alternator, it should have 12volts all the time, it can be tested while connected
No volts or low volts at B+ means Maxi-fuse or Fusible link is bad

3 or 2 wire connector on alternator
Remove connector from alternator
Yellow wire should have 12volts all the time, if no volts then 15amp fuse in engine fuse box is blown or fusible link is blown

Green wire should have 0volts with key off, 12volts with key on(battery light circuit), turn on key to test this
If there is no volts on Green wire with key on, then fuse in cab fuse panel is blown, check them, you will also not have Battery Light on dash with key on, which should come on with the key.
The Battery Light circuit is the ON/OFF switch for the alternator.

An alternator can not generate voltage just by spinning it.
You must supply voltage to the rotor FIRST, then when rotor spins that voltage will create a higher voltage in the 3 Fields in the outside case of the alternator.
But if you supply the rotor voltage all the time it will drain the battery while engine is off, so you need an ON/OFF switch, and that's the Battery light circuit, it supplies the rotor voltage(from the battery) when key is on.
Once the alternator starts to generate it's own voltage then battery light circuit(and battery power) is not needed, so battery light goes off.
If alternator can not generate any, or enough, power then Battery Light stays on or flickers after engine starts.


Alternator and voltage regulator
Alternator is similar to an electric motor but used in reverse.
For power generation you supply a lower voltage to the rotor, 7-9volts, and that creates a magnetic field.
As the rotor spins the magnetic fields spin and create AC voltage in the 3 Field Coils in the case of the alternator.
Each Field has 2 diodes that change AC volts to DC volts.
That part is the alternator

The voltage regulator can be external or internal(inside alternator)
The voltage regulator adjusts the voltage going to the rotor, the 7 to 9volts.
By raising or lowering this rotor voltage you can raise or lower the Output voltage of the alternator, output voltage comes out at the B+ terminal on alternator.

You test alternator and voltage regulator by starting the engine with volt meter connected to battery.
After starting the voltage regulator will be in "recharge mode", and voltage at battery will be above 14volts but below 15volts.
A battery will start to boil away chemicals if voltage stays above 14volts for more than a few minutes.
After about 5 minutes the voltage at the battery should drop down to 13.6v(approx.), this is the "maintenance mode", keeps battery charged but doesn't "cook it".

If you increase engine/alternator RPMs voltage will go up but should drop back down fairly fast as voltage regulator reduces voltage to the rotor to maintain the 13.6v
If you turn on head lights and fan blower to high voltage at battery will drop and then come back up as voltage regulator increases voltage to rotor to maintain the 13.6v.

A 100amp alternator only has 100amps available when engine RPMs are above 1,500, at idle, 700RPMs, it will only have about 60amps.
But no manufacturer puts in an alternator that can not power ALL the vehicle's electrics at idle.
So if your head lights dim at idle then your alternator has 1 failed field coil, NO that is not "normal", lol.
Because a vehicle spends most of the time above 1,000RPM the 2 fields remaining can keep system and battery working fine, but I would look for an alternator sale.




Testing Cliff notes
Battery voltage engine off: 12.3 to 12.8volts, steady not slowly dropping
Battery voltage engine running: 13.6 to 14.9volts

Wiring test, engine off
B+, larger terminal on alternator, Battery voltage, 12.3-12.8v
Yellow wire on alternator, Battery voltage, 12.3-12.8v
Green wire on alternator, 0volts key off, Battery voltage, 12.3-12.8v key on


A fuse uses a smaller wire inside a glass or plastic housing that will melt and separate if too many amps are pulled thru it, it is encased to prevent a fire
A Fusible link is a short, smaller gauge wire that acts the same as a fuse but has a higher tolerance for short "over amp" voltage spikes.
Fusible links are commonly used with alternator connections because short term power spikes are fairly common.
The gauge(size) of the fusible link determines the maximum amps it will tolerate before separating, it melts just like a fuse would and has a non-flammable coating so can't start a fire like a regular wire could.
A Fusible link is generally 2 gauge sizes smaller than the wire it is connected to, if you have a 10gauge wire from alternator B+ to battery then you would have/use a 12 gauge fusible link on that 10gauge wire, usually found at the battery end of that 10 gauge wire




One-wire alternators
These really do need only 1 wire to work, the B+ wire connected to the battery(with fuse/fusible link)
They have an internal voltage regulator AND an RPM switch
The RPM switch is the ON/OFF, so no Battery Light circuit is needed, used, or can be used
Above a preset RPM(alternator RPM not engine RPM) the switch closes and the voltage regulator gets 12volts from the B+ terminal
Voltage regulator then sends the 7-9volts to the rotor and alternator starts to generate the 13.6-14.9volts
RPM switch can be a bit finicky at low RPMs, which is why car makers don't use them, also no warning light(battery light) if there is a problem, if you have a voltage gauge, and you look at it, lol, then you could see if voltage was lower than 13.6v, i.e. 12.6v, so you would see that alternator was not working.

 

Awesome post! I'll have to bookmark this thread for later!

This should be a sticky.

 

Thanks :)

 

Hello RonD, I have enjoyed reading your posts. I have a 2000 B4000 and have been taking some voltage readings. At start I get 14.9. After the car is good and warmed up I get 14.5. While in drive and foot on the brakeI have turned on the fog lamps, head lights and blower on high to increase the load. It does drop to 12.5. When I take the load off and put in in park it goes back to 14.5-14.6 and idles around 950. Per many of your other posts I have cleaned the mass air flow sensor and the IAC. It seems like my particular problem might indicate a bad regulator AND a failed field. The alternator is a 95 amp rebuild that I put in 18 months ago. Should my truck have the 135? I also replace the 04' battery last Saturday that tested bad with a 590CCA Interstate. Thank you Ron for your help.

 

Hi,

Yes, it reads like 1 field is bad(or diode, same thing)

Yes, and voltage regulator is stuck at high volts
The 14.9v cold is from the higher idle, 14.5v lower idle

But 4.0l w/automatic should be idling at 750 after it is warmed up, in Park, in Gear computer should bump it up to 800 or so
Manual idles at 650

Yes, after a few minutes of running battery is Recharged so Voltage Regulator should drop system voltage to under 14volt, 13.5-13.8v is average, and it should stay in that range even if you raise RPMs.
Rule of Thumb is 1.1volt above Battery Voltage, so test battery voltage Key OFF, and it should be 1.1v above that after engine has been running a few minutes.

Another sign of stuck Voltage Regulator is voltage going up and down with RPM, it may popup briefly, when REVing but if you raise RPMs to say 2,000 and hold it steady, voltage should be the same as it was at idle.
A failing battery can cause the higher voltage because it is not recharging like it should, but it wouldn't cause the lower, 12.5v, voltage, that's fields

After engine is warmed up and idling in Park
Unplug the IAC Valves wires, RPMs should drop to 500 or engine may even stall, either is good, it means no vacuum leaks

If idle doesn't drop then there is a vacuum leak, look at PCV valve hose, also change PCV Valve if you haven't done that in the last few years, they can stick open a bit.

95amp alternator is fine for stock electrics, if you can find high amp for same price it won't hurt anything

 

Thank you for the great info Ron. I dropped the car off with my mechanic last night and he is going to "take notes" while starting it and driving it to his shop today. It is 15 degrees here in MN so it will be a good cold start testing morning as I have been having trouble getting it to start. I did the IAC test last week by unplugging it and the motor died instantly so that may indicate that I don't have any leaks. I also cleaned it the best I could but it is pretty dirty so maybe that could be cause of my high idle problem. I got the "moose howl" humming noise from the air box a couple times after I cleaned it but this seems to have stopped. Will keep you posted......Mike

 

Intake manifold gaskets. Contraction of cold parts resulting in vacuum leak. Alternator is also bad so I will be ordering and installing myself as that is a minor repair I can handle. Any suggestions on which brand to buy? Thanks!