I noticed when i first start my truck the volt meter only reads about 12 volts. When i get rolling and get the rpm's up it goes up to 15 volts. I never noticed it having a lower reading at start up before. Do i have a problem with my batteries or alternator?

 

I have brand new batteries on mine. I have noticed mine does the same. I have no idea, but gonna go out on a limb and say that it is normal. Maybe it's just the altenator not charging until the truck had run for x amount of time. Thus allowing in the cold the motor to have less of a drag on it?

 

That makes sense to me. Just strange that i never noticed it in the last 224k miles. Will keep an eye on it. Thanks

 

The "grid heater" comes on when outside air temps is roughly 60*F or below and draws some serious amps. It turns off when you reach 20 mph. Search for "grid heater" and you will have days and days of reading.

 

Same thing happened on all three of my trucks. It's normal and like others have noted, is due to the grid heater cycling.

 

Same thing happens to me and I just went out an got 2 new batteries also. I believe it's the grid heater.

 

It's an idiot guage and reads what the ECM says it should read not actual. You can see the lights dim slightly by the grid heaters but the guage doesn't track along with it.

 

You got it...
Mine gauge does the same thing every time I start it

 

its normal the engine has to hit time and rpm range before it will signal the alternator to charge alot of over the road trucks are the same

 

Yes, it's normal but there is no SIGNAL to the alternator. An alternator is an alternator, RPM; electricity generated. It's an idiot guage run by the ECM.

 

wow so why do they call it a signal wire u might want to read up on that it takes a signal to switch th alternator to activate

 

Feel free to provide the 'reading' you reference and I would be happy to look at it.

 

He is correct about a signal to the alt. It's called an Alternator exciter wire.
Read about it here, Alternator about 1/2 way down the page...

And yes there are 1 wire Alternators...

A conventional alternator draws full field current when the engine is
stopped. The reason it doesn't drain the battery is that the field
supply is switched off with the ignition switch. Since the switched
field power isn't available to the 1-wire alternator, engine stop
and start to turn the field off and on must be inferred from other
parameters. The 1-wire regulator detects engine stop by the
cessation of AC from the stator. This is reliable. Engine start
gets a bit more complicated. Since the alternator is not generating
until the field is applied, engine start must be detected by other
means. With the regulator I commonly use, this is done by looking
for the dip in voltage associated with engaging the starter motor.
If it sees a dip in voltage, it applies field and looks for stator
output. If no stator output, the field is cut off again.

The problem is, to be sensitive enough to detect engine starts under
all conditions (such as when the car is rolled off without engaging
the starter), the voltage dip detector has to be pretty sensitive.
In experiments I have done, I've discovered that the small dip
caused by switching on a single 50 watt driving light will trip the
field on. That means that the field will be momentarily turned on
from a wide variety of conditions other than engine cranking. It
won't be on long but it does consume some battery power.

Again, for the individual user, this isn't much of an issue. Most
1-wire alternators end up on hot rods and old cars with few
accessories and usually none that draw impulse current with the
ignition off. At most, if the dip detector ended up too sensitive
or the car has some load that trips it regularly, the only
consequence would be an occasional dead battery.

above from one_wire_alternators.html

 

OK, fair enough, there is definitely an exciter circuit in an alternator that could be used to control the alternator on and off and leaving it on would kill your battery so it is (or was for years) turned on with the ignition. The ECM could turn it on as a matter of starting the engine but I can't think of why you would delay it or even want to.

The 'signal' is on as soon as the key is turned to 'on' on my 03 and the voltage with a meter at the batteries does not track the dash guage. It is still an idiot guage.