Just installed a new alternator (95A) on my 1994 Mazda B2300 and took care my charging issue I had before. My battery reads 12.7V with engine off and 14.3V when the car is in idle. My voltage gauge also points to the normal position. Now my question is, sometime I notice when I step on the gas I see the needle on the voltage gauge moves higher than normal position and then goes back to normal position. I want to know if the alternator is no good or it is my battery (4 years old), because I still can return the alternator to the store at no cost by the end of the month. The truck starts and runs fine though.

 

An alternators voltage should increase with RPMs, but its the voltage regulators job to bring the voltage back down to between 13.5 to 14.0v, this can take a second or two but no longer

Above 14.0volts is normal just after startup when battery is drained, after 5-10 minutes or so voltage should be under 14volts

Quick "how it works"
An alternator has 3 Fields in its Case, these are small wires wrapped around metal cores
The alternator also has a Rotor which also has wires wrapped around metal cores
If you were to spin the rotor at 1,000 RPMs nothing would happen, except rotor would spin. lol

IF you were to apply voltage to the Rotors wires, it would become an electromagnet.
And now if you were to spin the rotor at 1,000RPMs, AC voltage would build up in the 3 Fields from the spinning magnetic field near them

They are called alternators because they produce Alternating Current(AC)
But vehicles use Direct current(DC) so each Field has 2 diodes, they convert AC into DC voltage

If you vary the voltage to the Rotor then you vary the Voltage in the Fields

If is send 8volts DC to the rotor at 1,000RPM I might get 14volts out of the diodes, at 2,000RPM I might get 15volts, 3,000RPM 16volts
So voltage goes up and down with RPMs
Over 15volts will cook a 12volt battery and is not good for 12v electronics, and under 12volts will drain the battery

This is where the voltage regulator comes in
Any electrical system has whats called electromotive force, I call it Pushback

If you turn on a light that bulb will draw a certain amount of energy and then pushback, it doesn't draw more, it only draws what it needs, if I add another bulb on that wire then pushback goes away until that bulb is lit up and then there is pushback again, no more energy is needed on that wire, it has what it needs

A voltage regulator works by push back
Just after startup battery is drained by starter motor so no pushback from battery only pushback from vehicle electrics because it has all its voltage now, everything is powered up, so voltage regulator send 9 volts to Rotor at idle, and you will see say 14.5volts at battery(anywhere in vehicle really)
If you raise RPMs voltage regulator should lower Rotor voltage to 8volts as vehicle systems pushback because they already had all the voltage they needed and battery can only take so much

If you turn on the head lights you should see voltage drop, so there is less pushback so voltage regulator sets rotor voltage at 8.5volts, then after head lights are all powered up pushback returns and rotor goes to 8.3volts.
If RPMs go up to 3,000 then pushback goes way up and rotor may drop to 7volts

As battery reaches full charge, 5 minutes or so, pushback goes up so voltage regulator lowers rotor voltage
As you turn vehicle electrics on pushback drops so rotor voltage goes up

Its actually a pretty simple system, general speaking you should see 13.5v to 13.9volts after engine has been running for at least 10min
It should be about 1.1 volt above engine off battery voltage, that keeps battery recharged but doesn't "cook it", cause fluid and plate damage

 

Is it normal to see the voltage guage fluctuating a little during the ride? The reason I asked because i don't recall it was like that before.

 

The reason you didn't notice it before is because you didn't pay as much attention to it before this problem as you think you did.

Before the problem you only took a quick glance and saw it was in it's normal spot, then thought no more about it.

Now after the problem you are looking at the gauge closer and paying much more attention looking for a problem.

 

Put voltage meter on the battery, start and REV the engine a few times, have someone turn on and off heater fan and headlights while you watch the volt meter on the battery, see if its displaying same up and down movement as dash gauge

 

Today I drove on the freeway for 45 mins. Right after I turned off the engine, I measured the voltage on the battery and it read 13V. Is it too high?

RonD, I will do your test later today.

 

No, not after just shutting off the engine, it will still have the residual charge from alternator
Best time to test battery voltage is after vehicle has been sitting over 4 hours or so, overnight, 8+ hours, is even better to get a true "at rest" voltage for a battery

 

You are right, now it reads 12.7V.

 

Assuming volt meter is correct that means its a fairly new battery

12.8/9v is a new battery
3 years old will be about 12.5v
5/6 years old 12.3v and time to shop for battery sale

12.2v and lower is done, less that 50% charge can be held in this battery, cold morning will get the "click, click, click" of not enough AMPs to crank the engine

If you test battery with volt meter and its voltage is slowing dropping then it has an internal short, thats the most common way a battery fails, it is usually caused by Overcharging or Draining the battery to low.
Car batteries are specific use, quick discharge quick recharge, thin plates inside, which can short from too high or too low a voltage

 

My battery is like 3.5 years old.

 

It is holding up well, only show 2 years old by at rest voltage

Some of the newer batteries are lasting 7 years now

 

Update:
The truck was fully warmed up (45 mins drive), I measured the voltage on the battery when it's at idle (900 rpm approx, I don't have a tachometer on my dash). The reading on my voltmeter jumped rapidly between 14.33V and 14.55V. Only thing was on was the headlights because it was at night. My dash voltage gauge was fluctuating a little as well. Should it be one constant voltage when the engine is at idle?

 

Something is wrong, voltage needs to be under 14volts or battery will be ruined, you measured voltage WITHOUT shutting off the engine right?
If you shut off the engine and then restarted it voltage may be OK

Do you know the model number of the new alternator?
The maker?

It does read like the Voltage Regulator in the alternator is not doing its job
After even a 15min drive the voltage should be stable, battery would be fully recharged, and unless you are running a 1,500watt stereo amplifier with BIG WOOFER thumpping away, lol, it shouldn't be changing that much

 

Yep, I think something is wrong. This morning, during warming up the engine I re-measured the voltage again and it was more stable this time staying at 14.6V. I then turned on A/C, blower, headlights, still stayed steady at 14.6V. I even revved the engine up to 2500 rpm and voltage stayed at 14.6V.

I bought this online ($65) and believe it's a re-built for that price. No name brand but looks solid and new.