bought a brand new 2G motorcraft alternator at Napa today.. it was working gr8 all day now.. suddenly the volt meter goes WAY!! upthere... almost at 18!!! i turned it off so it doesnt burn anything.. i go back now start it the meter shows fine at M from NORMAL and suddenly it start to go up.. and goes to almost 18 when it does it the little red battery light comes on... is this an bad alternator problem or something to do with the truck?


does my truck has an voltage regulator ? or is it in the alternator?

I think I'd be looking at both the battery, and the grounds. Both can cause an overcharge condition.

blacksnapon said:

I think I'd be looking at both the battery, and the grounds. Both can cause an overcharge condition.

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I agree with the grounds, but the battery? I need you to explain to me how a battery can fail, causing an overcharge condition.

Bob Ayers said:

I agree with the grounds, but the battery? I need you to explain to me how a battery can fail, causing an overcharge condition.

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An internal open, making the alt charge at its full potential, usually 110 volts. Can also make the battery explode (been there, done that). An open anywhere in the system will make the alt do this. Thats why its bad to remove one of the battery cables when the engine is running.

blacksnapon said:

An internal open, making the alt charge at its full potential, usually 110 volts. Can also make the battery explode (been there, done that). An open anywhere in the system will make the alt do this. Thats why its bad to remove one of the battery cables when the engine is running.

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Nope, I disagee! If the battery had an internal open, there is no way it will even start the engine (NO CURRENT)!

Next, the voltage regulator will limit the voltage, you can verify this by pulling a battery cable off with the vehicle running (A VEHICLE WITHOUT A PCM) and measure the voltage on the open battery terminal back to the one still on the battery. The huge voltage spike happens due to Di/Dt and wiring inductance when the cable is first removed, NOT the alternator.

never, never, never pull the cable ends off while the engine is running. I've been making a living in the automotive business long enough that I've had numerous vehicles, in my bay, that needed replacements of electronic components because of this. The voltage regulator will limit voltage, IF, theres a complete circuit (ground, and power). I had an old Chevvy van years ago, just out of high school, that I did just that. With the headlights on, and the engine running, I pulled the battery cable off, and promptly blew both headlamps (and there was no pcm). And the battery will start it if there is an intermittent open (you have heard of them, right?). How about a cell completely out of water?

ok but its not the battery its a brand new batt.. so i think the regulator is bad.. it came bad with the alt

or something else?

becuase when we start it first time with the new alt.. was vry good good voltage.. very steady.. i turned it off.. a few ours later.. go to the gym after gym wen to the gas station.. after filling up driving home near my house.. i found the dash ligghts where very bright.. so i looked at the meter and it was WAY up there.. just under the 18 ... so i turned it of. started it again it was steady... begin to drive again and again it went way up there..

got home turned it off.. restart it.. was steady again ... and i can see the needle keeps going up with no stop.. and if i rev it a little the needle jumps even higher..

yep, look at the ground cable, where it bolts up on both ends.

blacksnapon said:

never, never, never pull the cable ends off while the engine is running. I've been making a living in the automotive business long enough that I've had numerous vehicles, in my bay, that needed replacements of electronic components because of this. The voltage regulator will limit voltage, IF, theres a complete circuit (ground, and power). I had an old Chevvy van years ago, just out of high school, that I did just that. With the headlights on, and the engine running, I pulled the battery cable off, and promptly blew both headlamps (and there was no pcm). And the battery will start it if there is an intermittent open (you have heard of them, right?). How about a cell completely out of water?

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We are in agreement on NOT pulling a battery cable while the engine is running! However, we are NOT in agreement with what creates the huge voltage. Do you know what Di/Dt is?

A voltage regulator will regulate without a battery, try the measurement I suggested in a previous post. The reason for doing this experiment on a vehicle WITHOUT a PCM, is due to the possibility of blowing the PCM.

To answer the OP, I think your new alternator has a bad regulator.

This may help understand how an alternator works. Look at the "rectified output" waveform, which is without a battery connected. The output voltage is slightly above 12V, and is filtered (or smoothed) when the battery is connected:

http://www.bcae1.com/charging.htm


And to add to the alternator URL, here are a couple of pages from a Semiconductor Device course that I developed a couple years ago, that
explains half-wave, and full wave rectification in detail:

i took it back.. they gave me a new one no charge.. and i bought new wire harnas connectors for the alternator conectors.. i installed it this morning and its been working fine till now...