I have a 75' F-100 that has fairly decent 390 in it and pretty high power stereo. Well to go with a bigger alternator what do I have to change and who sells a decent priced alternator? I feel like my trucks not getting the voltage it needs. Any recommendations.

 

What is the current rating at idle and road speed of your present alternator? What is the current draw of your stereo? You need add up the current draw of all your electrical accessories (light, blower motor, stereo, etc.) and compare that number to the alternator output, particularly at idle.

 

My current alternator is a 61 amp max. My autometer voltage guage reads right at 14 volts at idle no accessories and it drops to about 12 volts with electric fan, lights and stereo on. I wanted to go to a 90 or 100 amp alt. but didnt no if what i would have to chnge to do so.

 

You need to calculate the current draw of everything. Voltage drop does not provide much information. A 90/100 amp alternator is an easy fit, but more info is need to determine if that is the real problem. It would also be helpful to know how the stero is hooked up.

 

What do you mean how stereo is hooked up?
Electrical is not my thing so how do you measure current draw with a meter?

 

Where does the stereo get its power from? Direct from battery/alternator terminal though a heavy gauge wire (at least 10 gauge) or from some circuit under the dash through a 16 gauge wire? What is the fuse rating?

As far as the current draw, I assume that all of your circuits are fused. Start by adding up all the fuse ratings and then take 3/4 of that number as your normal current draw.

I am goinmg to bed now (1:30 AM in Virginia) and will follow up with you Monday night if you are able to answer my questions.

 

Sorry for keeping you up but your help is greatly appreciated. The stereo is wired directly from battery using 4 gauge wire w/150 amp ANL Fuse. Amp is running at 1 ohm and is 1100 watts. All fuses added up equal 295 so I figure about 75 amp current draw or so. Correct?

 

WOW! I think you are seriously overloading your electrical system.

If we take the total of your fuse ratings (295 amps) and derate that to 3/4, that equals 221 amps. Even if we derate the 295 by half that is still about 150 amps. I assume that the 150 amp fuse for the stereo was included in the 295, so that says the rest of the system is drawing 109 amps (3/4 derating) or 73 amps (1/2 derating) with the stereo drawing 113 amps (3/4 derating) or 75 amps (1/2 derating). This tells me that even a 100 amp alternator may not keep up if you are running the lights, heater, and stereo at the same time.

PA Performance sells a 130 amp alternator for the 390 with an optional upgrade to 200 amps. Here is the link:

http://www.pa-performance.com/display.asp?sku=7

If you are having a problem with weak bass, you may want to consider adding an auxilliary 1 mircofarad capacitor or two to your stereo system. This helps hold the voltage up during deep bass notes by supplying extra surge capacity to the battery. Here is a link to one of the caps that Crutchfield sells, though any good car audio shop will have them:

http://www.crutchfield.com/S-rIPfz0W...rch=capacitors

Finally, you should go through your wiring harnesses and make sure all the connections are clean and tight and that the wire is of sufficient size to support each circuit.

Good luck with your mods.

 

Yes that included amp fuse and all original fuses under dash for lights, heater, etc.