loosing a volt at idle

I have a 2002 Ford Ranger "Tremor" I had a Mr. Alternator 200 amp after market alternator, that replaced the stock 120 amp alternator, the Tremors have an upgraded alternator because of the hugemongus stereo system, thats a joke btw, but they do have an upgraded alternator.

I have been having a problem where every time I come to a stop at night, my lights will dim way down, and the interior dash lights will dim so far down they appear to be off. Today we did some messing with it and checked the voltage. At idle, with no extra load, we are getting about 13 volts, down to 12.5 volts. With the light, and the electric fan on, it drops to about 11.9 volts at idle.

We took the after market alternator off and put the stock one back one. At idle now with no load it was pushing out 14 volts, with a load it was at 13.5 volts.

We swapped the Mr. Alternator back in, and it did the same thing as before, so we went back to stock, and now the stock one does the same thing as the Mr. Alternator. We unbolted the alternator and tried to change its position thinking that had something to do with it, to no avail.

There is no radio in this truck right now, the only loads were the lights and the colling fan.

Any ideas on what to cheeck.

check your battery connections, the ground on the alternator, the tension on the alternator, and is the pully on the new alternator smaller?

My buddy had this problem and it turned out that he didn't put it on right and there wasn't enough tension on the belt so it wasn't spinning fast enough.

We took the alt off, cleaned the contacts for the ground, and started it back up. Got 14 volts right off the bat. Started it up later, and were back down to 12.5 volts.

Unhooked the ground side of the battery cable with it running, and tested it to the end of the wire, away from the battery, back to 14 volts, touch the terminal to the battery, back to 12.5 volts. It is an Optima red top, and there must be something wrong with it, making it draw voltage down. Going to toss in another battery tomorrow and see what happens then.

i would first take off the alternator to a shop that can test it wright the start checking yourself

23 yr auto tech saying this   i[f you dont know you can blow]

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never stop