I just dropped in a 350 into my 86 transam. I've been driving for a couple weeks and the voltage ouput was good. The guage read a little over 13.....

Now starting this week it would be under 13 and when i drive at nights with my headlights on, the guage is showing 10.5-11.5....

So today, i put in a new alternator, the 102 amp one. and it has the same problem. now the power is a little worse too. is something else going out in the car. i disconnected my stereo to see if that was at fault but it wasn't.

also, my idle is start get jump around too? Is something draining the hell out of my car.?

I checked the voltage at the battery with a voltmeter and it said 14.6 V....?

What could be the problem....thanks for the help.

 

I'm going to guess it's a poor ground, but check all your connections.

The thick 10-gauge wire bolted to the back of the alternator should make its way through the wiring harness and eventually hook up to the battery.

The only way your alternator makes its ground connection is through its casing.

Most American cars have one large 4-gauge cable from the engine block to the battery ground, and a smaller one from the battery ground to the fender.

Most imports have one large wire (or braided strap) from the engine to the firewall or radiator support, and another one from the battery ground to the fender, firewall, or radiator support.

You can wire it up either style you prefer...but you need a good clean connection from the engine block to the battery for your alternator to do its job of charging the battery. And because the engine mounts are rubber and don't conduct electricity, you need another wire from the battery to a solid metal part of the car...the frame would be best. Both wires should be the thickest you can find, and make good solid connections, and if there's paint there, sand it away first.

 

if the battery voltage is 14.6, then thats good. sounds like a bad gauge. Try turning on the headlights and seeing how far it will drop. be sure to measure it with the voltmeter, not the gauge. then try turning the car off and with the ignition on measure the battery voltage and then look at the gauge...if they were different both times then its probly the gauge. i have a brand new 92 amp alternator and it only drops below 12.5 when i have headlights, windshield wipers, radio, rear defrost and vent on.