Electrical charging system question, Alternator? Options

[rjames]

I installed a brand new battery and a new voltage regulater last week in the car. My volt meter is showing 12.5 volts at the terminals. With the car running and the lights and fan on my volt meter fluctuates between 12.7 and 12.9, then slowly starts to dip, getting as low as 12.5 before I decided to shut the engine off because I could see that the battery was slowly draining.

Would it be logical that the alternator might need replacing? The alt. belt also seemed tight (not sure exactly how tight it should feel though.) I can check the electrical connections (have to find them first though.) The connections at the battery all look good though.

It doesn't look like a fun job to replace the alternator, but I'm up for the challenge. Anything else I should look for before I go this route??

Thanks!

PS: Forgot to add that the alternator light on the dash comes on when starting the car, but goes off right away once it's started and doesn't come back on at all when driving.

[rjames]

Check the ground circuit: measure the voltage difference between the negative post on the battery and the fan shroud. The alternator grounds through its case to the fan shroud, then the engine, transmission, up the ground strap and through the body to the battery strap and then the post. That is a long path with lots of connections. If you get much of a voltage difference, then start checking the ground straps.

Voltage is the same at bothe the battery and the fan shroud. Voltage alo measures the same at the cable ends as it does at the battery terminals: about 12.5 at idle. Measured for resistance at the ground straps both at the battery and at the transmission. Zero resistance. Removed them both anyway and gave them a good cleaning.

Voltage regulator is a new Bosch unit, although I still have the old non-bosch unit that came with the car. No difference between the two. I have 2 relay boards to play with- the one currently in the car, and another used one that I will test tonight. Other things I will try tonight:

plugging in the alternator directly into the voltage regulator. (this works, right?)
and/or shorting the D+ & D terminals on the relay board. Although a little leary as I don't wan't to fry my battery.

also maybe grounding the coil as suggested by Dr. Roger to see if the alternator kicks in when starting the car.


I drove around last night for about 30 minutes at high rpms after cleaning the ground straps- when I got home the voltage was still 12.5ish at idle.