Lilfur

What is the normal output from the alt. when running at idel ?
Is there always voltage at the alt., even when key is off ?
I am pushing about 12.92 volts when running at idel at the alt. , and 12.58 volts at the battery.

...Roger...

Idle is not a very good check point (variables)but I would think 12.9 at idle is ok providing that when you bring the rpms up the voltage goes up above 14.2.

GaryS

In my experience, GM alternators should run at about 12.5 volts +/- at the battery with the engine off. Idle voltage will slightly higher but you should see around 14.3 volts +/- when the engine is running above idle. What is your voltage at, say, 1500 rpm?

Relying on memory but you can check the battery at the alternator with the key off. That post is always hot, iirc.

joshtried

even at idle, first start should be 14 + as well. this may be a bit of a pain, but 2 bolts and a local parts store can check it for you.. for peace of mind

jb78L-82

Stock alternators should show 13.5 volts or higher at idle and should not move much when the rpm's are raised. High output alternators are a different story. My 120 Amp alternator does not produce much voltage above 12.5-13.0 volts at idle but will go just shy of the red zone on the gauge if full charge is required. The high output alternator also will put out 14.8-15.0 volts under heavy load such as when my euro halogen lights pull current especially with the 100 watt halogen high beams on. The manufacturer of the high output alternator (and I have seen from other sources) state that the gen light flickering with HO alternators at idle is normal, which mine can do at times. The original alternator showed very consistent voltage on the guage and my Tach/Dwell meter (ps-the gauge in my car is very accurate).

Lilfur

When i increase rpm does not go up much, still below 13 volts, at alt. ,and batter. Any ideas?

...Roger...

Do you still have the separate voltage regulator? In other words not internal regulator like the 69 has.If you do you can test the alternator separately.
edit-On the back of the alt. remove the plug with 2 wires in it.Make a little jumper to go from the "F" terminal to the "Batt" term.One end of the jumper will be a female connector and the other end will just be bare wire.Dont put the jumper on yet.Get your voltmeter hooked up,start the engine,set your idle to 1500,now put the jumper on the "F" term. and touch the bare wire end to the Batt term on the alternator(DONT GROUND THE JUMPER).Watch the voltmeter and see if your voltage starts to rise above 14v.Dont let your voltage get above 16v (alt is unregulated with the jumper in place).If your voltage went up then chances are the alt is ok and you have a voltage regulator problem or a wiring problem.Make sure your voltage regulator has a good ground.

Lilfur

This alt. is not orig. it is a newer type with 1 post, the regulator is seperate. I just started car and then disconnected the + post on the battery, and my volt meter on the alt. post and am getting about 10 volts, does this show anything?

...Roger...

I'm no expert but a 1 wire alternator should have an internal regulator I believe.If you only have 1 wire then the separate regulator is not connected to the alt..
As far as your 10 volts coming from a 1 wire alt. probably means no alt. output but I dont think that alt. will have output unless its connected to the battery.....I believe thats how it regulates itself.

Why are you asking these questions? Do you think you have a problem?Is your battery not staying up? One seat of the pants test I use is to watch the interior light,turn on the headlights and it should dim and then go bright again.If not raise the rpms a little and see how it reacts then.

Project80

A fully charged battery should be 12.6 volts.
Voltage at idle should be a minimum of 1 volt above battery voltage.
Voltage output can vary between 13.6 to 15.5 volts depending on the electrical needs of the vehicle. But, with a voltage higher than 15.5v, bulbs start to cook.

I agree with DWncchs, one wire alternators should have an internal regulator. Have the alternator tested.