1993 Grand Prix, 3.1 L Engine, when I start it up voltage regulator shows 12 volts , when I turn on the headlights it goes to 14 volts. I have checked the alternator & starter. Both ok.

with the engine running use a volt meter to check battery voltage with the head lights on, should be around 14 volts on a 12 volt system, if 12 volts or less the regulator or alternator is at fault . if around 14 volts there is a drain or short, the battery could also be faulty.

nothing wrong with the ECM unless it has the voltage regulator as part of the system
indicates faulty wiring and faulty alternator
have a load test done on the battery
have an auto electrician do a full charging system check
charging voltage should be 14.5-14.8 volts DC
battery voltage should be 12-13.5 volts dropping to 11 volts when cranking the engine
make sure that the drive belt tension is correct and the belt is in good condition

amps or volts
11 to 12 amps indicates nothing but if it is volts then it indicates that the alternator is not charging the battery
Battery voltage is 12 -13.5 volts and when charging is 14.5-14.8 volts dc
the rest of your statement indicates that the alternator or voltage regulator is faulty and you should have the charging system checked by an accredited auto electrician

unfortunately the alternator is only a part of the charging system and it should be tested by an auto electrician not a parts shop
replace the alternator and then take the vehicle to an accredited auto electrician for a charging system check charge rate should be 14.5-14.8 volts DC at around 1500rpm
they can check the alternator out put , voltage regulator or if the regulator is part of the ECM they will be able to test that as well especially when the system is hot as that is when the problem occurs

I'm assuming you mean the charging voltage indicator. This is an indication of an incorrect charging voltage (too high, should be from 12.5 to 14 volts). This problem is most likely caused by a bad charging voltage regulator. There are two variations of the location of the regulator: 1) regulator is outside of the alternator and located on right side of the engine compartment on the wheel-well, below the starter relay or 2) The regulator is inside the alternator (in which case you'd have to replace the alternator).

if you have a multi meter,
check voltage at battery with car running,should be at least 13-14 volts,
now turn on fan motor to high,flashers on,radio on, and check voltage at battery,regulator should have adjusted voltage to handle the load,should still be 13-14 volts,
now turn all that stuff off,and turn on headlights and recheck battery voltage,if below 13-14 volts replace regulator,if voltage is spiking higher than 14volts replace regulator.you do not need other belts.

Suggest that you recheck the connection on the voltage regulator!

Next, start the engine and place a good voltmeter across the battery terminals and read the voltage at idle with nothing on.
Next, turn on the headlights and read the voltage. If it drops more than 2 tenths of a volt (goes from 14.2 to less than 14 volts) the regulator is bad. If the voltage is always less than 13.5 with the engine running - chances are that you got a bad generator!!

Let us know!!

Check battery voltage should be 12v or better with motor off. start engine read voltage again should be 12.9 or better and then turn all accessories on...voltage should be same or slight y less but best would be at least 13v let it idle and volts should not creep downward..
Volts should never be below 12 and never at 14 except for a brief time after startup. Turn engine off and disable ignition system, then check across battery while craqnking. Volts should not go below 9.5. If so, have batt charged and load tested. If good, replace starter. No valid electrical computer testing can be done without a fully charged battery.

two issues here. one is the fuel filter. this filter amy be clogged or dirty. Two. The fuel pump may be malfunctioning or weak due to low voltage. this pump should have 14 volts running threw its circuit's and if its lower than 14 volts, it will not pump properly. i would check the fuse to make sure the pump is receiving the necessary current. if the fuse is good and the volts are within range, the fuel pump is dead and will need to be replaced. i woudl styart with the fuel filter first,

please rate this response and god bless.

Hello! I too am PA St Insp Mechanic.

Here are some suggestions:

Note that the Fuel injection computer starts to cease to work properly at 10.5 VDC. This leads me to a voltage issue. You mentioning that the veh has bright headlamps may be consuming more power than the alt can deliver, with the heater, the lights and the other "normal" draws.

Test the alt output with a voltmeter, plus to plus, minus to minus, at the battery. You should read 14 + volts with all of the above mentioned accessories on, at idle. 11-12 volts = bad alternator, unless the belt is real slipping bad. 14.4 is the spec.

Also, the Voltage Regulator in inside the alternator.

Glad to provide assistance to you; please rate my help with a FixYa, I would be very grateful.