Voltage drop
 

My 95 3500 Silverado maintains 13.4 volts when driving. When the headlights are turned on, in a matter of 1/2 hour the voltage drops to 12V.
New Alternator, new battery. Maybe ground issue?

Thanks for any help,
Tom toms63fury@gmail,com

Is the battery staying charged?

The battery stays charged to 12V, I have not driven long enough with lights on to see if it will go to total discharge.

It could be the system dosen't need charged. Or what you see without an electrical load, or small load is the alternator/voltage regulator just outputting what the system requeirs to keep the voltage at 13 volts or whatever. Reading static battery voltage is going to be lower than what it would be running.

Keep in mind, a fully charged battery should be reading at least 12.5 volts with the key off (if equipped, underhood lights can create enough draw to pull the battery below 12.5 so unplug it to verify reading). The alternator has to overcome that in order to charge the battery and power all the systems/accessories, so if it’s reading less than 12.8 volts at any time, I’d suspect a problem.

That said, there’s a few things to verify:

1) Is this a diesel? (I’m guessing no since you said “battery” instead of “batteries” but you didn’t state which engine you have and guessing doesn’t make for good diagnosis.)

2) Is the alternator/charging system light in the cluster coming on when the voltage drops? If not, verify it’s functional by watching for it during a bulb check (first few seconds after the key is turned on or the engine is started - however your truck might be too old for this function - I can’t recall what year that started) or unplug the connector from the alternator and ground the brown wire with the key on - the light should come on. If the light doesn’t come on, it’s powered by the gauges fuse once the key is on, so it might be the bulb in the cluster is out.

3) The internal regulator has a switch that closes when charging voltage is too low or too high which is what causes the charging light to turn on. I can’t find exact specs for what that range is (generally 12.8 - 15.0 is normal depending on load, but it’s not beyond aftermarket parts to have a wider range as updated components would allow for more precise voltage output) so if you still have the box/installation notes for the new alternator, see if it defines the range. If not, try calling/emailing the manufacturer and see if they can provide it.

If you don’t have that info or can’t obtain it, monitor the circuit with an ohm meter by back probing the brown wire at the alternator and grounding the other lead on the alternator case. With the engine running, turn on as many loads as possible and watch for the switch to close when the voltage drops to its minimum. When it closes you should see the ohm meter go from open (OL) to less than 1.0 ohm - if that doesn’t happen, particularly below 12.5 volts (or the minimum given you by the manufacturer), I’d say the replacement alternator is faulty.

4) When the switch inside the regulator is open (within correct charging range), the brown wire should be reading system voltage. Unplug the connector at the alternator and start the engine. Give the engine a minute to adjust idle RPM after start up and check alternator output at the battery, then check voltage on the brown wire - the readings should match within 0.2 volts - if not, high resistance on the indicator light circuit could be causing the internal regulator to reduce voltage.

Another test, idle at 13.5V A/C and fan on high 13V A/C off and parking lights on voltage drops to 12V within minutes same with headlights. Light wiring?

Voltage regulator is bad, that means replacing the alternator. Both one unit.

Then why doesn't A/C pull down voltage?

Try turn on loads opposite of your test, with ac being the last, my $ is you will have the same issue.

Finally found the problem, the issue is the volt meter. With high beams on the volt meter will fall to 12V and lower, Meter reading at the battery is 13.6-14V.
I will install a second volt meter directly from the battery. I also notice when the high beams are switched the oil pressure and gas gauge flicker.