Voltage Regulator Alternator Output ????

'95 K2500 454 TBI Suburban

Suspect low charging ....

Tonight, I was rolling down the road with headlights on high. I noticed the volt meter was well below 14, near 11.5. With high beams on and AC on max, volt meter down to 11.



I felt it should have been much higher (near 14), but question the accuracy of the gauge.
I want to test the alternator output with my multimeter.
Where is the best place to test output, at alternator or battery?
What numbers are acceptable at idle?
With revs?

Also, is the regulator in the alternator like older GM alternators or in the computer?

Any help for a newbie with his first gmt400 alternator/regulator question?

I've always checked at the battery, alternator will feed the battery. I want to say these trucks run around 13.5V when the battery is happy. I know my electric fan really puts a hurting on my stock alternator, I should upgrade to that fancy alternator like you have in your Burb. Regulator is on the alternator, same as all these trucks. I know 98+ had a measure of PCM control, but the regulator and rectifier was still on the alternator.

13.5 at the battery. Thanks.

Maybe I'm just paranoid since my only other GMT 400 experience is my '99 6.5L Turbo Diesel. Dual batteries and the volt meter never even flinched with both A/Cs, high beams & wipers on .....

Buy a cigarette lighter voltage meter and plug it in whenever you question the OEM voltmeter. I currently have a faulty one and use my RF transmitter that is plugged into the lighter. It only displays the voltage on powering it on for a couple seconds. Ordered a more permanent aftermarket and waiting for it in the snail-mail. https://www.ebay.ca/itm/DC-12V-24V-...var=502776502071&_trksid=p2057872.m2749.l2649

I've checked directly at the alternator. A few months ago my K3500 5.7 had a hard time starting and checked and was getting something like 5V. Last year same thing happened to my 99 NBS Silverado and I was getting something like 1V! Both turned out be faulty alternators.

However, the gauge reading on your dash looks like both of mine. I wouldn't be concerned, but just check with a voltmeter to confirm that all is well.

Finally tested it.
12.2 at battery not running
14 at battery when idling cold.

Tried it with the AC on (both) and the dash voltmeter showed well under 13, near 12.... but showed 14 at the battery again. Guess it's fine.

bugdewde said:

Finally tested it.
12.2 at battery not running
14 at battery when idling cold.

Tried it with the AC on (both) and the dash voltmeter showed well under 13, near 12.... but showed 14 at the battery again. Guess it's fine.

Click to expand...

What does it read when hot though?

I was getting 11v with ac and headlights on. Decided it wasnt worth screwing with it and updated to an ad244 (NBS style alt). Cost about the same as replacing the stock one.

I went out and tested it last night.... Both A/C on high and head lights on.
Dash gauge plunged around 10-11 and never rose.


However, the voltage at the battery fluctuated a good bit (digital multimeter) from 10-14 at idle.
Seemed to level out at 13.5 for 3 or 4 seconds before varying again. Dash gauge never rose.
Maybe the dash gauge pulls voltage not directly from battery but somewhere downstream of the system showing large draws???? Just my thinking......

Update:

Roadtrip fiasco.

We took the 'Burb on a 620 mile trip to the Outer Banks, NC.
Hey, I drove it 1800 miles home from Arizona a few months ago (when I bought it) ...... it'll be FINE!

About 100 miles in on I-40 (up the mountain just 50 miles west of Asheville), the whole electrical system shut down. Yep, running 70-75 mph, at night, the head lights, ignition, EVERYTHING, shut the frick off!!!
Kicked it up in neutral and attempted a restart.... Nothing. Pulled over in the emergency lane and attempted a restart again.
This time I remembered I had to hit the alarm "unlock" button before it will crank. When I did, it started right up... Lights good. Good to go.
About a mile down the road, the dash alarm light starts flashing. It shuts off again after a minute or so of flashing. So, I hit "unlock" on the fob & it starts right back up and I'm on my way again.
Every 60 seconds, the dash alarm light flashes, this time I hit the "unlock" button BEFORE it shuts off. Good to go ..... For 60 more seconds. Then hit it again when it flashes and again and again... Etc.
Now I'm just before Asheville (exit 44) at the ******* Barrel parking lot and I shut the engine off, and arm/disarm the system several times hoping to figure something out ..... Hoping to Google a way to unhook or bypass the alarm which seems to be shutting everything off on me. .....
After 20 minutes of finding nothing helpful on google ..... It fires up with no other issues while we each take turns to go pee. No flashing dash alarm light, no shutting down. OK ..... We're off again !
Oh, ..... She's not real happy.....
No issues, stop about 1 am near Raleigh for a room & some rest.
Off at it the next morning..... Within an hour, or so, it shuts off again .... Even setting off the alarm in the process. WTF????
Google says to remove the battery terminals (both) & turn ign to run and attach terminals to re-set it. Worth a try.
So upon removal of batt terminals the alarm keeps kicking on and off. Noticing the underhood light flashing as well as contact is made/lost. (Doing all this with a multi-tool) ... I left my tool bag at home saying "any issues, we'll just use AAA" . lol .... My life, right?
Upon inspection of the terminals, they were a bit corroded/eroded. I used the fish scaler/file on the multi-tool to get some bare lead/metal showing .... and removed what seemed an excessive amount of anti-seize type gunk off the bolts/terminals.
DROPPED THE FRICKIN' MIDDLE TERMINAL SPACER DOWN INFRONT OF THE BATTERY ....WAY DOWN THERE.
UNREACHABLE!!!!!
After 30 minutes (in the hot sun) of failed attempts at reaching it with the LONG dipstick, she meekly suggests bending a hook in the dipstick with the multi-tool. That's just crazy talk right there. Ain't never gonna happen, babe.
Well........ It happened. It worked. It got bent back (not pretty).

Re-attached the terminals (ign in run position the whole time) ...... And all is well again.

Now, with the lights on, the A/C on high (both) .... Dash Volt meter stays near 13 - 13.5.

My original issue (and alarm issues) seemed to be a bad connection at the battery terminal. The terminals on the cable ends were somewhat eroded with actual holes ate through the flats around the bolt hole.
The added corrosion (hard black gunk) and anti-seize goo probably didn't help much either.

Oh, the starter really spins fast now!!!!
Cheap, off-brand Multi-tool to the rescue!!!!!

Enjoying my stay in OBX where 4wd is required to get to the rental beach house. No paved roads up here to these houses.

Do a Ground Voltage drop test from battery to Alternator; one lead on battery Neg other on alternator body, should be .2V or less, if higher check for dirty, loose or missing ground.
Do a positive voltage drop test : output wire of Alternator to Positive battery terminaal, again anything more than .2V means high resistance.
do test with engine running