voltage meter question

T_ClarkEx39 · August 29, 2017

2017 2500HD, 6.0.

On my old 03 Duramax, my voltmeter, when the truck was running, ALWAYS sat on the mid-point (14 volts). Now, this new truck, sometimes it sits on 14 volts, and sometimes its down at the 12-13 marker. Ive tried turning items on/off trying to see if its something thats drawing it down, and nothing effects it. Sometimes its just up at 14, and sometimes its down.

Is this something specific to the "smart" technology on these new trucks that it adjusts the charging voltage or something? Nothing runs differently with the truck, it actually runs flawlessly so far (only 2K on the clock), but its just something i happened to notice the other day (just bought it 4 weeks ago).

tbarn · August 29, 2017

Output is computer controlled. Just drove it.

Motor City Rick · August 29, 2017

Volt meters gauges in any motor vehicle are general indicators of voltage. True / accurate voltage readings are from Digital Volt / Ohm meters (DVOM). Every vehicle will indicate general voltage, ...not exact voltage.

Now seeing lower voltages of 12 to 13 volts, is usually caused by the AC blower on max, compressor engaged, electric engine fans, rear window defoggers etc. Then if the motor is at lower rpm's, yes you will see shockingly low voltages. Run the motor from idle to 2500 rpm's, if the voltage is above 13.5 volts full load, your doing okay. Also, check voltage comparison between the DVOM and the dash gauge and keep a "mental note" of what the actual voltage is.

MikeNH · August 30, 2017

Normal. GM has been doing computer-controlled alternator output for like 10 years. It'll go up and down as needed.

8lugCT · August 30, 2017

Hmm. Mine sits slightly above 14 and I don't think I've seen it budge.. maybe bc mine had the auxiliary battery?

T_ClarkEx39 · August 30, 2017

Volt meters gauges in any motor vehicle are general indicators of voltage. True / accurate voltage readings are from Digital Volt / Ohm meters (DVOM). Every vehicle will indicate general voltage, ...not exact voltage.

Now seeing lower voltages of 12 to 13 volts, is usually caused by the AC blower on max, compressor engaged, electric engine fans, rear window defoggers etc. Then if the motor is at lower rpm's, yes you will see shockingly low voltages. Run the motor from idle to 2500 rpm's, if the voltage is above 13.5 volts full load, your doing okay. Also, check voltage comparison between the DVOM and the dash gauge and keep a "mental note" of what the actual voltage is.

did all that, when it was showing at 14, i turned everything in the truck on high, at idle and at speed, no difference, did it again when it was showing low, and again, no difference. Hence why i was asking if its something to do with the computer on these new vehicles.

Motor City Rick · August 30, 2017

T_ClarkEx39,

That is why I pushed for the usage of a DVOM. Seeing how electrical based gauges work over the last 50 years, I trust an accurate instrument over any gauge in any car. Seen too much error in dash readings to be nearly accurate.

Whomever designed the voltage reading coming from any computer controlled brainbox is a massive idiot. In electrical theory and circuit design, all voltage / amperage taps "must" be the closest to the power source before any electrical loads, not way down the line in an ECM / BCM. It should be labeled ECM/BCM voltage, not voltage for the vehicle. If the computer is controlling the voltage reading, then what is the actual voltage at the battery which is before any electronics??? Somebody flunked electrical theory and circuit design.

sk · August 30, 2017

Normal. GM has been doing computer-controlled alternator output for like 10 years. It'll go up and down as needed.

This. GM calls it "Regulated Voltage Control":

https://acdelcotechconnect.com/pdf/imtn_V12I305.pdf

Also a good explanation here:

http://www.agcoauto.com/content/news/p2_articleid/350

Motor City Rick · September 1, 2017

SK,

Read both articles, I understand where your coming from. I see the concept GM was / still is trying to do.

In actual electrical circuits, every connection, ...every device has some level of electrical resistance including all batteries. Basic electric circuit fact since the days of Tesla and Edison.

Where this 20 something electrical engineering flunkie forgets, the actual voltage tap must be the first connection from the battery which is the B+ terminal off the alternator (or power distribution center), not coming out of a BCM with it's own internal resistance values / variables though a half a dozen electrical connectors making an "intended voltage calculation" sent to the dash gauge.

It's on the same basic concept of voltage / amperage in your home. Your home doesn't tell the power station what the voltage / amperage should be, nor does it tell you what amperage is allowed, it's the power station (electrical source) making the call on load demands. Also the voltage must be at a steady state or everything in your home will fail.

Second, creating 10 multiple duty cycle levels plus temperature conditions is an over complicated way to get something done. Diagnosing out in the field would be a boondoggle. It will and has caused numerous customer complaints (like this one here) because this "flunkie" changed the voltage demand methodology on how to supply a "steady voltage" to the electrical system under various load conditions. The regulating of voltage is better run tight by hi / low voltage limits and amperage demands on electrical usage. Not having 10 voltage performance levels for system requirements. Radical voltage levels on all automotive electronics / electrical items will cause premature component failures. Let the ECM work in conjunction with the alternator's regulator having bi-lateral data communication to speed up or slow down alternator performance at lower alternator performance speeds to maintain a steady voltage level.

Looks like a gimmick to prematurely trash out electronics at a given time frame from radical voltage fluctuations.

sk · September 5, 2017

I agree with you. I didn't design the circuits -- I was just pointing you to GM's explanation.

As for your load example, just remember your home power is AC which behaves a little differently than DC under different load demands.

Motor City Rick · September 6, 2017

Oh I agree with your statement sk. It's GM's infinite wisdom to try something new bypassing prototyping the bugs out, then creating one massive recall over something very stupid to save a few bucks. Love watching all the dumb azz recalls that car makers have.

redwngr · September 10, 2017

There's also a blurb about voltage changes in the Owners Manual.

If the computer determines that the voltage is outside of the expected range it will also turn on a 'charging system' light and/or display a DIC message.

SierraHD17 · September 10, 2017

Been doing this since the GMT900 trucks. It's just demand based... hence they have a CT to monitor current load on the cable. You are fine.

T_ClarkEx39 · September 10, 2017

T_ClarkEx39,

That is why I pushed for the usage of a DVOM. Seeing how electrical based gauges work over the last 50 years, I trust an accurate instrument over any gauge in any car. Seen too much error in dash readings to be nearly accurate.

Whomever designed the voltage reading coming from any computer controlled brainbox is a massive idiot. In electrical theory and circuit design, all voltage / amperage taps "must" be the closest to the power source before any electrical loads, not way down the line in an ECM / BCM. It should be labeled ECM/BCM voltage, not voltage for the vehicle. If the computer is controlling the voltage reading, then what is the actual voltage at the battery which is before any electronics??? Somebody flunked electrical theory and circuit design.

i completely agree!

35 year electrician here. I have a pretty good grasp on this stuff, thats why i had a feeling this was "computer generated voltage". LoL