Proper voltage reading questions

[Husjer63]

My '06, 5.9l Turbo Diesel was struggling to start a few times in last couple days (real warm here), and then the check engine light came on. Took to dealership and tech told me batts were going bad.... Said would take about $550 to replace, with labor, included diag charge, etc. Said no thanks, i'll replace myself.
Went to AutoZone, folks there looked up the specs on the truck and game me 2 new batts (750 amp, 12v (I assume)). I replaced the oldies and started her up. No Prob. Now truck volt meter reads about 12v though. Used to constantly read about 15-16 volts. Any idea why it's now reading about 12 when it used to read about 15-16 volts? Is this ok?
Any help is appreciated.

Husker63

 

[RikDavis]

Sounds like an alternator problem. If it is reading 12 volts, actually less than 13 + with the engine running you are not charging

 

[Regcabguy]

Mine's read 15-16 from new.

 

[Husjer63]

I can't believe the dealership would not have checked the alternator. I'll have to take the truck back in and raise hell with em if it is the alt....
Thanks for the info folks.

 

[SierraRam]

The voltage regulator is probably shot.

 

[Prairie Dog]

Something is very wrong - dont run the truck that way. I would disconnect the batteries and check them individually for voltage or take them for load testing.
I brought home two brand new batts form NAPA and found one of them was shorted internally. So it can happen!

 

[lpennock]

The voltage regulator is probably shot.

That would be pretty bad since the regulator is built into the ECU. Doubtful it is the ECU.

 

[TJJEEPER]

The only real test is to have an accurate volt meter. Start the truck and either put it on high idle or have someone idle it around 1300 to 1500 RPM. First, with everything off, you should read about 12.5 to 12.7 volts at fully charged. Start the truck and the voltage should go up to around 14 volts and settle around 13.8 after a few minutes for a properly operating system. It a simple check before you go to the dealer and get their, possibly expensive, test.

 

[Husjer63]

Greg,

I'm really not sure of anything at this point. I agree that taking back to the dealer is expensive, however, I've been on the phone with them and asked about this. I also asked if I would be charged again for service that may have been overlooked (should have been checked the first time) and I was assured I would not be.
So, with the old batteries the voltage guage consistently displayed about 16 volts. Now with new batteries the guage consistently displays about 12 volts. The truck starts fine and seems to run fine. It ran fine before, even though it struggled to start (only two times in over a year-here in N. VA)
So I wonder, can it be a bad guage? Can it be alternator, volt regulator? I would like to know FIRST, what should the voltage guage be reading? What is normal? As I said, from date of purchase (Nov 2012) it has consistently read about 16 volts. Even after having trouble starting, it would read 16 volts. It NEVER waivered. It never slowly dropped, etc. Now with new battereis it's consistent at about 12 volts.
Any thoughts?

 

[Moose]

It is not unheard of for the brushes in the alternator to wear out. Worn brushes = no charging. It is very easy to replace the brushes. You can snag an anternator rebuild kit (with brushes and bearings, probably dont need to install the bearings though) from genos for $35.

 

[cerberusiam]

At this point it is worth reiterating that the gauge is a dummy, it is NOT neccessarily a true reflection of what is going on in the charging system. It is simply the ECU telling you what it thinks you want to know.

Check the voltage at the battery with the engine on high idle and no grid heater draw and it should be between 13 and 14 volts. If it is lower than 13 volts suspect the alternator or battery temp sensor.

With bad batteries or a dirty battery temp sensor the ECU might have been setting the voltage high to note there was a problem. Now, after changing batteries and possibly disturbing the temp sensor it is telling you something different. You have to check actual charge to see if the gauge reading is a problem.

 

[RDeFayette]

At this point it is worth reiterating that the gauge is a dummy, it is NOT neccessarily a true reflection of what is going on in the charging system. It is simply the ECU telling you what it thinks you want to know.

Check the voltage at the battery with the engine on high idle and no grid heater draw and it should be between 13 and 14 volts. If it is lower than 13 volts suspect the alternator or battery temp sensor.

With bad batteries or a dirty battery temp sensor the ECU might have been setting the voltage high to note there was a problem. Now, after changing batteries and possibly disturbing the temp sensor it is telling you something different. You have to check actual charge to see if the gauge reading is a problem.

agreed BUT also check the cable ends to make sure they are CLEAN and TIGHT.
battery charging is read at the drivers side battery temp. if the ecu does not see the correct temp good chance you will cook the pass side battery.
Charging read at drivers side battery, alternator/charge goes to pass side battery then through cross over cable to drivers side.
Bar crossover cable = cooked battery.

 

[Prairie Dog]

Master Cerb, whom which I have the utmost respect for, has argued this with me in the past but from what I've seen from the volt meter, it is extremely accurate and it's not a show like the oil pressure gauge puts on. The meter is able to respond very quickly to voltage changes, like several times a second. If it's at all different from what you read at the batteries, it's because it's taking the voltage reading someplace else like at the ECM - where there could be a bit of a voltage drop.
I'll reiterate what I said before, if all was fine and now after replacing the batteries you have low voltage, one of your batteries is bad. Keep driving it that way and it will cost you big!

 

[zohaa3492]

When did the "new style" recirc doors start being installed in trucks??

 

[Prairie Dog]

When did the "new style" recirc doors start being installed in trucks??

Mine broke when my truck was a week old in Oct. 2002 and the first generation of updated doors was installed then. But many of those broke as well and I think there was at least one if not two more updates in following months.
I guess I was lucky in that mine have not failed since.