Understanding 12v Readings...

[n1265]

I put a new battery in the truck today, It is a ever start max - 79 . It has 1000 CA / 840 CCA and when putting it in I decided to clean all the connections and take some readings at different points. I figured by doing so I might get an idea of the condition of the wiring and ALT gauge.

The problem is, I wouldn't know a good reading if I seen one and I'm conserned some are a little high.

This is what I got, please tell me if you think I may have a problem somewhere:


Key Off
battery = 12.9V
starter relay = 12.9v
fuse box = 12.6v
Alt lug = 12.8v

Running ( multi meter bounces around a little )
battery = 15.2v - 15.5v
starter relay = 15.2v - 15.5v
fuse box = 15.3v - 15.5v
alt lug = 15.3v - 15.6v


Many Thanks,

[Gordon Maney]

Your voltages are fine. The high voltages you are seeing result from the voltage regulator setting of your charging system.

Historically, GM was 13.8 to 14.2 and others were 13.5 to 14.5.

Battery terminal voltage not running will normally be somewhat above 12 volts.

Connection quality at various points will affect all your readings.

Good questions.

[MoparFreak69]

The 0.3V drop between the battery and fuse box is probably a bit of corrosion that should be cleaned up before it gets worse. Its not a bad drop, but it will only get worse as time goes on.

Otherwise all your voltages look fine. Actually 12.9 at the battery means the battery is a good strong one. 15.6 is a tad high, but as long as it isnt boiling the battery out due to too much heat I wouldnt worry about it one bit.

[n1265]

Am I thinking right when I think that the .3v drop at the fuse box would indicate corosion at the fuse box itself , ( maybe where the main wire plugs into in the back ) because there is only a 1v drop at the ALT lug ?
( when not running )


And would it be safe to say that because there is only a .1v drop from the battery to the ALT, that the ALT gauge is good ?

what is the proper way to test the ALT gauge ( if any ) ?


Many Thanks,

[Gordon Maney]

I remember having a Sun VAT 40 charging system tester. It had an expanded voltmeter scale. If I remember correctly, full scale was one volt. You used it to evaluate connections.

Sun made beautiful equipment. I had a marvelous engine analyzer, too. I did a little searching and I don't see that they have been in business for a long time. Very sad. Great stuff. Allen also made good test equipment.

[n1265]

Is that the same company that makes the gauges ( oil, temp, volt ) thought I seen some at advanced auto the other day....

[Gordon Maney]

Years ago there was a company that made tachometers. Sun tachs. Highly regarded. I don't know if gauges bearing Sun today are made by the came company, or if the name has been diluted.

I do not know if the test equipment was from the same company. I am inclined to say not, but admit to not knowing.