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Old 01-05-2012, 11:22 PM
RickM5X3 RickM5X3 is offline
Join Date: Mar 2009
Location: U.S.
Posts: 317
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Originally Posted by we350z View Post
During the drive home, even at highway speeds 55-85mph the voltage still wandered around a lot - typically from 12.4V - 13.5V and sometimes moved as much as 1V in either direction. Is this normal? I was told that when the engine/alternator is running the charging system should regulate the electrical system to be at a constant 14.4V? Upon stopping the voltage almost always nose dived below 12V than came back up to fluctuate between 12-13V.
I monitor voltage every now and then using the OBC test function. When my alternator was on the way out, voltage was always in the 11-12V range.

With a good alternator, it never dips below 13.4 or so and usually is 13.9 to 14.1V according to the OBC.

To view the OBC test:
) Key to ignition pos 2 (no need to start engine).
2) Fasten the seat belt to get rid of the “Fasten Seat Belt” message.
3) Press the right button on the instrument panel, and hold it until “TEST-NR. 01″ comes up (5 to 10 seconds).
4) Press the left button on the instrument panel, and the vehicle id comes up (last 7 digits of VIN).
5) Add up the last 5 digits to get the “unlock code”, e.g. the unlock code for “AB12345″ would be 15 (1+2+3+4+5).
6) Repeatedly press the right button until “TEST-NR. 19″ appears.
7) Press left button -> “LOCK : ON”
8. Repeatedly press left button until “LOCK : xx” appears, where “xx” is your unlock code.
9. Press the right button. All modes are unlocked now.

Test 9 gives the voltage. My personal favorite is Test 7, which shows coolant temp.
2016 GMC Sierra 2500 HD All-Terrain, Duramax 6.6

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