strange volt gauge reading

mark'stb

Original poster
Member
Dec 17, 2011
94
Yesterday about 40 miles in to a 200 mile trip, I noticed the needle on the volt gauge was about 1 line to 1 1/2 lines to the left of the 14. Later it would be 1 line to the right of the 14. Did this the whole trip. Sometimes it would stay at 14 I've never noticed it doing this before. It did this throughout the whole trip.I checked the voltage with my tech 2 when I got home, it was showing 13,7 volts on ignition 1. Any thoughts? Reading the shop manual I found this: f the generator has field control circuit failure, the generator defaults to an output voltage of 13.8 volts. Could it not be the generator but the generator battery control module that is on the negative battery cable? This may not be related, sometimes, not always, but when dark, the security light on the dash is on very dim. Security system and keyless entry seems to be working properly.
 

Matt

Member
Dec 2, 2011
4,019
IIRC, the alternator is a supply and demand type, so the voltage will fluctuate based on need. I've monitored it via Torque on trips and see anything from 13.4v to over 14.1v.

If you have any doubts, get it tested and find out for sure.
 

budwich

Member
Jun 16, 2013
2,027
kanata
pull the connector on the control module and see what happens to the voltage. IF it goes / stays at the default, then you know that the alternator is doing what is expected. As suggested, the system is designed to "fluctuate" depending on various conditions / states.
 
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mrrsm

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Oct 22, 2015
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It might be a good idea to stop by an Autozone and have the Battery Load Tested AND the Alternator tested as well. With the Very Cold Weather just around the corner... now would be a good time to know that it is in nominal shape.
 
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Dree

Member
Jun 30, 2017
1
Netherlands
Had the same wobbling Voltage meter after changing the battery. Turned out to be a loose connector on the negative battery (screw worn out, not getting full contact all the time). Perhaps a connection that is corroded somewhere?
 

mark'stb

Original poster
Member
Dec 17, 2011
94
Reading through the shop manual it seems if the generator has field control circuit failure, the generator defaults to an output voltage of 13.8 volts Sounds about right since my tech2 is showing 13.7 volts on ignition 1. With over 214,000 miles on the original generator, I think I will just change it along with the battery control module. Battery is new last Saturday.
 
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mark'stb

Original poster
Member
Dec 17, 2011
94
Changed the generator. After a week of driving, the volt gauge is pretty much staying a 14 or a little above and not going below the 14 much. I changed the battery control module at the same time. Not sure what GM was thinking. I couldn't get the thing past the middle grounding lug. I ended up trimming the lug so the module ring would go past it. Would have made it easier if the ring would slide past the battery end of the cable instead of having to remove the negative cable.
 

DrData

Member
Nov 8, 2014
17
Mark, I'm struggling with a similar issue on my 2008 GMC Yukon Denali, 201k miles. Voltage on dash gauge continues to operate erratically, now mostly staying left of top center, sometimes returning to 14+.
Question: When you made the two changes listed above, were you able to discern whether the solution was the new alternator OR the battery control module?
 

Chickenhawk

Member
Dec 6, 2011
779
How old is your battery? I had the common voltage fluctuation issues on my Trailblazer for years. Changing the alternator fixed it but only for a few months. But since I changed out my six-year-old AC Delco Professional battery for a new AGM battery, it has been rock steady.
 

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