Fluctuation Voltage

[David Smith]

I have a 2001 Tahoe and here recently, the voltage gauge has been dancing around like crazy. It seems to have started when I ran a new, heavier gauge wire from the alternator to the battery. Also started around the time that it got really cold outside.

I put a voltmeter on the battery, and it fluctuates from 14.4 to 14.6. Should it fluctuate that much?

When I am inside, the gauge bounces from 14 to about 16.5. Literally just bounces back and forth quickly. Sometimes, it drops to about 13 for a second, then goes back to bouncing around.

Any ideas? Bad gauge? Alternator still good if producing 14.4-14.6 volts? Should it be more consistent?

Thanks!

 

[David Smith]

Did not do it yesterday afternoon or this morning. Maybe because temperature was over 45?

Does cold weather change how an alternator puts out? I have noticed in the past that on cold morning, the amperage was higher than on a warm morning. This was before I had the fluctuating issue.

 

[iamdub]

An alternator's output decreases the hotter it gets. Higher than 14.xx when first started or when operating in cold weather is perfectly normal. 14.4-14.6 isn't much of a change at all and is perfectly normal. The alternator was probably charging at 14.6, but maybe you had the headlights on or A/C blower running or even something small such as the PCM changing the duty cycle of the injectors so the voltage dropped that .2 volts- all perfectly fine.

The voltage shouldn't be drastically bouncing around from 14-16 that quickly, though. Make sure all of your cable connections are clean and solid. You ran a heavier positive gauge, but did you upsize the gauge of the ground as well? This is known as the "Big 3 upgrade". Upsize the wire from the alternator to the battery positive, from the battery negative to the truck's frame, and from the engine to the frame.

 

[David Smith]

An alternator's output decreases the hotter it gets. Higher than 14.xx when first started or when operating in cold weather is perfectly normal. 14.4-14.6 isn't much of a change at all and is perfectly normal. The alternator was probably charging at 14.6, but maybe you had the headlights on or A/C blower running or even something small such as the PCM changing the duty cycle of the injectors so the voltage dropped that .2 volts- all perfectly fine.

The voltage shouldn't be drastically bouncing around from 14-16 that quickly, though. Make sure all of your cable connections are clean and solid. You ran a heavier positive gauge, but did you upsize the gauge of the ground as well? This is known as the "Big 3 upgrade". Upsize the wire from the alternator to the battery positive, from the battery negative to the truck's frame, and from the engine to the frame.

I plan on upgrading the grounds this weekend and maybe installing a 160 amp alternator. HOPEFULLY that will eliminate the issue. It has not fluctuated at all today, but it has been warmer today than usual. It does seem to only happen when it is colder outside.

 

[iamdub]

I plan on upgrading the grounds this weekend and maybe installing a 160 amp alternator. HOPEFULLY that will eliminate the issue. It has not fluctuated at all today, but it has been warmer today than usual. It does seem to only happen when it is colder outside.

Maybe the regulator is failing and during the colder weather, the alternator puts out more juice and the regulator is constantly on it's threshold of restricting and permitting the higher voltage, so it bounces.

An electrical circuit is only as strong as it's weakest link. Upgrading the charging cable will do nothing if the rest of the circuit can't carry as much current. The grounds (actually almost all the non-data circuits in these things) are known to be marginal at best.

 

[David Smith]

Maybe the regulator is failing and during the colder weather, the alternator puts out more juice and the regulator is constantly on it's threshold of restricting and permitting the higher voltage, so it bounces.

An electrical circuit is only as strong as it's weakest link. Upgrading the charging cable will do nothing if the rest of the circuit can't carry as much current. The grounds (actually almost all the non-data circuits in these things) are known to be marginal at best.

The regulator is built in to the alternator, correct?

I will be adding/replacing/upgrading those grounds this weekend. Probably go ahead and throw on the 160 amp alternator as well.

Only need to add battery to frame and frame to block, correct?

 

[iamdub]

The regulator is built in to the alternator, correct?

Correct. IIRC, the PCM senses the state of charge and load on the electrical system and controls the regulator in the alternator. It's what those small wires plugged into it are for.

Only need to add battery to frame and frame to block, correct?

Yup. ThIs will complete the circuit. The alternator is grounded through it's body to the engine block that it's bolted to, so the engine block needs to be just as well-grounded to the frame. The battery grounds to the frame, so it also needs to be equally as well-grounded. Be sure you have the cable terminals on clean, unpainted/uncoated steel. You can paint over or otherwise coat the connections after you bolt them down to prevent rust.

 

[royesses]

Check the connections at the PCM and the BCM. My 2002 GMC Yukon Denali had the dancing voltmeter syndrome for quite a while. The other day I removed and reinstalled the PCM and BCM harness connectors and fixed the problem.

Roy

 

[David Smith]

Put a new alternator on yesterday and have not seen any fluctuation in voltage. Hopefully that fixed it. Still going to be adding the extra grounds soon.