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Old 04-03-2012, 09:45 PM   #1 (permalink)
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Declining Voltage

I have an issue that I'm running out of ideas on how to fix...! This is for a '99 Tahoe 5.7L

My resting voltage is 12.4 and when I start the truck I'm at 14.5....however, as the truck runs, it slowly drops down to 13.8. When the truck cools down and I restart at a later time, it starts @ 14.5

If I'm driving for awhile and start to hit stop/go traffic, I've seen it hit 12.* volts.

I have a OEM 140amp alternator that I've had bench tested and it passed all their (Advance auto) tests. I also just installed a new 220amp and it passed as well...and the voltage dropped just like the 140amp!! Even if everything was turned on, my only big draw is my efan (mark VIII) and 1500watt (draws 60amps @ max) sound system.

I've turned off all accessories, lights, etc and the exact same thing occurs with the dropping voltage; so I know I'm using nowhere near the alternator's capacity. The battery is grounded using OEM + 0gauge wire to the body and the alternator is wired using OEM + 0gauge wire. The battery is also a dual post Interstate that is <1yr old.

Thinking their might be a bad ground from body to frame, I bought a big grounding strap and through bolted it on each side....no improvement. I also checked all my OEM grounds in the engine bay and couldn't find an issue with any of them.

Any recommendations to fix this??
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Old 04-03-2012, 09:55 PM   #2 (permalink)
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and youve preformed a parasetic draw test?
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Old 04-03-2012, 10:23 PM   #3 (permalink)
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and youve preformed a parasetic draw test?
Not yet - i figured with standing and starting voltage in check, parasetic draw was not the culprit.

I've left this truck sitting for over a week and starts up with no issue.

But I'm no expert with electricity, so my logic could be flawed.
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Old 04-03-2012, 10:33 PM   #4 (permalink)
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are you depending on the guage on the dash? they are horribly innacurate. try checking with an actual good digital volt meter.

FYI: its perfectly normal for the guage to vary from 13.5-14.5 volts when the engine is running at 1500+ rpm. charging voltage depends on many many things and it will vary all the time.

if your battery starts going dead or the charging voltage is below 12.0 volts then you need to check things out.

depending on load, under certian situations the alternator will be 'maxed' out, and your charging voltage will be in the 12's.

now please move this to the General Repair questions area. this is for 4wd related quesitons only.
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Old 04-03-2012, 10:40 PM   #5 (permalink)
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are you depending on the guage on the dash? they are horribly innacurate. try checking with an actual good digital volt meter.
I used both dash and a digital - both are surprisingly in synch!

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Originally Posted by sjracer View Post
FYI: its perfectly normal for the guage to vary from 13.5-14.5 volts when the engine is running at 1500+ rpm. charging voltage depends on many many things and it will vary all the time.

if your battery starts going dead or the charging voltage is below 12.0 volts then you need to check things out.
Thank you for the intel!

While the truck is running, it's never dropped below 12 volts so maybe my attention is on a non-issue.
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Old 04-04-2012, 12:29 AM   #6 (permalink)
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I just had a similar problem. Mine was really low like 10v and then it would be normal after a while 13-14v. The problem was a loose ground to the gauge itself.
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Old 04-04-2012, 06:16 PM   #7 (permalink)
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I used both dash and a digital - both are surprisingly in synch!



Thank you for the intel!

While the truck is running, it's never dropped below 12 volts so maybe my attention is on a non-issue.
Nothing is wrong with your truck.
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Old 04-04-2012, 08:03 PM   #8 (permalink)
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Quote:
Originally Posted by Sparg93 View Post

But I'm no expert with electricity, so my logic could be flawed.
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Nothing is wrong with your truck.
If it aint broke fix it till it is.
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Old 04-04-2012, 09:30 PM   #9 (permalink)
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go to a garage that has a tech 2 and have them do a sweep test on your guage cluster. clusters go bad all the time.

you could also check your belt and tensioner to make sure they arent worn out and slipping.
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Old 04-04-2012, 10:12 PM   #10 (permalink)
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So..... It always starts, and powers everything you added "1500W stereo", WTF is the problem?

I'd say you don't have a problem. Except you're ANALizing waaaay to much. Relax and spend the extra time you have on something worth while, theres a video game called "World of Warcraft". People like you love it!
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Old 04-05-2012, 05:31 AM   #11 (permalink)
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If it aint broke fix it till it is.
That is very true!

Brookes - belt is new, tensioner is working. I also ran an electric gauge and both gauge & OEM panel are in synch.

Cooter - today I'm swapping the 10bolt for a 14 bolt - it's been a long time since I've had time for video games!
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Old 04-06-2012, 02:14 PM   #12 (permalink)
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13.8 volts will keep the battery happy in warmer weather. I assume you are checking the voltage at the battery terminal? 12 volts while the engine is running at speed is too low to keep the battery fully charged.I've seen stuff like this happen because the battery had a internal problem.Swap the battery if you can and see what happens.
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Old 04-06-2012, 02:42 PM   #13 (permalink)
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It's normal for charging system output voltage to be reduced after some time of running. There is no reason to run at 14.4 volts if the battery is charged. All you're doing is cooking the battery and wasting gas.

Also...either you got ripped off on you "1500 watt stereo" or you're not telling us the whole truth.

60 amp * 14.4 volts = 864 watts assuming 100% efficiency. Typical efficiencies for a class A-B amp is about 50% while some class D amps can get a little higher, but only power sub bass frequencies. Enjoy your 400 watt stereo.

FWIW...a true 1500 watt stereo would draw about 175 amps assuming an average efficiency of 60% (class A-B and class D combined).
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Old 04-06-2012, 03:25 PM   #14 (permalink)
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It's normal for charging system output voltage to be reduced after some time of running. There is no reason to run at 14.4 volts if the battery is charged. All you're doing is cooking the battery and wasting gas.

Also...either you got ripped off on you "1500 watt stereo" or you're not telling us the whole truth.

60 amp * 14.4 volts = 864 watts assuming 100% efficiency. Typical efficiencies for a class A-B amp is about 50% while some class D amps can get a little higher, but only power sub bass frequencies. Enjoy your 400 watt stereo.

FWIW...a true 1500 watt stereo would draw about 175 amps assuming an average efficiency of 60% (class A-B and class D combined).
That's just crazy talk
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Old 04-06-2012, 07:13 PM   #15 (permalink)
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Dont bring Ohms Law into this thread this dude wont sleep for days...
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Old 04-06-2012, 08:50 PM   #16 (permalink)
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Do you have your E-fan positioned in a way to blow the hot air right on the alt? An alternator getting hot will drop output to reduce heat. I found this out when the bushings on mine started going. It was dropping the output down to 12V after a short time of running and getting real hot.

I also have a dual E-fan set up on mine and I run mainly the driver side fan. If I run the pass. side fan it blows hot air from the radiator/condenser right on the alt and the voltage will drop more than it does normally. You might notice that it runs a lower voltage in real hot weather too than in cold.

Be glad he doesn't have a class A amp. You could use that as an aux. heater. Class D can run as high as 90% efficient too. It uses a digital on/off signal instead of linear so it's way more efficient. Bad side though are that the MOSFETS they use can't switch fast enough for higher frequency so they are limited to mid/low to bass. Yeah I had to be a complicated smart ass.
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Old 04-06-2012, 08:59 PM   #17 (permalink)
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There is nothing wrong with his charging system!
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Old 04-07-2012, 01:49 PM   #18 (permalink)
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Quote:
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Be glad he doesn't have a class A amp. You could use that as an aux. heater. Class D can run as high as 90% efficient too. It uses a digital on/off signal instead of linear so it's way more efficient. Bad side though are that the MOSFETS they use can't switch fast enough for higher frequency so they are limited to mid/low to bass. Yeah I had to be a complicated smart ass.
Even at 100% efficiency he's only at 864 watts. 60 amps*14.4 volts=864 watts

*edit* But I bet the amp says 1500x4 super awesome when-struck-by-lightning max watts.
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Old 04-07-2012, 03:18 PM   #19 (permalink)
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What part of queens are you working on the truck in? I could swing by and take a gander at whats going on..Im usually in woodside, maspeth, or jamaica most of the time..
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Old 04-07-2012, 05:39 PM   #20 (permalink)
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I don't know if it was already implimented in '99, but GM with its ecm controlled alternator will reduce voltage when the draw is minimal (and the battery is fully charged), and kick it back up as required. Somebody can correct me on this, but I think they let the system go down as low as 12.8 volts. All in the name of saving a drop of fuel by reducing the alternator load on the engine.

If you have a tow/haul mode you can check this quick, with tow/haul mode engaged the alternator will revert to standard operation (ie: 14.x volts).

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Old 04-07-2012, 05:43 PM   #21 (permalink)
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13.8 volts is a perfectly acceptable charging rate. GM is the only manufacturer that routinely runs 14.4 volts. There is nothing wrong with this truck. The charging system warning indicator is not coming on and there are no DTCs. Put a piece of tape over the voltmeter so you stop paying so much attention to it.
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Old 04-07-2012, 07:26 PM   #22 (permalink)
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13.8 volts is a perfectly acceptable charging rate. GM is the only manufacturer that routinely runs 14.4 volts. There is nothing wrong with this truck. The charging system warning indicator is not coming on and there are no DTCs. Put a piece of tape over the voltmeter so you stop paying so much attention to it.
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Old 04-09-2012, 08:50 AM   #23 (permalink)
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Thanks guys for the intel...this thing took on a life of its own over the wkd.

4x - good observation, my voltage does go lower in the summer and my efan (from a Lincoln Mark VIII) does blow hot air towards the alt; it's almost the size of the radiator, so hot air is blown across the entire front of the engine.


GMGuy - one of the guys in the garage is a long time mechanic and mentioned this was what was going on in my rig. Voltage essentially drops until there is a load placed on the system.

I stopped thinking about this week after the first few posts...everything seems ok after everyone chimed in.
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