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Voltage dropping

electrical problem

Voltage dropping

Postby Puhly [OP] » Nov 12 2011, 3:13pm

I have a 2003 sierra Z71 5.3

i just put a new alternator and battery in the truck

Any time i put any kind of a load on the alternator the voltage meter drops down to as low as 9-10ish. the truck doesnt die. just as soon as a turn off what ever is drawing the power it goes right back up to where its supposed to be at 14-15ish.

wondering how to determine if i have the 105 amp or the 145 amp alternator>?,,,, i put the 105 amp in the truck and i thought maybe it was just the wrong alternator.? .... im not sure


i was wondering if anyone had a similar problem and could help me out

Thank you
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Re: Voltage dropping

Postby zacharyh687 » Nov 13 2011, 5:32am

I would check and clean the connections to the battery and your grounds. A bad or dirty connection to ground will cause this to happen.
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Re: Voltage dropping

Postby Puhly [OP] » Nov 13 2011, 5:02pm

Any tips on getting to the ground on the engine. lol


how to tell what alternator your supposed to have either 105amp or 145amp
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Re: Voltage dropping

Postby JennaBear » Nov 13 2011, 5:48pm

The Z71s had a 145amp alternator. Sounds like you put a smaller amp one in, and that would definitely cause the problems you are having.
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Re: Voltage dropping

Postby country1911 » Nov 13 2011, 7:07pm

Do some searching for RPO codes. My 02 Z71 came with a 105amp alternator. It could be the alternator is going bad or is bad, especially if you bought it from a store like autozone. I have read they have a lot of problems with things like alternators and starters. The other thing it could be is a voltage regulator, but those are internal on our alternators IIRC.
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Re: Voltage dropping

Postby Larry64 » Nov 13 2011, 7:22pm

Also check your standing battery voltage. Under a load your alternator can't take you go to your battery and if it is big enough and fully charged it should be OK. Even new you should fully charge your battery before you use it. I use an automatic charger like a battery tender to finish charge a new battery. New batteries also can get plate damage it shipping and handling.
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Re: Voltage dropping

Postby Puhly [OP] » Nov 13 2011, 8:54pm

okay i cleaned the starter terminals just because they were corroded.

i cleaned the bumper ground

i cleaned the braided ground on the firewall. but i don't know where that one goes too. the other end..looks to go down by the bellhousing. but i cant see it lol

i just charged the new battery all the way, i just put in a 145 amp alternator,

it still dims the headlights and the dash lights and the voltage meter drops but not nearly as bad as it was.
any other ideas? other grounds im missing ?
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Re: Voltage dropping

Postby country1911 » Nov 13 2011, 9:01pm

The other thing to keep in mind is that alternators at idle barely produce enough to keep the battery charged. You should be testing this at around 1500 to 2000 rpms as that is normal operating range.

In my truck when I engage the turn signal I can watch my voltage gushed drop with each click when I am at idle. When driving at highway speeds, this not nearly as noticeable.
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Re: Voltage dropping

Postby Explorer » Nov 13 2011, 9:43pm

Yeah, you have an issue. My 2002 with original alt and battery at least six years, with every load I can put on it, even factory foglights and AC, never moves the voltmeter or dims lights. You need to find both ends of your ground wires and clean. Look for any swelling in insulation that indicates corrosion creeping back under it. Too small of a battery when you changed it could be a factor. While I did not look close, one ground does go towards bumper and another towards engine block. Firewall gound should go to block also. Test was made at idle.
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Re: Voltage dropping

Postby Larry64 » Nov 13 2011, 9:57pm

Ya, under a load at idle your alternator in not going to keep up. You have the lights on, the radio goin, the heater fan on and you take your foot off the gas at a stop sign. Your lights dim. Sounds normal.
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