charging system question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

lostmya · 03-20-2009, 09:39 PM

I recently installed a voltmeter in my pickup and noticed that anytime turn on my headlights, wipers, turn signal, etc. etc., my voltage instantly drops from about 14volts to about 10 or 12volts. I have a brand new optima red top battery in it so I doubt it has anything to do with that, so could this mean I need a new alternator?

**Green Machine** · 03-20-2009, 10:40 PM

Might depend on where you voltmeter is wired into. Could have some bad grounds, body to frame, engine to cab, battery to frame or body etc, common problem, you can never have too many grounds. Get a good handheld voltmeter and check voltage right at the battery. You should hold about 14.5 volts at fast idle, load or no load, maybe a little less at idle depending on the alternator capacity. If not, then you have a charging problem.

Longhorn Man · 03-21-2009, 05:24 AM

you need to actually test it on the battery for troubleshooting.

Andy4639 · 03-21-2009, 05:52 AM

What kind of alternator do you have...external or internal. Do you still have the old external voltage regulator hooked up. Checking the charging system is best done at battery as stated. Have you checked the belt tension on the alternator lately? These are all things to check and the grounds too!

Wrenchbender Ret · 03-21-2009, 04:50 PM

Sounds like you have a low output alt. or it has a bad diode. You should have at least a 65 amp. The amp. rating is stamped on the drive end housing near the ear for the adjustment bracket.

ChevLoRay · 03-21-2009, 05:06 PM

Didn't these trucks have 37, 42 or 61-amp alternators when they were new? I know there are higher-amperage alterntors, but anyone of these are going to be stretching it to keep up with the stuff we like to use. A/C, stereo systems and the normal stuff all will load the electrical system. Like suggested earlier, grounding is important. Rust and dirt that build up between the lugs and the frame will function as an insulator if the dirt and rust is allowed to fester. The heater motor could have some issues and draw more current than it did when new, as could the wiper motor. Every ground on every light should be checked, too. After 37-42 years, we can't expect all of this stuff to be like it was when it was newly assembled. Yeah, it sucks but we like our trucks, huh?

Longhorn Man · 03-21-2009, 05:12 PM

It is normal to have voltage drop between the battery and the fuse box. When everything draws power from the fuse box, you will have voltage drop, period.
Example my longhorn (and every other truck I've owned) will be running at 14.6 volts, but the guage says 13, then when I turn everything on, the actual output is 13.2, give or take .1, but the guage is reading closer to 10.
That amount of voltage drop is a bit extreem, but it's nothing to be too worried about either.

03-20-2009, 09:39 PM	#1
lostmya Registered User Join Date: Dec 2008 Location: Dallas, Tx. Posts: 19	charging system question I recently installed a voltmeter in my pickup and noticed that anytime turn on my headlights, wipers, turn signal, etc. etc., my voltage instantly drops from about 14volts to about 10 or 12volts. I have a brand new optima red top battery in it so I doubt it has anything to do with that, so could this mean I need a new alternator?

03-20-2009, 10:40 PM	#2
Green Machine Senior Member Join Date: Jul 2004 Location: Estherville, Iowa Posts: 3,371	Re: charging system question Might depend on where you voltmeter is wired into. Could have some bad grounds, body to frame, engine to cab, battery to frame or body etc, common problem, you can never have too many grounds. Get a good handheld voltmeter and check voltage right at the battery. You should hold about 14.5 volts at fast idle, load or no load, maybe a little less at idle depending on the alternator capacity. If not, then you have a charging problem. __________________ 1968 C10 307 3spd Long Fleet ------ http://67-72chevytrucks.com/vboard/s...d.php?t=201103 1970 C10 305 Super T10 Long Fleet --- http://67-72chevytrucks.com/vboard/s...d.php?t=202285 1971 C20 383 TH350 Dana Posi ----- http://67-72chevytrucks.com/vboard/s...d.php?t=206894 2001 GMC Sierra 1500 C3 6.0

03-21-2009, 05:24 AM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: charging system question you need to actually test it on the battery for troubleshooting.

03-21-2009, 05:52 AM	#4
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: charging system question What kind of alternator do you have...external or internal. Do you still have the old external voltage regulator hooked up. Checking the charging system is best done at battery as stated. Have you checked the belt tension on the alternator lately? These are all things to check and the grounds too! __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

03-21-2009, 04:50 PM	#5
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,208	Re: charging system question Sounds like you have a low output alt. or it has a bad diode. You should have at least a 65 amp. The amp. rating is stamped on the drive end housing near the ear for the adjustment bracket.

03-21-2009, 05:06 PM	#6
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: charging system question Didn't these trucks have 37, 42 or 61-amp alternators when they were new? I know there are higher-amperage alterntors, but anyone of these are going to be stretching it to keep up with the stuff we like to use. A/C, stereo systems and the normal stuff all will load the electrical system. Like suggested earlier, grounding is important. Rust and dirt that build up between the lugs and the frame will function as an insulator if the dirt and rust is allowed to fester. The heater motor could have some issues and draw more current than it did when new, as could the wiper motor. Every ground on every light should be checked, too. After 37-42 years, we can't expect all of this stuff to be like it was when it was newly assembled. Yeah, it sucks but we like our trucks, huh? __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

03-21-2009, 05:12 PM	#7
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: charging system question It is normal to have voltage drop between the battery and the fuse box. When everything draws power from the fuse box, you will have voltage drop, period. Example my longhorn (and every other truck I've owned) will be running at 14.6 volts, but the guage says 13, then when I turn everything on, the actual output is 13.2, give or take .1, but the guage is reading closer to 10. That amount of voltage drop is a bit extreem, but it's nothing to be too worried about either.



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