High Voltage Problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

70GMCer · 09-03-2005, 04:07 PM

I just finished rewiring for an internal reg. alternator and am having voltage problems, I think. I rewired as described in the FAQ. I'm reading 17.5 volts at the battery at idle and 18.5 volts at higher RPM. I don't think this is right as this is overcharging the system. Isn't it only supposed to be around 14 volts.

This is a new reman alternator that I installed.

Hope someone can let me know what's up, if this is normal or?

magooster · 09-03-2005, 05:04 PM

Yeah, somethings definetly screwy. Your right, the battery should read somewhere around 14 volts at idle. Either your new alternator's internal voltage regulator is defective or your wiring is incorrect.

red71cheyenne · 09-03-2005, 05:05 PM

What was the condition of the battery before you started it back up? What are you reading the voltage with and where? Need those answers, but 18.5 while charging, if the battery is low might not be bad, as long as it drops down after its recharged. If you are reading that at the alternator output, check it somewhere else in the circuit to see if it is dropping down once its in the system. If you have a voltmeter, read the battery terminals with it shut off and then again with the truck running. Normally, you will see about 14.5 volts with it running and a normal charging situation gonig on(that is, battery not fully discharged.) Jeff

70GMCer · 09-03-2005, 05:13 PM

I was checking the voltage at the battery with my dig. volt meter and set for dc volts. The battery is getting old, (about 7 yrs) and I have had to charge it recently. I've just never had a volt spike like this before. I'll try and take a reading at the alternator and see what I get......Like I said, I just bought this alt from Advance Auto today.....

neonlarry · 09-03-2005, 05:22 PM

You said the magic words, reman & Advance. Its probably a bad alt. either that or you didn't get an internally regulated alt.

Longhorn Man · 09-03-2005, 10:14 PM

I have never heard of an alt putting out more VOLTS to charge up a low battery....they will however put out more [/i]AMPS[/i] to recharge a dead player though.

IO think neon larry hit the nail on the head there...either not an internally regulated alt...or it's a worthless one.

68C15 · 09-03-2005, 10:33 PM

I am leaning towards the "junk alt" side. I personally REFUSE to buy an alternator or starter from anyone. they are so easy to do yourself. I spent about $60 in parts to do my 100A junk yard jewel (get one from an 85ish Olds with heated rear window)

if you had that thing running with any stereo or other higher tech (non-antique) equipment you may have blown that as well

ChevyMex · 09-04-2005, 01:44 AM

I am reading 12.4 at idle, new alt , new engine harness, external regulator, whats happening?

Longhorn Man · 09-04-2005, 07:19 AM

You aren't charging. Need to check out which component isn't working.

70GMCer · 09-04-2005, 07:21 AM

I've rechecked the wiring conversion I did, and is correct according to the FAQ. I'm going to start the truck up again, and while it's running, disconnect the + bat. cable and see what that (maybe junk) alt. is putting out then. I got a feeling that the alt. is junk. I wanted to buy a cheap one ($29.99) just to see if this change would work. Gotta feeling that I will be taking it back, at least I'm hoping that's what's wrong with this, and not any wiring problems......thanks for all the help guys......

69Goody · 09-07-2005, 12:55 PM

Put a new high amp alt in my ride...replacing an old one. It required a high amp wire kit (came with my painless harness) to reduce the load. Gives me one hot wire to the fuse block like normal and another off to a Maxi-fuse off the battery/starter.
Before I did this, I fried some wire and the Flex-o-lite control on my fans...thank goodness for warranty (they were real cool about my stupidity). My voltage was through the roof too.

70GMCer · 09-07-2005, 03:27 PM

I took the reman alt back to Advance Auto to check. This alt was putting out so much voltage, that it cooked one of their connections. That was funny. I told them to give me my money back. Didn't want another one from them. I then went across the street to Auto Zone and bought one of their Duralast with lifetime warranty for $10.00 more. Put it on the truck. Charging at 14.1 volts. I should have done that in the first place. I just hope I didn't cook any of my electrical things, like maybe the HEI mod. I'll wait and see......Never again, Advance!!!!

Thanks for all the help guys.............

Longhorn Man · 09-07-2005, 05:18 PM

Normally that is the other way around. Auto Zone has junk alt.

09-03-2005, 04:07 PM	#1
70GMCer Senior Member Join Date: Apr 2005 Location: Joppa, Maryland Posts: 4,408	High Voltage Problem I just finished rewiring for an internal reg. alternator and am having voltage problems, I think. I rewired as described in the FAQ. I'm reading 17.5 volts at the battery at idle and 18.5 volts at higher RPM. I don't think this is right as this is overcharging the system. Isn't it only supposed to be around 14 volts. This is a new reman alternator that I installed. Hope someone can let me know what's up, if this is normal or? __________________ 1997 GMC Sierra Ext 1965 Mustang

09-03-2005, 05:05 PM	#3
red71cheyenne Back in the sticks Join Date: Jan 2001 Location: Fordland, MO Posts: 3,188	What was the condition of the battery before you started it back up? What are you reading the voltage with and where? Need those answers, but 18.5 while charging, if the battery is low might not be bad, as long as it drops down after its recharged. If you are reading that at the alternator output, check it somewhere else in the circuit to see if it is dropping down once its in the system. If you have a voltmeter, read the battery terminals with it shut off and then again with the truck running. Normally, you will see about 14.5 volts with it running and a normal charging situation gonig on(that is, battery not fully discharged.) Jeff __________________ 1971 Cheyenne C-10 w/700R4 and Tuned Port Injection 1969 K5 Blazer w/Tuned Port 2010 2SS/RS Flaming Orange Camaro 2011 K1500 Suburban 2014 K1500 Pickup 2008 Nissan Altima? The wifes' hoopty

09-03-2005, 05:13 PM	#4
70GMCer Senior Member Join Date: Apr 2005 Location: Joppa, Maryland Posts: 4,408	I was checking the voltage at the battery with my dig. volt meter and set for dc volts. The battery is getting old, (about 7 yrs) and I have had to charge it recently. I've just never had a volt spike like this before. I'll try and take a reading at the alternator and see what I get......Like I said, I just bought this alt from Advance Auto today..... __________________ 1997 GMC Sierra Ext 1965 Mustang

09-03-2005, 05:22 PM	#5
neonlarry Registered User Join Date: Feb 2002 Location: Vacaville, CA Posts: 2,745	You said the magic words, reman & Advance. Its probably a bad alt. either that or you didn't get an internally regulated alt. __________________ 70 C/10 Light Red 350/TH350, HEI, Duals w/40 series Flows, 91 seat, LED taillights 99 Pontiac S/C GTP, SLP Ram Air hood, GMPP Konis & springs 95 Neon ACR, MP PCM, AFX UDP, 3.0 CAI

09-03-2005, 10:33 PM	#7
68C15 blood type; Retumbo Join Date: Jan 2004 Location: next to my reloading bench Posts: 10,269	I am leaning towards the "junk alt" side. I personally REFUSE to buy an alternator or starter from anyone. they are so easy to do yourself. I spent about $60 in parts to do my 100A junk yard jewel (get one from an 85ish Olds with heated rear window) if you had that thing running with any stereo or other higher tech (non-antique) equipment you may have blown that as well __________________ Man rule #77...if you own a 67 stepside with a caddy 472 you will never be in danger of loosing you man card

09-03-2005, 05:04 PM	#2
magooster '65 GMC Project Join Date: May 2005 Location: Abbotsford, BC Posts: 653	Yeah, somethings definetly screwy. Your right, the battery should read somewhere around 14 volts at idle. Either your new alternator's internal voltage regulator is defective or your wiring is incorrect.

09-03-2005, 10:14 PM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	I have never heard of an alt putting out more VOLTS to charge up a low battery....they will however put out more [/i]AMPS[/i] to recharge a dead player though. IO think neon larry hit the nail on the head there...either not an internally regulated alt...or it's a worthless one.

09-04-2005, 01:44 AM	#8
ChevyMex Registered User Join Date: Jul 2005 Location: Mexico Posts: 92	The oposite I am reading 12.4 at idle, new alt , new engine harness, external regulator, whats happening?

09-04-2005, 07:19 AM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	You aren't charging. Need to check out which component isn't working.

09-04-2005, 07:21 AM	#10
70GMCer Senior Member Join Date: Apr 2005 Location: Joppa, Maryland Posts: 4,408	I've rechecked the wiring conversion I did, and is correct according to the FAQ. I'm going to start the truck up again, and while it's running, disconnect the + bat. cable and see what that (maybe junk) alt. is putting out then. I got a feeling that the alt. is junk. I wanted to buy a cheap one ($29.99) just to see if this change would work. Gotta feeling that I will be taking it back, at least I'm hoping that's what's wrong with this, and not any wiring problems......thanks for all the help guys...... __________________ 1997 GMC Sierra Ext 1965 Mustang

09-07-2005, 12:55 PM	#11
69Goody Registered User Join Date: Jul 2004 Location: Southern Tip of OHIO 45680 Posts: 49	Put a new high amp alt in my ride...replacing an old one. It required a high amp wire kit (came with my painless harness) to reduce the load. Gives me one hot wire to the fuse block like normal and another off to a Maxi-fuse off the battery/starter. Before I did this, I fried some wire and the Flex-o-lite control on my fans...thank goodness for warranty (they were real cool about my stupidity). My voltage was through the roof too. __________________ Only the Mediocre are always at their best... '69 GMC LWB 406, TH400, 3.73 Posi' Things I've learned from my truck...If I was an automaker, my vehicles would be overpriced, run rough, and get 5MPG.

09-07-2005, 03:27 PM	#12
70GMCer Senior Member Join Date: Apr 2005 Location: Joppa, Maryland Posts: 4,408	I took the reman alt back to Advance Auto to check. This alt was putting out so much voltage, that it cooked one of their connections. That was funny. I told them to give me my money back. Didn't want another one from them. I then went across the street to Auto Zone and bought one of their Duralast with lifetime warranty for $10.00 more. Put it on the truck. Charging at 14.1 volts. I should have done that in the first place. I just hope I didn't cook any of my electrical things, like maybe the HEI mod. I'll wait and see......Never again, Advance!!!! Thanks for all the help guys............. __________________ 1997 GMC Sierra Ext 1965 Mustang

09-07-2005, 05:18 PM	#13
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Normally that is the other way around. Auto Zone has junk alt.



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