Can you help me diagnose electrical problem? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

ReeseBobby · 10-20-2013, 10:25 PM

Hello,

A week ago I found my truck wouldn't turn over in the morning. I replaced the battery, and the next day the same thing happened. Here is what I've done so far to diagnose. I think its the alternator, but would like some second opinions.

-Battery alone sits around 12V.

-When I connect the battery cables, the charge immediately drops to 9.5 or so.

-I pulled each fuse individually and nothing changed.

-I removed the alternator wires from the alternator, and the voltage drop stops.

-I jump started the truck, and at about a 1500 rpm idle, the battery charged up to 13.5, so the alternator started to charge the battery. However, at lower idles, the battery drops back down to 9.5v.

-I took the positive cable off the battery with the truck running. The truck ran for about 15 minutes, but the lights and such got dimmer until the truck finally died.

Are these all classic alternator problems? Is there a definitive test I can do at home?

Thanks a ton for your advice.

SKELLY4x4 · 10-20-2013, 10:38 PM

I would charge the battery with a battery charger with the battery cables disconnected. Then hook up the positive cable and with a voltmeter set to amps check the current going between the negative terminal and the negative battery cable. If it's more than an amp, disconnect the alternator and see if the current drops to near zero. If so, the alternator has an internal short and will need to be replaced. Good luck.

71swb4x4 · 10-20-2013, 10:49 PM

I would take the alternator off and have it tested at a parts store. Almost all of them have a tester.

They can test batteries too. Don't just throw new parts at it guessing what's wrong. This hobby is expensive enough without replacing good parts.

68gmsee · 10-21-2013, 10:11 AM

Agree with above but here's another possibility: You don't say if it's an internal regulator or external regulator. If you have the external regulator, that could be the problem. If it's internal, have it checked out or just get a new one.

Wrenchbender Ret · 10-21-2013, 05:05 PM

Sounds like the battery is bad. Put a good charge in it & load test it.

ReeseBobby · 10-21-2013, 07:36 PM

Got an external regulator. Looks original. Maybe I'll try that first. Is there a way to test it?

**bigmac73** · 10-21-2013, 07:47 PM

not for sure on the test, but i will say this i went through 2 new ones from napa ( premium Line) that never worked out of the box then i spent a little extra money at rock auto and got a AC delco one that works great

oklalawnboy · 10-21-2013, 09:45 PM

Try tapping the regulator and see if it starts charging. I have seen the contacts stick and it won't work correctly. It sounds like the Alt has a short in it. IMO.

10-20-2013, 10:25 PM	#1
ReeseBobby Registered User Join Date: Apr 2013 Location: Portland, OR Posts: 38	Can you help me diagnose electrical problem? Hello, A week ago I found my truck wouldn't turn over in the morning. I replaced the battery, and the next day the same thing happened. Here is what I've done so far to diagnose. I think its the alternator, but would like some second opinions. -Battery alone sits around 12V. -When I connect the battery cables, the charge immediately drops to 9.5 or so. -I pulled each fuse individually and nothing changed. -I removed the alternator wires from the alternator, and the voltage drop stops. -I jump started the truck, and at about a 1500 rpm idle, the battery charged up to 13.5, so the alternator started to charge the battery. However, at lower idles, the battery drops back down to 9.5v. -I took the positive cable off the battery with the truck running. The truck ran for about 15 minutes, but the lights and such got dimmer until the truck finally died. Are these all classic alternator problems? Is there a definitive test I can do at home? Thanks a ton for your advice.

10-20-2013, 10:38 PM	#2
SKELLY4x4 Registered User Join Date: Jan 2001 Location: RAMONA, CA, USA Posts: 94	Re: Can you help me diagnose electrical problem? I would charge the battery with a battery charger with the battery cables disconnected. Then hook up the positive cable and with a voltmeter set to amps check the current going between the negative terminal and the negative battery cable. If it's more than an amp, disconnect the alternator and see if the current drops to near zero. If so, the alternator has an internal short and will need to be replaced. Good luck. __________________ 1972 chevy k20 long bed 4x4 1990 chevy K5 blazer 2001 chevy 2500 hd 4x4 1999 chevy 1500 4x4

10-20-2013, 10:49 PM	#3
71swb4x4 Senior Member Join Date: Nov 2003 Location: Brookings, SD Posts: 10,497	Re: Can you help me diagnose electrical problem? I would take the alternator off and have it tested at a parts store. Almost all of them have a tester. They can test batteries too. Don't just throw new parts at it guessing what's wrong. This hobby is expensive enough without replacing good parts. __________________ Some people are like slinkies, they aren't good for anything, but you can't help but smile when you see one tumble down the stairs.

10-21-2013, 10:11 AM	#4
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Can you help me diagnose electrical problem? Agree with above but here's another possibility: You don't say if it's an internal regulator or external regulator. If you have the external regulator, that could be the problem. If it's internal, have it checked out or just get a new one. __________________ 68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...

10-21-2013, 05:05 PM	#5
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,229	Re: Can you help me diagnose electrical problem? Sounds like the battery is bad. Put a good charge in it & load test it.

10-21-2013, 07:36 PM	#6
ReeseBobby Registered User Join Date: Apr 2013 Location: Portland, OR Posts: 38	Re: Can you help me diagnose electrical problem? Got an external regulator. Looks original. Maybe I'll try that first. Is there a way to test it?

10-21-2013, 07:47 PM	#7
bigmac73 Senior Member Join Date: Mar 2012 Location: Chesapeake, Va Posts: 1,275	Re: Can you help me diagnose electrical problem? not for sure on the test, but i will say this i went through 2 new ones from napa ( premium Line) that never worked out of the box then i spent a little extra money at rock auto and got a AC delco one that works great __________________ Mike 72 C-10 Cheyenne off frame resto and Upgraded to 4 wheel disk, Tilt, Dakota Digital Dash / Rear slider. 421 SBC / TH350 3000RPM Stall Progression Ignition /Holley 750 DP/3:73 gear Eaton Limited Slip unit / 2 1/2 exhaust glasspacks

10-21-2013, 09:45 PM	#8
oklalawnboy Registered User Join Date: Jan 2012 Location: Glencoe Alabama Posts: 510	Re: Can you help me diagnose electrical problem? Try tapping the regulator and see if it starts charging. I have seen the contacts stick and it won't work correctly. It sounds like the Alt has a short in it. IMO. __________________ 72 Cheyenne super 10 08 3500 HD 4dr dually 12 ford flex (wifes ride) 76 Monte Carlo 83 Chevy short wide Silverado 96 Mazda Miata, 2000 Allegro Bus

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