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Alternator or Short?
#793451 Sat Oct 29 2011 09:53 PM
Joined: Dec 2003
Posts: 3,953
S
Shop Shark
Truck died after leaving a parking spot during a rain storm. Lights and wipers (electric) were on. Got a jump start and it was good but driving home the lights and wipers became intermittent. Pulled the battery and had it checked out okay. At full voltage overnight and by morning the meter read 11.88v. Next morning it read 11.72v. Pulled out the ALT and the battery still read 11.72v later that day. Took the ALT to a few shops to get it checked but all were closed for the weekend. Like to think it is the ALT (worn brushes?) and not a short in the old wiring circuit.
With the ALT still hooked up and the BAT reading 11.88v and then 11.72v makes me think there is a short in the ALT.
Any ideas would be welcomed.


"Sedgewick"
1949 Chevy 1/2-Ton
1989 Caprice

Re: Alternator or Short?
Sedgewick #793488 Sun Oct 30 2011 12:13 AM
Joined: Feb 2004
Posts: 20,608
H
Boltergeist
Can you check the specific gravity of the battery? It sounds like it's only partially charged. A fully-charged battery will have an open-circuit voltage of 12.6 or more. Put the battery on a 5-10 amp charge overnight and check the S.G. in the morning. Each cell should have a specific gravity of 1.265-1.300, with no more than a 25-point difference between cells, the lower the difference the better.

A fully-charged sealed-top battery which cannot get a specific gravity check should have a MINUMUM terminal voltage of 12.5 - - - -12.6 or more is better.

Once you're sure the battery is fully charged, start the engine and run a jumper lead from a battery voltage source such as the big charging output terminal of the alternator to the field terminal. The voltage should jump up to 14 volts or more if the alternator is OK.
Jerry


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Re: Alternator or Short?
Hotrod Lincoln #793489 Sun Oct 30 2011 12:26 AM
Joined: Dec 2003
Posts: 3,953
S
Shop Shark
I had it fully charged and put it back with the ALT still hooked up and next day it showed a bit lower voltage. At the moment the ALT is out and I have connected the battery and taken a volt reading. As I cannot get the ALT checked before Monday morning I will take another volt reading then and see if it has dropped at all. If so it would indicate, I think, a short in the wiring somewhere.


"Sedgewick"
1949 Chevy 1/2-Ton
1989 Caprice

Re: Alternator or Short?
Sedgewick #793959 Mon Oct 31 2011 03:58 PM
Joined: Jan 2000
Posts: 124
F
Shop Shark
If you want to check your alternator to see if it is working....hold a screwdriver or pocket knife (if you are allowed to carry one) onto the smooth bearing surface on the back of the alternator (the raised portion in the middle of the alternator on the backside) and with the engine running that surface will be magnetized if the alternator is working. Adding more electrical load... headlights etc will make the magnetism stronger leaving little doubt the alternator is working. No magnetism....no working alternator. By the way...this works on any alternator.

Last edited by fifthave; Mon Oct 31 2011 03:59 PM.
Re: Alternator or Short?
fifthave #794400 Wed Nov 02 2011 12:20 AM
Joined: May 2005
Posts: 7,263
B
General Purpose
Great tip Fifthave. OLBLU49: your last post is confusing. Battery back in but alternater out but hooked to battery????????
Anyway if you charge the battery on a battery charger to 12.6V and let it sit over night, not hooked to anything, and it goes down, it's the battery. It's also very easy to check output of an alternator. Hook up voltmeter to batt, should read 12.6V, start truck, should read (at fast idle of 2500) 13.5-14V area. Do this first with no accessories then with. Should not go below 13.5 much. The intermittent wiper does sound like Alt. But my motto has always been: It's always the battery! If you take the alt to get tested, take the battery also.

Last edited by bartamos; Wed Nov 02 2011 12:23 AM.

I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.


Re: Alternator or Short?
bartamos #794408 Wed Nov 02 2011 12:38 AM
Joined: Dec 2010
Posts: 292
G
Shop Shark
Disconnect the battery after you start it. If the alternator is working it will stay running. Also is it a alternator or genorator? Have you checked all your connections to make sure they are clean and grounded and that no acceserys are left on.


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Re: Alternator or Short?
GrandpasTruck #794434 Wed Nov 02 2011 01:45 AM
Joined: May 2005
Posts: 7,263
B
General Purpose
That's true grandpa but it is somewhat dangerous and should NOT be done on a modern type system, it will burn out the ALT.


I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.


Re: Alternator or Short?
bartamos #794655 Wed Nov 02 2011 09:23 PM
Joined: Dec 2003
Posts: 3,953
S
Shop Shark
Sorry for the confusion. Meant to say the ALT is out of the truck but the battery is still connected to the cables and the truck. I used the battery as a power source to see if there were any electrical problems in the lighting circuit.
Anyway long story made short I took both the battery and the ALT to an auto electric shop and both checked out but there was a lot of belt dust on the ALT pulley. Seems the belt was simply not tight enough. It squealed on occasion and that seemed to be the problem.
Here is a follow up. When a light bulb is burned is it still drawing power?
Thanks for the help.


"Sedgewick"
1949 Chevy 1/2-Ton
1989 Caprice

Re: Alternator or Short?
Sedgewick #794767 Thu Nov 03 2011 02:15 AM
Joined: May 2005
Posts: 7,263
B
General Purpose
No


I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.


Re: Alternator or Short?
bartamos #794995 Thu Nov 03 2011 10:54 PM
Joined: Dec 2003
Posts: 3,953
S
Shop Shark
Thought not!


"Sedgewick"
1949 Chevy 1/2-Ton
1989 Caprice


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