O/T alternator or battery??

[old]

In my 1980 chev truck the volt gauge reads only around the 12 volt level when 2 weeks ago it would read at around 13 volts. If I kick the RPM up it reads at 13 or so. So is it the battery going or the alternator??
Thanks just need a few good guess on this one
Hobby farm

 

[Bob]

Did you check the belt and pullies? A slipping belt can cause that.

 

[old]

Belts good and tight, so I know thats not the problem. Thats the first thing I checked because I figure that might be the problem and its not. Guess it could also be a bad battery cable also. Funny thing is it starts up just fine but the gauge doesn't read as it did a couple weeks ago
Hobby farm

 

[Walt Davies]

Alternator output should read 13.8 volts.
Walt

 

[old]

It does if I rev the engine up but if idle it reads around 12 volts

 

[Old Ford Mechanic]

Old,Could be the bridge rectifier letting go.

 

[T_Bone]

Hi Rich,

Any alternator should put out 13.5v at idle rpm. Since you cleaned the cables, tried jumper wire?, and tighten the belts, sounds like a free testing would be next at a chain store.

T_Bone

 

[GeneMO]

Hey Rich how you doing? Hey if it works dont screw with it. That's my motto.

See you, Gene

 

[MarkB_MI]

Sounds like you've got a blown diode. There are six diodes in your alternator and it only takes two good ones for it to put something out. (The alternator is three-phase, one pair of diodes rectify each phase.

Also, I believe your alternator has a "diode trio" that operates the idiot light; that can go bad too. If it does, and you have an idiot light, the idiot light will glow faintly.

 

[john in la]

A battery is only a store house for power. If you can come back 24 hrs later and the truck starts the battery is most likely good.
Since you have MUCH more power going out the cables to crank the motor than you have going in when charging; I would say your cables are good.
So I would think you have a alternator problem. Most likely a diode gone bad.

 

[El Toro]

You should be seeing 14 to 14.5 volts at your battery terminals with the engine running above a fast idle. I would have your local shop to check the alternator. Hal

 

[Allan in NE]

Set of battery cables cured mine about a month ago. Had the same identical symptoms as you're talking about.

I've also gotta run a new feed wire from the temp sender to the guage due to high resistance (old). Darned thing has been reading "half" of normal for 20 years now.

Someday, I'm gonna get around to doing that job. :>)

Allan

 

[hay]

voltage should be about 14.2-14.4 at fast idle startup when cold in the morning. a bit less on a hot engine. make sure the battery terminals are clean and tight. the ones on my 1996 chevy keep getting loose and the gauge shows when they need tightening. must be the connection on the side terminal battery.

 

[Allan in NE]

The Chevy boys have a "fix" for that.

A little lead, square-shouldered spacer washer at the battery connectors.

Had to have my wife's car given the treatment. It always acted up at just the wrong time. :>(

Allan

 

[John T]

Rich, Im sure you have seen me post this a hundred times but is there electrolyte above all the battery plates and no cells have a gray or milky color?????? I bet you have a battery hydrometer see what each cell reads...... Its easy n free for Auto Zone or Pep Boys or Advance Auto etc to stick a load meter on the battery PLUS indicate the alternators condition. Let them do it lol while you pick up some oil etc.

Id say an accurate volt meter on a good charged battery shuld read 12.6 setting bur rise from 13 to 14.5 or so depending on RPM and state of the battery n alternator butttttttt I wouldnt be all that concerned so long as it rises some at least cuz maybe the regulator is smarter then you think n just doing its job....I would expet it to charge more right after start up but drop off soon afterwards. Id also checked for a glazed belt n that its good n tight although thats less an issue on those serpentine types.

Tale care my Old friend

John T

 

[JDemaris]

You can play guessing games - or you can stick a amp meter on the system to find out for sure what is going on.

Voltage only comes up after the amperage flow and demand slows down. If a battery is going bad, and is constantly absorbing a charge of current, the voltage cannot rise properly.

Not much different that a water system that can build high pressure with the faucet held wide open.

If the alternator is not working properly, it is still sort of the same situation as a bad battery. Not enough amperage is being provided to satisfy the need and let the voltage rise.

You could also have a situation where the battery and alternator are working fine, but there is an excess current draw on the system.

Very often, Delcos lose part of the diode trio and lose some charge capacity and can cause this - as well as many other things.

The whole deal could be diagnosed in 10 minutes with an ampmeter.

 

[Anonymous-0]

Pull the battery, so the test is without cables and corrosion, etc. Oriellys or almost any of them will do a true load test on the battery. You can compare actual amps to rated amps on the battery. This is free and will end all speculation about the strength of the battery.

If the battery test OK, then pull the alternator and have it bench tested free. If both test OK, then check cables, wiring, and your guages. Tom

 

[Anonymous-0]

Check that belt again...if you can turn the alt. pulley by hand or with light load with a wrench it's not tight enough or the belt is worn.
I've lived with that on my "77" 350 since it was new. Just doesn't seem to be enough belt wrap after it passes over the smog pump pulley.

 

[old]

No hydormeter in the place. Cell full and the stuff looks good. I guess I'll pull a battery out of one of the tractors and see what happens. The battery was put in some time in 05 so it is getting out and I do work it hard some times with the 2 wenches I have on the truck.
Hobby farm

 

[Anonymous-0]

some people claim their batterys last 8 yrs. or more. How come I never have that luck?

 

[old]

Only battery I have had that lasted that long was never used for 9 of those years and kept in a heated room LOL

 

[Anonymous-0]

Learned this after replacing alternator, battery, etc. A "good, tight" belt that is more than three or four years old may slip a bit when the engine gets up to rpm. The centrifugal force stretches it out, making it 'bigger' and thus slipping a little. Generally speaking solid state electronics will either work or fail, no in between.

Belts, batteries and battery cables can degrade over time causing reduced performance.

 

[Anonymous-0]

Paid 89 cents for my last hydrometer.Ony way to catch tired batteries.Uneven cell readings mean trouble.