The Stovebolt Site Forums The Shop Area The Electrical Bay Alternator pulley size ?

Chasing an electrical gremlin that has been dogging me for a while now....I am killing the battery while driving ! Have relatively new battery & alternator so I am confident they are not the issue. I can drive the truck for weeks on end with no issue, then, like last night, we go for a 2 hour cruise and the darn battery dies - had to limp it home. Is it possible the Alternator is not spinning fast enuf when I am doing a lot of slow / low RPM driving ? Any other suggestions ?
Thanks guys !

It's not likely the alt. isn't turning fast enough. If you have a lot of acess. like electric fan & A/C it could be too much load for the alt. Make sure the belt is good & tight. Even a new alt. can develope a shorted diode & it's output will be greatly reduced. Put a volt meter across the bat & check the charging voltage. It should be about 14 volts. Turn on some access. & rev it a little. It should go back up to 14 volts. This must be done with a well charged bat.

Hy britcon, what are you seeing on your ammeter or voltage gauge? Any load large enough to deaden a battery while driving should provide other symptoms, something not working [lights, air conditoning] or smoke leaking out of a wire somewhere. Batteries can develop an internal short, battery cables can rub through on something hot or sharp and cause a pretty good short circuit. As Wrenchbender Ret. says alternator diodes can short out and give you little or no output. Charge the battery up and then load test it, if it passes the load test, then re-install it and as Wrenchbender Ret. says turn on some loads and see what voltage she charges at, at around a 1000 r.p.m. More batteries die in the heat of summer than in the cold of winter, hope that helps.

Thanks guys......have the charger on it now - will wait for a full charge and check the voltages. I also installed a ground wire from the alternator case to the block to make sure that is working right ( Alternator & brackets are chrome plated / may be a poor ground ? ). Will keep u posted.

It's almost impossible to find a bad diode with only a voltmeter. The voltage will appear normal, but the amp output will be a little less than 1/3 of the normal capacity. The simplest way to find a diode problem is to put a high-discharge load test on the battery with the engine running at fast idle, and check the amount of current the alternator can deliver to the battery. It should be within a few amps of the rated capacity of the alternator- - - -for instance, a 65 amp alternator will put 55 or 60n amps to the battery when a load tester is applied to it. Most alternators with diode problems will only put out 15 amps or less, enough to keep a battery up in the daytime, but not if the lights, wipers, A/C compressor, etc. Any overload the system puts on the alternator will be made up by the battery, resulting in a slow discharge.
Jerry

OK..battery fully charged - getting 12.8v. Truck running getting 13.8 to 14.2v. Took it out for a run / checked it on return, getting 12.6v stopped / running readings bouncing between 6v.and 9v. Wondered if electric fan was drawing too much ( fused @ 15A ), so I pulled the fuse to shut fan down - got 14v / plugged fuse back in to run fan - got 14v !
Alternator is rated at 80A. Is that rating calculated at idle or at " full speed " . Is it possible when fan is running - maybe taking 10 or 12 Amps, that Alternator is not large enuf ?
Geez I hate electrical stuff ! *g*

Thanks guys.

Hy britcon, alternators are rated at maximum output around 2000 engine r.p.m.. That alternator should be able to handle all your loads on at once. I don't like that bouncing reading you were getting with the engine running, that is not correct at all, it sounds to me like your alternator has a problem, either in the voltage regulator or diodes or something. You could remove it and take it to a good electrical shop to get it checked out, or you could just get an exchange one from your favorite f.l.a.p.s.. Or you could bring it over and I could look at it for you. Is it one of those fancy smancy one wire jobs, I can't remember?

Thanks Bobb. I tightened the belt up some more to ensure it wasn't slipping on me when it gets hot. Just foolin' with the carb tonite, then I will try electrical stuff again. Might have to take you up on your offer. Have you got that yellow peril off your hoist yet ?

Hy britcon, yellow peril still on hoist, but hopefully not for much longer. But that won't stop us will it?

Hi Bobb.......took truck for a 2 hour run last night. All seems OK. Getting 14.2v when running before & after cruise - Electric fan running & headlights on. Getting 12.9v from battery when switched off. Keeping fingers crossed. I ended up using a pry bar to get the alternator belt tight enough ( 1/2" play ). This is a notched V-Belt - wondering if when things get hot under the hood and Alternator loads up, if it was slipping on me ??
Time will tell. Thanks again for the offer.