The Stovebolt Site Forums The Shop Area The Electrical Bay A simple alternator question...

My alternator light came on the other day, so I disconnected the batt while running, died immediately. I have always been told if the truck dies with no batt, it’s the alternator...but I wanted to verify anything else I can check before taking the alt down to the parts house.

my understanding is without a battery [dead or alive] to complete the circuit the engine won't run - I'd do a proper static and load test of the battery before pulling the alt, although it may well be a bad alt

Bill

In a 12 volt system, an alternator charging the battery will output 14.3 volts. A battery by itself will output 13.6 volts. A better test would be to reconnect the battery, and start the truck. Before performing and testing, rev the engine up to about 2500 RPM. Many alternators need a good rev to start generating power. Once you rev it, just let the engine idle. Place a voltmeter between the two terminal of the battery. If its 14.3 Volts, your charging system works fine, if it's 13.6 or below, your alternator is no good. If it's between the two, your alternator is questionable. I'm sure other members of this board could tell you what the two different test voltages are for a 6 volt system. I do not know off the top of my head for 6 volt.

I think removing the battery with the motor running is a bad idea, as it acts like a large capacitor, and regulates the voltage in the vehicle in addition to the voltage regulator either internal or external to your alternator. Your alternator generates A/C power, which is then converted to D/C power internally. This conversion process is not perfect, and results in a non-constant output current that is sort of D/C. The regulator alone can only lower high voltages. The battery can provide current to avoid low voltages. Basically, the battery is in parallel with this "sort of D/C" that the alternator is generating, and the result of this is a steady D/C power source for the vehicle. Based upon how good the conversion circuit in the alternator is, which will vary from seemingly identical alternators, running without a battery may work in some systems, it may cause the engine to die in others, and in some instances, I think it may damage the rectifying diodes in the alternator.

Doug

I'm by no means an expert but I have some anecdotal information. As a lad it was commonly preached that a good way to check for a bad alternator was to pull a battery cable just as Sixtyfive did. I remember that being done on occasion. Also, although big trucks may be different, on last season's Ice Road Truckers, one of the drivers lost his whole battery box, including the batteries, in the middle of nowhere and his truck ran fine until he shut it off & it wouldn't restart.

But disels will run without power, he was running the electrical from the alt.A gas engine needs the battery to complete the curicit.I'd never pull the battery cables while running these day's,Had it happen once that the alt died and I put in a new one jumped the car and unhooked the jumpers.Prompty burned out the new alt.Come to find out the batt should be charged at leat 75% or it burnes em up.Note to self:anytime changing out alt,charge and test batt to make sure it'll hold a charge.
Tom

If the 65 has the original system it has an external regulator.
So you could have a problem with one or both. Do the tests with a meter and isolate the problem, it's much cheaper that way than throwing parts at it.
I saw the ice road episode and since it was a diesel it could run without batteries, probably had no lights though. Alternators do not create electricity like the older generators did, they need power input to get something out.
Fred

Most of the run was at night & he didn't know he'd lost the battery box. If he hadn't had lights I think he'd have figured he had a problem.

Once the alternator is "excited" from an external voltage source..the battery, it will produce power and self excite the rotor (field). This is the basis of the idiot light/diode scheme when using a alternator. I vote with Tiny and the Ice Road gents.


Stuart

Let me get this straight. You're going to watch a reality show on TV and let them tell you, all you need to know about automotive electronics.
IT'S television for *%$#@ sakes. Sensationalism at its best or should I say at its worst.

The correct way to troubleshoot any device is with the correct tool. In this case the correct tool is called a volt meter.
As cide1 said, disconnecting the battery can blow the diodes. If the alternator wasn't bad before, there is a good chance it will be afterwards.

If you pull the battery and the engine dies, what was proved?
You may have proved that it wasn't charging, but why wasn't it? Was it a bad alternator, a bad regulator or a bad wire?
You proved squat and may have damaged something else in the process.

I can't see how pulling the battery cable one time would hurt anything...its not like I did it several times. This is no different than a battery going dead. My question was more towards where to I start to test anything else in the system before taking the alternator to the shop only to find that it’s ok. I wasn't going to take what someone saw on a TV show as the only truth, but they were only trying to help. I have a volt meter, just wasn't sure where to test. I'll just take the alternator in for testing...start on that end. Thanks for the help

Standard quick test for years was to pull a battery cable. If it died it was the alternator or regulator. Before internal regulators you could isolate which by running a 12v jumper to the field terminal but just for a moment because it threw the alternator into max output, if it was good, causing it to growl and usually the fan belt to squeal. I have been told one should never do this on computer controlled systems. On many occasions I took the battery out of my 68 shop truck with it still running and traded places with a run down one on a vehicle we were picking up. By the time we got back to the shop it's battery would be charged up if it was good.

Jeeze man, did you get up on the wrong side of the bed? Lighten up. It's OK to have a different opinion without all the chest beating.

Tiny, I wanted to clarify that you are not talking to me. I hope I did not come accross that way, if I did I'm sorry.

Find a local starter and alternator shop, if you cant take the truck down there then take the alternator and reg. with you. They should be able to test the alternator in a test stand for proper Amperage output and if they have a good tester a little diode light will come on if one of the pos. or neg. diodes is out. They should then be able to test the reg. inline with the alternator. If they have the proper equip. they will test it under load etc. They will also be able to put your battery on a charge, load test it etc. If you cant get it worked out in Tyler you can ship the alternator and reg. to me, Im just south of Dallas in Midlothian.
Ken
www.classicgenerator.com

Tiny,





Reality-Stu

Nope, not you man. It was the post above yours. I guess meby I shudda used the quote function.

You CAN fry the diodes in the alternator or some of the other electronics in the vehicle if you pull the battery cable because the voltage will overshoot until the regulator can pull down the alternator output, which takes a small amount of time while the field collapses in the rotor. If you have the headlights on it is less likely to happen because the power has some place to go. It is still a bad idea. It may not hurt anything immediately, but it will shorten the life of something.

Buy a DMM, they cost less than $10. Much cheaper than replacing your alternator.

Ok...I took the alternator down to the parts house...tested badly. I then figured it had to be that so I purchased a new one…replaced, and the "Gen" light on the dash is still on. Could there be something in gauge cluster I should check to see if it’s something with the light? How do I test the regulator while it’s on the truck? Or should I just go buy a new one? A new one is only 15.99 or so. Is it just me or is nothing ever as easy as it should be on these old trucks. Mine just sat too many years for the easy stuff to be all that is wrong. Sorry for the venting. Any help with this would be greatly appreciated!

sixtyfive c-10,

I think terminal #4 on your voltage regulator supplies ground to your idiot light when the key is switched on. This allows the light to illuminate. When your alternator begins to charge, terminal # 4 either becomes charged, or a relay opens...I don't remember which. In either case, this extinguishes the idiot light.

If your alternator checked bad at the store, you replaced it and the problems still exist, your first chore would be to make sure it is operating properly on the rig. The very simplest way to do this is with a voltmeter.

A battery reading is taken with the truck at rest. The engine is then started and the RPM's are increased. This should result in a increase on the voltmeter...high 13 volts or so.

If this all happens, it means the voltage regulator is telling the alternator to charge, but whatever internals connect to #4 are not functioning.

The odds would be that the voltage regulator itself is probably bad and the alternator won't charge properly.

The alternator worked at the store (hopefully)...will the alternator work on the truck. If it doesn't', it's the voltage regulator, or a broken or loose wire. The idiot light might be telling the truth.

Stuart

I have the parts store test the new alternators and starters when purchasing them.had more than one over the years bad right out of the box.Saves a bit of hassle going back for a new one and saves ya the hassle of them saying you must have installed it wrong.Tom

Sixtyfive, pull the connector off the regulator. Hook a jumper wire from the hot term on the connector (i think it is red) to the field term. (blue). You should get a little blue spark. Start the engine & the alt should charge real strong. Use a voltmeter or watch for lights to flare up. If it is working then there is something wrong with the circuit or the regulator is bad. The other wire/term on the connector comes from the ign. & should have curent when the key is on although it won,t be much because it comes through the light or a resister.

Crank the truck, stick a screwdriver to the rear of the alternator just where the rear bearing is, careful not to touch anything else, and if it sticks the alternator is charging. If it doesnt stick, its not charging. Replace the Voltage regulator. Your external regulator may be what burned up the alternator to begin with. You should always replace the regulator when replacing the alternator, otherwise you run the risk of the same thing happening.
Ken
www.classicgenerator.com

Ok, I just now got around to doing the magnet test, and 3 metal roll pins stuck to the back of the alternator while the truck was running, and then they fell off when the engine was turned off. So I take it this means its charging, but the "Gen" light is still on. Could this be a ground inside the gauge cluster? Or something else? Thanks

I helped a friend of mine with a similar problem on his crane several years ago. He also replaced the alternator without checking the voltage regulator. Then replaced the voltage regulator when things still didn't work. After 3 alternators and 2 regulators he was still not willing to listen to the idea that something was shorting both components and he needed to replace several sections of rotted wiring and both components at the same time.
He took it to an electrical specialty shop where they were happy to replace several feet of wiring and install a newer Chevy alternator with internal voltage regulator for several hundred dollars. I was glad to learn from his experience on that one.
Bottom line is check the whole system out before putting in any more components to avoid frying the new stuff.

Won't a one wire alt fix the problem and clean up your eng bay?