Alternator woes...

[Mister Sunday]

I have the Prestolite 60 amp, 28 volt alternator on my deuce. My voltmeter in the dash started reading in the yellow, so I decided to check it out. At the back of the alternator, if I put my multimeters negative lead on the obvious ground wire, and te positive lead on the big wire under te cover plate, I get a reading of about 100 volts. But, at the batteries, I get a reading of about 26 volts. This is all while the motor is taching about 1200 rpms. Can I get some suggestions on what to do, I would prefer a less expensive idea as I have more time than money right now. Thanks in advance for all the comebacks, I now hafta go get ready for a wedding.

 

[kc5mzd]

What is the reading at idle?
Does the voltmeter decide whether it is reading ac or dc on its own?

When the diodes go out it will cause the alternator to put out AC instead of DC. The diodes usually go out when you run the engine without a battery. If you jumpstart it and don't turn the main power off before disconnecting the jumper cables when there is no battery or a very dead battery or a battery with a open cell it will also fry your diode pack.

The best way to test for bad diodes is to check for AC voltage at the alternator. You have to use a volt meter that lets you select between AC and DC.The smarter the volt meter the harder it is to do.

If it is the diode pack they are relatively cheep. I would recommend replacing the brushes also as they are also cheep.

 

[glcaines]

I have the Prestolite 60 amp, 28 volt alternator on my deuce. My voltmeter in the dash started reading in the yellow, so I decided to check it out..

Just because your meter has started reading in the yellow doesn't necessarily mean that you have an alternator problem. If the battery has been taxed, the meter will indicate a higher reading, until the battery has been recharged completely. Likewise, if you increase the load on the batteries while driving, such as using headlights, the demand will go up slightly. Also, as batteries start to fail, it is common for the alternator to put out more in an attempt to recharge the batteries. If the batteries are starting to fail, they may never fully re-charge.

To fully check out the alternator, you need to confirm the proper voltage output and current. Voltage is only part of the story. Some alternator testers will work with 28V systems, many will not. An A&P at an aircraft repair facility should have an alternator tester that will work, as most aircraft today have 28V systems. You may also want to check the specific gravity of the electrolyte in your battery cells after fully charging with a battery charger. This is easy to do and will tell you a lot about the condition of each cell in the batteries. You could have a bad battery cell.

 

[3rdmdqm]

Should the guage be in the green all the time or just when the truck is running? Reason I ask, my Alternator is putting out the correct 28 votls when running. With the truck off, the guage reads in the yellow. When the truck is running, the guage reads in the green by the white mark where it is supposed to.

 

[Mister Sunday]

As far as I can tell when the truck is not running it will read in the yellow because the voltage will be the 24 you are getting from the batteries. whenthe truck is running the voltmeter should be on the little white mark that is in the green area (the white tick mark indicates approximately 28 volts).

 

[rlwm211]

What condition are your batteries in? I am not being a smart alec.
The reason I ask is that if you have a battery going bad, or has a short in a cell it can pull the system voltage down like a resistor. I would also check your electrolyte levels as a low cell can reduce conductivity even if the truck still starts and runs.

One other test is to idle the truck, check the voltage at the batteries and turn the headlights on and see if the voltage drops, and how much. An alternator that is failing can still charge but adding load can cause the voltage to drop below where it is actually adding voltage to the truck at all. A healthy alternator may drop slightly at idle in terms of voltage but as soon as you raise the idle it should have more than enough ability to make up for the headlight load in terms of electrical capacity.

As it was pointed out in a previous post the alternator can produce AC when the diodes fail, and the battery would tend to moderate the effect so the test is more accurate at the alternator itself for AC. An occiloscope would tell immediately that the diodes are either good or bad but not many car techs have one and know how to use it.

RL

 

[Mister Sunday]

Well, I took the alternator off and took it to a shop. The old man there has been doing business for quite some time, and my Pops told me he would be honest with me. After checking it he told me it was pegging his meter out and that he only had two employees and didn't really have the time to look for what he needed to fix it. So any good deas now? I'm thinking about looking for a modern one wire alternator and getting some Optima batteries and being done with it.

 

[stumps]

Did he know it was a 28V alternator?

-Chuck

 

[markmontana]

So any good deas now? I'm thinking about looking for a modern one wire alternator and getting some Optima batteries and being done with it.

I'm sure it's been suggested many times: this kit from one of our SS members solves the problem:

M35 2.5 Ton Military truck alternator upgrade kit from Wolverine Technologies LLC

"We now have a 12 volt add on kit for your multifuel engines in 2.5 and 5 ton trucks, 189 amps, kit is designed to fit with our 24 volt alternators"

 

[Mister Sunday]

Did he know it was a 28V alternator?

-Chuck

I hope so the man has been in business for decades, and he does starters/generators/alternators for everything from your lawnmower to your earthmovers. I think he has pictures of him rewinding some kind of motor in the belly of a ship.

 

[glcaines]

After checking it he told me it was pegging his meter out and that he only had two employees and didn't really have the time to look for what he needed to fix it.

I wouldn't assume that he knew it was a 28V alternator. I would give him a call and ask the question.

 

[stumps]

I hope so the man has been in business for decades, and he does starters/generators/alternators for everything from your lawnmower to your earthmovers. I think he has pictures of him rewinding some kind of motor in the belly of a ship.

No way that I'd make that assumption. 99% of the alternators out there are 12V, and without a strong pair of reading glasses, us older folks can't read labels worth a darn.

Ask him. It's kind of important.

-Chuck

 

[Mister Sunday]

I remember our conversation, and when he said, "It pegged out my meter.", I said," It's a 28 volt alternator.", To which his reply was,"Well, it's putting out way more than 28 volts."

 

[Hunter2506]

I'm thinking about looking for a modern one wire alternator and getting some Optima batteries and being done with it.

It's feasible, though you'll have to run a lead to each battery. I run one wire 65 amp GM alternators on just about everything once I have a regulator failure. They're less efficient, but they're cheap, very reliable and available anywhere. I even have one on my fuel injected '87 Daytona, installed after the field circuit in the computer failed (couldn't find another computer).


But the factory deuce alternators are pretty good units. I had mine rebuilt about 2 years ago when the bearings seized. New bearings, new brushes, new diodes. IIRC, it was $185 total. Not bad. NOS is closer to $400 plus shipping.

 

[jonesal]

Is this a regulator issue? It's an easy check for an electrical shop to perform. <Insert normal and customary warnings about safety here> Put the alternator on the test stand and take the regulator all the way off (note location of wires in case the numbers fell off the wires). Now you need to figure out which brush is grounded. You either guess (not recommended) or slide a piece of paper between the brushes and slip rings and use a VOM to see which one is grounded. Once you know, spin up the alternator and apply 24 V to the brush with normally applied voltage (the one that is not grounded). This will essentially run the alternator WIDE OPEN. Do this only for a moment! These alternators can produce so much amperage wide open that it could damage the testing machine. If it has a large amperage draw (I usually get 80+ amps from these 60 amp alternators when I do this check), you know the alternator is OK and it's the regulator. If you have nothing, then you need to start performing checks on the rectifiers, stator, rotor, etc. This is always the first test I do on the test stand when one of these go bad. All the parts are on ePay for CHEAP! (regulators for around $100, rectifier kits for $12.50, rotors for $50, bearing and seal kits, etc.). I think the 60 amps alts are a fine unit and easy to work on. The problem is they are not very forgiving when abused (see previous comments in this thread).

Good luck.

Allen
'70 M35A2
'61 CE106AC
'55 M38A1

 

[Mister Sunday]

It's feasible, though you'll have to run a lead to each battery. I run one wire 65 amp GM alternators on just about everything once I have a regulator failure. They're less efficient, but they're cheap, very reliable and available anywhere. I even have one on my fuel injected '87 Daytona, installed after the field circuit in the computer failed (couldn't find another computer).


But the factory deuce alternators are pretty good units. I had mine rebuilt about 2 years ago when the bearings seized. New bearings, new brushes, new diodes. IIRC, it was $185 total. Not bad. NOS is closer to $400 plus shipping.

I just plan on getting the kit that I see in the classifieds. I don't understand how it could be less efficient when it is putting out the same voltage and ampherage.

 

[markmontana]

I just plan on getting the kit that I see in the classifieds. I don't understand how it could be less efficient when it is putting out the same voltage and ampherage.

Heads up- when you order the alternator kit, ask to have it sent UPS, not USPS. I ordered mine on April 12, shipped the next day via USPS Priority, and its still not here (May 18th)! Ridiculous.

 

[Mister Sunday]

Heads up- when you order the alternator kit, ask to have it sent UPS, not USPS. I ordered mine on April 12, shipped the next day via USPS Priority, and its still not here (May 18th)! Ridiculous.

Typical government efficiency.

 

[Hunter2506]

I don't understand how it could be less efficient when it is putting out the same voltage and ampherage.

They suck more crank horsepower due to less efficient regulation. Total power output (horsepower/watts) and efficiency (kinetic or heat energy used vs. power generated) are not the same thing.

That's why you don't see them used on all cars. to keep MPG ratings up, automakers do everything they can to unload engine accessories. Computer regulated charging is one of those things. Digitally monitored and controlled alternators are far more efficient than simpler, cheaper analogue units, whether internally or externally regulated


As far as use on your truck, you really need to run two of them. The 60 amp 28v alternator you have has a peak output of 1,680 Watts. The 65 amp 14v one wire has a peak output of 910 watts, slightly more than half. It won't be able to keep up under heavy electrical load.

 

[doghead]

It looks like at post#7 you removed your alternator but have not tested the other components of the electrical system in the truck. If indeed your alternator is bad, it may have gone bad because of another problem, like a bad battery.

You can send the alternator to CME, they rebuild them.

I would repair the original or replace it with the same thing.

Before I replaced it, I would read the -20 TM and diagnose the trouble.There is step by step instructions to testing and repairing them. Also there is a TM that specifically is for the alternator. TM 9-2920-225-34P