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Alternator keeps cooking itself

Finca SDR

Well-Known Member
Joined
Dec 5, 2017
Messages
246
Location
Costa Rica
Hola everyone,

Would anyone know anything about things that can cause an alternator to repeatedly cook itself? '97 JD 310D. The alternator is relatively new, perhaps six months. I bought it from the expensive commercial electromechanic shop because they have the parts on hand and are pretty knowledgeable, if very expensive. A couple weeks ago my machine wouldn't start, batteries were dead. Checked the alternator, wasnt charging. Took it to the shop, $50 later they gave it back with new brushes and bearings, and it was charging again. Batteries wouldn't stay charged, so I got a new battery. (down to one big battery from two smaller batteries).

Charged the new battery for a bit before installing just to make sure, 12.7 volts. Checked for a drain on the system at the negative terminal just out of curiosity, and it was registering a 12.7 v drain with the machine turned off, tracked it to the alternator. Not enough amperage in the drain to turn on a bulb-style circuit tester though. Took the alternator back to the shop, they replaced a $20 diode or plate or something and gave it back to me. Still had the same drain. They said it's always gonna be there, something about a condenser or something and that the amperage is negligible on a battery. Vocabulary is translated from spanish, we're in Costa Rica.

Machine started great, strong turnover, charging at 14+ volts. Dug a small ditch at my house, couple days later went to do another job nearby. That day smelled the hot electrical smell while working, touched the alternator and it was hot enough to give me a blister burn on my hand. Let the machine cool down a bit, disconnected the alternator and drove home on battery power. Alternator was always charging but clearly not right.

Now they "fixed" it again and charged me $200. This is starting to be a pain in the ass. This time they replaced the regulator, diodes, plate and one phase of the bobbin. remember, spanish translation if it doesn't make sense. The people at the electromechanic shop say that only battery related things can do it, that if were caused by a short circuit it would register a significant battery drain while the machine is turned off. They're saying my old batteries that wouldn't accept a charge cooked it the first time and that my new battery has a defect or something and is low on charge or something and that's what cooked it this time. They got a "specialist" from a local battery shop to come and check the acid levels and he said they're low, I left it at his shop so he could do more extensive testing overnight. I'm sure they're all in cahoots.

The battery is a big 950 amp Mack battery from the local CAT dealer, can't imagine that's the problem but exhausting all their explanations before raising hell. However, if they were trying to give me a runaround, I'd expect them to tell me that my hoe's hodgepodge electrical sytem is short circuiting and that's to blame. That would be a definite possibility because previous owners have done who knows what with it, most fuses aren't connected to anything. However, the alternator shop says that's not really a possible cause.

Anyone have any insight? sorry for the long winded explanation but figured it's better to know the whole story.

Thanks!
 

Ronsii

Senior Member
Joined
Jun 26, 2011
Messages
3,464
Location
Western Washington
Occupation
s/e Heavy equipment operator
A master disconnect is cheap insurance against dead batteries from sitting and fires also!

As for your problem.... hard to say... just because you don't have much draw when you test it don't mean something doesn't change when you're not looking ;) especially with a 'hodge-podge' system.
 

Finca SDR

Well-Known Member
Joined
Dec 5, 2017
Messages
246
Location
Costa Rica
Thanks. I've been considering disconnecting the battery at night as a general policy. Yeaaahhh... I dunno that much about electrical problems except that they can appear and disappear like ghosts ...
 

repowerguy

Senior Member
Joined
Mar 18, 2015
Messages
810
Location
United States southern Ohio
Occupation
mixer truck mechanic
I have a 310 that had a Motorola alternator and had nothing but little issues with it and I used to rebuild alternators at one time. I finally got disgusted and installed a 1 wire 10SI Delco for what a new Motorola regulator would have cost. I have never looked back. I may have had to modify the pulley by boring it to fit the Delco shaft, can’t remember now.
 

hosspuller

Senior Member
Joined
Aug 27, 2014
Messages
1,869
Location
North Carolina
I would suspect a short that is intermittent. Meaning, it's okay sometime, but when the machine is moved / working the short appears. Then disappears again with movement. That would overload the alternator AND drain your battery. The battery Could be a problem too. Depends on the alternator and the battery charge rate. A larger discharged battery may demand more amps than the alternator can deliver. That would over heat the alternator and blow coils, diodes, etc. To protect the alternator, add a fuse on the output. The main disconnect suggested earlier, sounds like cheap insurance against a fire.
 

Delmer

Senior Member
Joined
Jan 3, 2013
Messages
8,864
Location
WI
I've never heard of a "Mack" battery, but I do have a pair of "Gato" boots from the Cat dealer:D so I'm equally suspicious of all of this. Plus, they haven't fixed this so far. There's no saying what killed it first because the batteries are dead and so was the alternator, no witnesses. Why would they keep dumping money into it? because your check hasn't bounced yet? You don't need experts as expensive as these guys, any junkyard mechanic that says he can replace an alternator should be able to do this.

I can't say if these guys are in cahoots or not. I sure would NOT go back to either of them if at all possible. Any more problems, put a delco alternator on that, it could be one wire, or wired in to turn on with the ignition and function just like the original. The W terminal is just tapped into one of the three phases to drive the tachometer if you have a W terminal.

If you want to test your battery, charge it up, leave one cable disconnected for a week, then if it holds 12.7V or better and starts the backhoe, it's fine. If one cell was shorted, it could still start the backhoe and overload the alternator POSSIBLY, but it woudn't hold 12V in that case. The alternator overheating while running after starting normally seems to me the alternator went bad. Were you routinely jump starting this? or putting a charger on it to get it to start? A short would show up quickly, as a fire, or it would burn out the wire, smoke the fan motor etc. I don't believe you have a short outside of the alternator. There's not that much electrical system on this backhoe, is there? do you have AC? fans? Nice sound system?

Don't forget your grounds, measure the voltage drop from the alternator B+ terminal to battery + terminal when it's just started, then from the alternator case to battery - terminal, those should not have a measure able voltage drop.
 

edgephoto

Senior Member
Joined
Aug 13, 2019
Messages
727
Location
Stafford, CT
I would suspect you are getting rebuilts with junk parts. I know more than one electrical rebuilder who has stopped working on alternators that he can't get OEM parts for. They say the cheap Chinese parts that supply the aftermarket don't hold up. I tend to believe him. I had an alternator rebuilt several times and it would last about 3 months. Finally broke down and retrofitted a GM 10Si 1 wire unit and went years before I sold the vehicle.

If you have an amp clamp you could measure the load when running. The alternator, if running at max output could fail over time as it is worked hard.

As for a battery draw you need to measure the draw. Anything more than 50 mA and you have an issue. However a draw will not cause an alternator to fail. A low or dead battery will not cause an alternator to fail.

The alternator failing is a function of junk parts or an internal issue.

EDIT: Even if your demand for power is greater than the output the batteries would just start to discharge.

Think of it like this. If you have a container of water and want to pump it dry the pump can only move so much water. The pump won't fail if the container has more water flowing in than your pump can handle.
 
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