How do I find which is bad, alternator or voltage regulator?

[maple flats]

I have an issue with my tractor, the battery seems to be good, because when I connect a charger to it and the battery gets fully charged, the tractor starts well for a few days but then it begins to crank slower and finally won't start.
The tractor is a 2012 Mahindra, 36 eng HP diesel. How can I test the alternator output and the voltage regulator? I figure one must be bad. Any mechanics out there? I'm certainly not one, but I do fix most issues on my tractor when needed.
The battery was new this past spring and it's higher cold cranking amps than the original was.

[TroyC]

I'd suspect the battery if it is more than about 4 yrs old. Take it to a parts shop and have it load tested if you do not have a load tester. Load testers can be had cheaply at HF for about 20 bucks or so. From what I read it appears the battery is taking a charge but not holding it which is an indication of a cell deteriorating.

If the battery is strong, you can check the alternator and voltage regulator easily with the tractor running. Get a simple volt meter, hook it up to the battery post, you should read close to 14.2 volts DC. Also, check all of the battery terminal connections and make sure they are clean, corrosion free, and tight.

[Wlmedley]

I would check the battery out first especially if it is 5 years old or older.Tractor should have a charge indicator light or a ammeter.If light goes out after starting alternator is probably charging.If you have a voltmeter you can probe between negative and positive at the battery and should read 13.5 to 14 volts with engine running high idle.Might take a minute or two after starting.If it has a small permanent magnet alternator like most small tractors they hardly ever go bad unless bearings fail.

[ljohnsaw]

Based on your initial post, I don't think your battery is bad since you get a "few days" of starting out of it. Check the alternator output with a meter and I think you will see it is around 12v, what you are actually reading the battery voltage.  I know that AutoZone has an alternator tester that will at least tell if the whole unit is bad (do you have a separate voltage regulator or is it internal?).

I thought my pair of Interstate batteries were going bad on my SkyTrac.  I know they are at least 10 years old.  Turns out the One-Wire (GM) alternator that was installed long before I got it had a stranded wire that only had a couple strands left at the alternator - so it couldn't get enough amps to the batteries.  I lucked out on both sides - batteries and alternator. 

[Erik A]

Check your voltage at the battery with it not running. Then start it and check the voltage at the battery. If the voltage goes up, check the battery, if not check the alternator.

[Hilltop366]

Don't forget to check the alternator belt/tension if you haven't done so yet.

[Tom King]

Without typing out a long explanation here, there are plenty of youtube videos.  I didn't watch this one, but if you don't like this one, google "checking charging starting auto system youtube"  and you will find all you want.

[scsmith42]

Most modern equipment has the voltage regulator built into the alternator.

The easiest solution that I know is to remove the alternator and take it to a local alternator repair shop for testing.  

[maple flats]

The battery is only 4 months old and when I put the charger on it, the tractor starts good for 4-5 days before it shows signs of cranking the engine slower. The battery is higher cold cranking amps than the original.

[maple flats]

We used to have a couple of alternator repair shops, there doesn't see to be any left anymore.

[red]

Back in the day , 40 years ago we would remove the negative battery cable with the vehicle running
If the vehicle continues to run your alternator is charging if the vehicle dies it is not charging 

[rusticretreater]

Given the age of the alternator, it is likely to have gone bad.  It is possible for an alternator to put out the required voltage, but not high enough amperage(regulator gone bad).  This can cause the issues you are experiencing.  So checking the voltage delivered to the battery is not a complete test.

However you could also have a parasitic drain.  This causes the same symptoms as the bad alternator regulator. To check for a parasitic drain, you simply disconnect the positive cable and hook up your voltmeter in between.  Any reading on the voltmeter shows the electrical drain.  To try and isolate the problem, pull your fuses for circuits one by one to see if that circuit is causing the drain.  A likely culprit is the fuel sending unit.  Rusty ground connections, wires rubbing are also common causes.

[TroyC]

Back in the day , 40 years ago we would remove the negative battery cable with the vehicle running If the vehicle continues to run your alternator is charging if the vehicle dies it is not charging

 
Do NOT do that with an alternator. On a relative newer vehicle you will unleash a bunch of demons. That was a shade tree test with a generator (think VW Beetle) but you will kill your alternator if you disconnect the battery with it running. Generators have permanent magnets, alternators do not. The alternator has to 'see' voltage in order to work. Another thought- I've seen bad diodes in an alternator drain the battery over time.

[TroyC]

The battery is only 4 months old and when I put the charger on it, the tractor starts good for 4-5 days before it shows signs of cranking the engine slower.

OK, without testing the battery, let's assume it is good and working properly. The problem now looks like a drain (parasitic as mentioned above) on the battery or the alternator is not charging. You really need to test voltage on the battery terminals with the engine running.

[maple flats]

I won't get back there until Monday at the earliest, if raining it will be after that. The tractor stalled in what seemed like a fuel outage, but the gauge shows over half. The tank holds 7 gal total and I topped it off about 2 hrs before it quit, I was brush hogging an overgrown field, that uses just under 1 gal/hr. For that I suspect the fuel filter, and I have a new one to put in, I always keep every filter spare ahead of needing it. This tractor has never been fueled using a fuel can, I have 2 tanks of fuel one at each of 2 locations, my blueberry fields and my sugarhouse. They are .7 miles apart. I had fueled from the tank where the tractor quit, I wonder if it might have gotten some condensation in it. I'll test using water finding paste to verify. I never had an issue on that at either tank in the past. I also have a water filter on the tank I used, (just in case), and a half micron fuel filter too.
When I get back to the tractor Monday or beyond, I'll have my multi meter and will test as suggested, after I change the fuel filter. The bowl showed no signs of water on Friday when the tractor stalled.

[rusticretreater]

Ah, I have too many days like that.  You might also want to test for a bad fuel pump.

[jcbrotz]

quick and dirty test if the back of the alternator is magnet then its charging. Simply put a screwdriver to the center of it and if it sticks then the alt is OK then time for a multi-meter or load tester.

[maple flats]

The forecast for tomorrow looks good, I'll try following the above suggestions, except I won't pull the negative cable off. 
I have a new fuel filter ready, then after I change that I'll try to start the tractor, if it starts I'll follow the suggestions, if it won't start I'll try to drive a F350 diesel 4x4 in to where the tractor stalled and jump it. If that fails my brother has a large skid steer track loader, he can pull me out. If we hadn't had so much rain lately, I'd be able to drive in without issue.
I'll post what happens tomorrow.

[maple flats]

It's not typical of me, but I took a picture of every reply in this thread so I'd recall exactly what each suggestion was when I get to the tractor.

[rusticretreater]

Good idea.  I have printed out various forum threads in the past to have handy.  Lots of ones from auto forums.