best way to test an alternator?

We are having some electrical problems with a john deere tractor, and received an error codes 61f, 018, 023, 026, 049, and 051. These error codes are not cleared when the problem goes away, and from memory, the first few meant low fluids at some point, and one of the later ones is indicating the output of the alternator is outside of normal operating range.

The problems experienced:
* The tractor would start, and "lunge" forward but never actually go any go further than 4 feet.
* reverse does absolutely nothing.
* hydraulics work fine
* Engine would start in any gear, when it should only start in neutral.
* Replaced a few dead fuses and a missing relay but these did not change anything important.
* oil / hydraulic fluid was very low, we fixed that.
* this lunging behavior was intermittent before, only requiring some wiggling of the gear shift.
* requires jumping in order to get started. Does not always crank fast enough by itself to start on its own.

Today, we plan to test the alternator to see what's going on there, It is rated to deliver 115 charging amps to a huge 12V battery. what is the best way to test this with a basic multimeter or some basic around the house parts? I measured the voltage across the battery rise from 12V up to 14V, so it would seem that it's fine, but I'm going to take that with a grain of rice. Is there a better way to measure the alternator? 

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steveastrouk5 months ago

What happens if you crank it with another battery, does the engine crank at the correct speed, or is it still slow ? I'm thinking the transmission is at fault.

-max- (author)  steveastrouk5 months ago
It starts perfectly fine when jumped. The transmission felt strong just before the fault, and are fairly confident it is an electrical problem, given that the relays that put the transmission in-gear to go forward or reverse are clicking on and off as the thing nudges.

What happens to the battery voltage as you try and make the tractor move ?

-max- (author)  steveastrouk5 months ago

I've not measured that, but the voltage rises from 12.3v to 14v when started, and remains fairly stable. There is at least one other smaller wire coming from the alternator that measures approx. 0.4v below the 12V battery while the tractor is running, and about 0.4v above ground when shut down (from memory)

Jack A Lopez5 months ago

There exist machines for testing alternators, and sometimes these machines can be found in stores that sell auto parts. As an example, I have seen theses machines at Autozone(r) stores. I don't know if you've seen that name before. Autozone(r) is well known auto parts selling chain, with probably thousands of locations all over the Former United States.

Similar auto parts stores with different name, with similar alternator testing machines, may exist, in other countries where cars are in widespread use, which might be almost everywhere, on this planet, Earth.

So I claim these alternator testing machines (ATMs) exist. I don't know if that part of the story is hard to believe or not. The really remarkable part of my story is I claim that it is possible to walk into one of these stores, carrying a broken or suspect alternator, and ask one of the friendly sales associates, to use this machine for to test your suspect alternator.

Then, if all goes well, the friendly partsmonger will take your alternator over to the ATM, strap it in, put a belt on its pulley, and attach some electrical connectors. Then he or she, will fire up the ATM. Of course the mojo the ATM does is all automated, and after several seconds of vrooming noises, its computer display will present, unto the partsmonger, and you, and anyone else looking at the display, a summary of what the testing revealed. You know, maybe the screen has a green border if the alternator is "good", or a red border plus some more specific error messages if the alternator is "bad".

Oh! I forgot to mention the most remarkable part of this story. The most amazing part is that the partsmonger dudes will typically do this test for free! I mean, if you've already gone to the trouble of pulling the alternator out of your car, or tractor, or whatever, all the associate partsmonger has to do is strap your alternator into the magic machine, close the hood, and press a button. It is not a huge amount of labor on their part. Plus there is a pretty good chance that if your alternator is bad, that they'll be able, and eager, to sell you a new one.

I know this story might sound crazy, so I went to Google(r) images to see if I could find some photo evidence of this. So you can get some context for the attached pics, the pages I stole these pics from are linked below.

Story containing picture of an Autozone(r) store's alternator testing machine:


Story containing picture of custom, homemade, alternator testing machine:


By the way, I have no idea what else is wrong with your tractor, but it kind of sounds like it has problems other that just a bad alternator.

-max- (author)  Jack A Lopez5 months ago
I don't think this typical car alternator, so they may not know how wire it. There is at least one other smaller wire coming from it, that measures approx. 0.4v below the 12V battery while the tractor is running, and about 0.4v above ground when shut down (from memory)

We got a part #, so we will research the specs of this alternator, and I will consider pulling it out if we suspect it's bad. Thanks!

The alternator in most older tractors are different than car alternators and cost about $300 and up rebuilt.

We swap out 12 volt tractor alternators for GM Delco alternators $120.

Newer tractors can have the same alternator as cars and trucks.


Now since you mentioned codes I can assume your tractor has a computer with sensors. a bad computer or bad sensors can cause all these problems.

Low voltage can make the computer send erroneous codes and do the wrong things.

To test the alternator start the tractor and test the voltage at the battery, should be 14 to 16 volts.

Then disconnect the battery, the tractor should keep running if the alternator is good. If the tractor stops when you disconnect the battery the alternator is bad.

Should only start when the clutch is pressed, sounds like the clutch switch is bypassed or the sensor needs replacing. If the switch is bypassed it will start in any gear.

Sounds like reverse gear is busted, the linkage is damaged, or the reverse sensor needs replacing.

rickharris5 months ago

Your alternator charges the battery. the tractor will run happily without it until the battery goes flat.

i would get a mechanic to have a look as it sounds like there is something seriously wrong in there somewhere.

-max- (author)  rickharris5 months ago
It costs $350 just to have omeone look at it, and they will likely say a bad transmission and try to get the most $$$ out of us. This is not an option. (Or rather, a very much last resort option)