kubota kh new alternator overcharging

[John V]

Hello, I could use some help figuring out what's going on with a new alternator I put in my kubota kh70 excavator. The old alternator wasn't putting out any juice. I took it apart and the rotor showed open circuit between the slip rings. So I bought a new cheap one off amazon. Installed it, a new voltage regulator (external denso 6 pin), and a new fan belt for good measure (turns out the one on there was too big according to the manual). Cranked up and saw 14+ volts and was happy. Went to check it the next day just to make sure things were good, and I saw ~15.5V at the battery terminals with the motor running. Motor off, battery reads about 12.5V no load.

Started doing diagnosis according to this old d1402 engine manual I have, but one of the tests was a no battery no load test, which I've read elsewhere is bad for alternators. Manual says that should see ~14V... I did it briefly and saw about 36V.

Not sure where to go. Could be a bad alternator... it was a cheap one off amazon after all. But it could be just fine and I should be looking elsewhere. Anyone have good source for bench testing an externally regulated alternator? It has a B+ terminal and a 3 prong FEN terminal.

 

[Delmer]

nothing wrong with the alternator! You bought a new external regulator, do you still have the old one? swap it back and see what happens. I'm not familiar with denso external voltage regulators, but how wrong can I be?

 

[007]

Delmer is probably 100% correct.
The alternator field circuit is supplied externally and has been most likely crossed in the plug of the new regulator or a faulty regulator.
Running the alternator open circuit is a strange test with little to be learned but can be done for a few moments without damage.
In your case what would be worth doing is pulling the plug with the small wires off the alternator and confirm the alternator stops charging.
An alternator can self excite and may have something screwed up happening inside but would be surprised if it was.
Your greatest danger being over voltage to your machine with it over charging.
Your old alternator can most likely be fixed also if you find an old school auto-electrician.
We used to press the rotor apart to get to the wire coil in the old days before it became cost prohibitive.

 

[John V]

Is there a way to bench test the voltage regulator? I have a DC power supply and have tried running the DC+ lead to the VR's IGN terminal and DC- to ground, then playing with the voltage to see the contacts move, but have yet to see any action. I've put a 12V test light in parallel with the VR to give a small load... nada. The dash charge light relay responds to voltage on the N terminal, but I haven't figured out how to electrically test the VR. Manually pressing on the switch with my thumb confirms approximately 11-12ohm between IGN and field terminal, which is spec.

I'll try your suggestion of unplugging the alternator wires and checking if it stops charging.

 

[John V]

Confirmed unplugged the field wire stops charging. Shorting F to B makes it jump to ~16V. So something must be weird with the wiring such that the voltage regulator isn't seeing that 16V on IGN. Or it is, and isn't adjusting the field voltage appropriately.

 

[John V]

Turns out the B wire in the VR connector was buggered. My understanding of this 6 wire VR was that the B, N, and L terminals didn't affect the alternator, they were just for the dash charging light, but like many other things, I was wrong. Original VR works great.

 

[007]

Hi John, I did not know you had the old style regulator, I just assumed it would be electronic type.
The coil style is hard to adjust not being attached to the alternator.
For various reasons they will often have series and parallel coils wound on the bobbin you are placing your thumb on.
So you have to have the same field current flowing to get it operating correctly.
The load has to be in series not parallel by the way.
Just to add to the complexity they are position sensitive and has to be in the same orientation as mounted on the machine when operating.
When your adjusting you would normally run them for at least ten minutes as they often have a resister mounted underneath which heats up.
As the battery becomes fully charged the field current reduces and the resister changes its resistance and drops the charging voltage slightly.
Sounds like more trouble than there worth but when they are working they go for years with very few issues.
There is a real art to adjusting them also which i can walk you through but sounds like your up and away anyway.
Cheers

 

[John V]

Fortunately no adjustment necessary! On my bench test I tried tying IGN and B together and I saw some small sparks/action on the field coil in the VR, which made me think maybe the B pin to the VR wasn't working right. Low and behold it was corroded. Tried returning the new VR to amazon but they said keep the VR *and* here's your refund. It's my lucky day.