Need to test alternator on B7100

[andrewj]

Hi folks...I have a 7100 made around 1994 and I'm trying to figure out of my alternator is charging my battery, which has been dead last few attempts. The kubota guy said it doesn't produce voltage since it is not an alternator, but a dymometer, whatever that is. It is a magnetic disc attached to the fan hub.

There are two wires coming out. All I have is a regular volt meter. How does one check it is charging the battery if it doesn't produce voltage?

any help is appreciated...

 

[KU Gator]

I don't see how anything could charge a battery if it doesn't produce more voltage than the battery would produce. You should be able to put your voltmeter on the batter, red lead on the + post and black lead on the - post. Normally a 12 volt battery will read about 12.5 to 13 volts. When you start the motor, and it should be charging, the volatage shoudl go to about 14 or so volts. I have not had reason to check mine, but that is the steps I would take.

 

[andrewj]

that's what I first thought, but eh service guy said I should see 6 amps, not voltage since it was a dynomoter. I don't have any information about that, so thought I'd ask here. Can someone with a 7100 check voltage at the battery with the engine running?

 

[nostep]

The service guy needs to go back to school. An alternator or dyno will ALWAYS develop a higher voltage than with it off. You can only push current when you create a voltage potential from one point to another. What you should see is the voltage go up after it's started. You should charge the battery good, then remove the charger. Measure the voltage. Should hold about 12.5 volts, give or take). Then start the machine (hopefully it starts good without the charger). Then just monitor the voltage. It would be a touch less than the 12.5 volts that you started with since you just used some power to get it started, but it should slowly grow over time to 13 or 13.5V. If the battery is good and the voltage doesn't climb (might take many minutes), then the dyno is probably bad. Also check for loose or dirty connections. Does it have a good ground, etc?

 

[Phoenix Farmer]

"Dyno" is just an old British name for a generator. Here is a site that explains the "dyno" - A QUICK PRIMER ON GENERATORS/DYNAMOS

 

[ibnxe]

I agree that the service person you spoke with needs to go back to school.

If the B7100 dynamo is anything like the unit on my tractor (BX23), it is actually a perment magnet alternator. There are two wires comming from the unit and the output is actually AC current. This is routed to a device up under the dash, that acts as a rectifier and shunt regulator. The regulator/rectifier is housed in a small alloy block with cooling fins. The rectified output is then passed on to the battery.

My tractor battery measured about 12.6 volts, after setting for a couple of days. When I started it and brought the revs up to about 2000 RPM, the voltage climbed to about 14.7 volts. The dynamo is able to generate about 15 amps with the engine reved up. If your battery voltage doesn't reach at least 14.5 volts, running, you most likely have a problem with either the dynamo or the rectifier/regulator unit. There are no adjustments you can make to either of these components. They either work or they don't.

 

[KU Gator]

I would suggest pulling the battery and charging it and getting it tested. I have been fooled thinking I had a charging issue and actually had a battery problem. Once started, assuming your not running lights, the engine should draw very little current. So I would think unless you have something drawing it down when it is turned off, the battery should stay up for awhile even if the generator isn't helping you much.

 

[B2400]

I believe that there is also a regulator behind the dash if I'm not mistaken.An alternator develops ac current and requires a rectifier and a regulator to provide a charge for battery and even if the dynamo puts out dc current, there still has to be a way to regulate the voltage. You will need to verify output from both if battery doesn't see a voltage jump when running.

 

[andrewj]

I really appreciate the replies, you guys really know your stuff! Thanks - I will check and report back.

 

[andrewj]

Can't seem to find that regulator! Any tips where to look?