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?

 

[nostep]

Go to Messicks.com. Look up parts for your B7100. In the electrical group, you can see the panel group. In the panel group there is an item #20 which is the regulator. See if you can find that behind the dash panel.

 

[Phoenix Farmer]

Look on the back side the dash for a finned aluminum block about the size of a pack of cigarettes.

 

[KubotaSam]

Go to Messicks.com. Look up parts for your B7100. In the electrical group, you can see the panel group. In the panel group there is an item #20 which is the regulator. See if you can find that behind the dash panel.

Apperently some b7100 tractors have a regulator behind the dash and some have a recitifer behind the dash. On the Messicks site if you look at a B7100D-P part #20 is listed 2 times once as a regulator and once as a recitifier. I would guess that it depends on the age of the tractor. I know what is behind the dash on my B7100 (1977 gear drive-old style) is a recitifier that changes the alternating current produced in the dynamo into dc current for the battery and lights. The voltage regulation on this type of system is built into the light switch. It is a manual type of voltage regulation. When the lights are off it is a low charge rate and when the lights are on it is a high charge rate. Kind of similar to the old Farmall tractors. My Dads Super A has a switch that I think had 3 positions low charge, high charge and lights.
On the Kubota you can not seperatly controll low or high charge unless you bypass the light switch and add a seperate hi/lo charge switch like I did on my tractor.

I have to assume that at some point in production (perhaps the new series of the B7100) Kubota changed to a regulator mounted behind the dash and the rectifying (changing ac to dc) is either done inside the dynamo like an automotive alternator or perhaps the rectifier is included inside the regulator.