# Deere 870 Battery not charging



## rovo (Feb 10, 2018)

The battery is good and holds a charge well but it isn't being charged when the 20 year old tractor is running. Alternator belt is tight and alternator is spinning. Dashboard light indicates a "no charge" condition. Verified with a voltmeter that the battery is not being charged.

I figure there are several possibilities for the source of the problem:
1. Faulty alternator (part # MIA10338)
2. Faulty remote voltage regulator (part # MIA881279)
3. Wiring between elements.

I suspect the alternator puts out AC volts and the rectification takes place in the regulator. The alternator has two (2) blue output wires and I think is grounded via the mechanical connection. I measured the output of the alternator and blue wire #1 has about 15VAC . Blue wire #2 has 0.35VAC. I can't find a schematic of the alternator internals.

The regulator terminals are identified as: BAT+, space, ACin, ACin, LAMPout, IGN SW. Since there are two ACin terminals on the regulator, I suspect that both blue wires should read about the same AC volts. 

My next step is to disconnect the alternator from the wiring harness and measure the AC volts again to see if the external circuit is loading down the one output. I am currently in a wait mode due to rain.

Does anyone know about this particular alternator with respect to expected output voltage from the terminals?


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## sixbales (May 18, 2011)

Howdy rovo, welcome to the tractor forum.

I have a small Kubota G1900 18HP diesel engine tractor that has a dynamo and regulator similar to yours. In fact, the dynamo has two blue output wires, same as yours. I measured the AC voltage across the blue wires at 24VAC. Both ways. DC voltage at the battery 14VDC (engine running mid throttle). 

The largest problem that I have had with my Kubota charging circuit is burned out contacts inside connectors. When I find burned out contacts within connectors, I cut the affected wires and install jumper wires around the connector. I recommend that you check your fuses and connectors before condemning the dynamo or regulator.


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## rovo (Feb 10, 2018)

sixbales, Thank you for the "welcome" and for taking the effort to measure the voltages. I now know what voltages to look for when things are fixed.

I've had very few problems with my Deere 870 which I bought new. I am on the second alternator as the first one froze up mechanically due to bearing failure. It would be easy to just replace the alternator and the regulator but for sure a waste of money (which is always in short supply). 

I haven't been able to get back on this project because it is still raining. I should have left the tractor under its carport cover and worked on it there instead of moving it near the tools.

If it is OK with the forum members, I will continue to post my findings as it may help someone else in the future.

While waiting for breakfast at the diner this morning I sketched up what I think the charging circuit would look like if I was designing it. I'm a retired Electrical Engineer and specialized in computerized machine control. After sketching the circuit I then looked for what element would need to fail to give me the voltage readings I am measuring. The problem is most likely the regulator, a failed diode.


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## sixbales (May 18, 2011)

Keep us posted rovo. It's always good for everyone to learn what the problem was.


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## rovo (Feb 10, 2018)

I haven't forgotten this post but I have not been able to accomplish very much. I did find a couple of things yesterday. I disconnected the alternator plug (engine off) and measured the resistances. There appears to be a single coil inside the dynamo and both ends are isolated from ground. The coil resistance was too low for my meter to measure accurately as it appears to be less than 0.1 ohm. My gut feeling is the alternator is fine based upon the measurements. I also took the opportunity to squirt off the engine compartment and engine with water as it was a little messy. Since I have to get to the regulator I wanted to clean everything a bit. Afterward the dash battery light was no longer illuminated but a measurement showed the battery was still not charging. Battery measured 12.4 VDC and should read 14.4 VDC if it was charging. There might be some connector problems since the water seemed to change things.


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## rovo (Feb 10, 2018)

One other item. DB Electrical has some good pricing on a new regulator, $20.23. Does anyone have any experience with this firm. Also, their alternators seem low priced compared to other sites.


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## sixbales (May 18, 2011)

Hi rovo,

Check ebay and you will find a new (aftermarket) regulator at similar prices plus some priced higher. Probably the same regulator. The best price that I have found is $20.22 with free shipping. I purchased an alternator/regulator combo for my Kubota and it has been operating for many years. 

I have no experience with DB Electrical. I like to buy on ebay because if as problem arises with your purchase, they will assist with resolution.


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## rovo (Feb 10, 2018)

I had some time today and the weather was okay so I brought the Deere 870 near the garage and started to go a little deeper. After start-up the "Battery" light was on again. First thing I did was to hook-up a battery charger to make sure the battery had a full charge. These tractors use very little current when running and I think I could run it for several hours with a full battery. I decided to remove the regulator and take some measurements on the bench. All I could determine was that the control circuit works on the negative side of the diode bridge. I always like to work on the high side but a lot of OEMs use the negative side since it can save them a few pennies. 

I expected two diodes from each AC(in) terminal. I confirmed a good diode from each AC(in) to the "Battery" terminal. The negative side gave me crazy readings and this is why I think the control circuit is on the low side. 

While the regulator was disconnected I checked the other terminals on the harness side of the connector. The Ignition switch supplies +12VDC to the regulator when in the on position. The "Lamp" terminal goes to the battery lamp, the other side of the lamp is grounded. These were as expected but I wanted to confirm the connections.

Since I couldn't determine whether or not the regulator was good, I cleaned all the terminals and the housing and re-installed the unit. Upon start-up the "Battery" light was not on so it may have been some dirty terminals (connections). I still think there is a problem but as of now everything looks good. Time will tell.


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## rovo (Feb 10, 2018)

Was it really in February that I started this thread?
The tractor still exhibits the same problem but I haven't looked at it very much. I changed out the alternator and regulator but this didn't help the problem.
I used a clamp-on AC current meter to check the alternator and when the dash light is out the alternator showed an output of 15 amps. When the light came on the amp meter showed 0 amps.
I now suspect a problem in the wiring harness, probably one of the blue wires from the alternator is broken. I've made a bunch of jumper wires to allow me to selectively bypass the wires from the regulator but I have not installed the jumpers as of now. Maybe in the next day or two if the weather holds.


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## jacobptierney (2 mo ago)

I'm having the same problem almost word for word on my 870. Have you gotten it figured out yet?


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## pogobill (Jan 31, 2012)

jacobptierney said:


> I'm having the same problem almost word for word on my 870. Have you gotten it figured out yet?


He may have sorted it, this is an old thread.....


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