Bad Dynamo?

CBH

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Jun 24, 2014
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With my volt meter set on AC Volts, and touching the 2 wires from the dynamo, I am getting a readout of 16.6 at wide open throttle, and 11.2 at idle. Am I correct in thinking that I need a new dynamo?
 

BruceP

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G5200H
Aug 7, 2016
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With my volt meter set on AC Volts, and touching the 2 wires from the dynamo, I am getting a readout of 16.6 at wide open throttle, and 11.2 at idle. Am I correct in thinking that I need a new dynamo?
What part of these numbers suggest to you that your dynamo is failing? I assume you know that most electrical systems do not have the ability to charge the battery at idle speed.

Are you measuring the AC volts OPEN CIRCUIT? (or connected to the tractor) If this is open-circuit readings, they may be low.

I am assuming you are using a meter which measures RMS (RootMeanSquare) AC volts (not PTP - PeakToPeak) When considering the conversion to DC volts to be used in your electrical system... those voltages you are reading should be satisfactory.

Try measuring the DC voltage at the battery with engine at operating RPM. (14.8v is considered a good charging voltage)
 
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100 td

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Are you having charging issues?
How old is your battery?
Ideally if you get the opportunity, measure DC volts at your battery, WOT, after 1/2 hour of tractor use so the battery has had the opportunity to have accepted a charge.

Your AC may be OK, depending on the regulator and battery status.
If your meter has a 10 amp AC scale, you can swap your meter leads to the correct current measuring position and break a single wire from the dynamo and measure the current in the circuit.
 

85Hokie

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With my volt meter set on AC Volts, and touching the 2 wires from the dynamo, I am getting a readout of 16.6 at wide open throttle, and 11.2 at idle. Am I correct in thinking that I need a new dynamo?
16.6 AC is low - what tractor is this on? Not that it really matters, but typical AC output BEFORE the rectifier is higher than that.

I have 2 B7100's - both with dynos, ONE I had to ....well, whats the word, rebuild from scratch.......I placed it on the bench and place a cordless drill to it...got something above 30 volts....AC!

At idle - I get a very little charge DC from the dyno......like maybe 12.6 or so.....rev it up, and she'll climb high, 14.5 easy!!!

Have you checked belt tension and all the little things first?

As mentioned, the correct test is to measure battery voltage, off, idle and WOT.....off should be something between 12.2-12.6 and idle might not make a dent, but WOT it should be close or over 14 v/dc. 14.8 will cook a battery over time if it stays that high for long.

here is a post from another forum.....
notice post #4

http://www.tractorbynet.com/forums/kubota-owning-operating/240618-b7100-dyno-troubleshooting.html
 

CBH

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Well, I'm getting those ACV readings at wide open throttle and at idle. I was told it should be 20 to 24. The DC volts at the battery (while running 3/4 throttle) is about 13.2 at start up and then climbs slowly to 15 or more.
When I'm mowing...the battery light will flicker, and eventually the engine will die....unless I disengage the mower deck, which has an electric PTO.
Battery is a couple months old. This D640 engine is on an older Cub Cadet
 
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100 td

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Forget the AC if you are getting good DC readings, it is charging, your regulator may be faulty, your battery may be faulty, if it dies "electrically", it's mostly not the dynamo, but a battery related problem because the battery can not sustain the load, as it should be able to even if the dynamo isn't working.
 

100 td

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What engine/model are you referencing the question to? As one would suspect it's a diesel, the electric PTO should not pull the system down unless you have another issue. I would suspect a fuel/load related issue, or a loose electrical connection or bad battery.
Edit:I see your edited post with model.
Remove battery terminals, clean and replace, also ground connection. Check fan belt. Do a temporary bypass on seat switch.
 
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CBH

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Here is what I have today.
Belt is tight, grounds are cleaned and reconnected. Battery cables removed, cleaned and connected
1) Battery volts before cold start up.... 13.2 V
2) I started it and let it run wide open for 5 minutes....Volts at battery were then 14.83
3) After 15 minutes of wide open running, the AMP (Battery) light came on. Volts at battery reading 15.73
4) I engaged the PTO, the volt reading at the battery is reading 16.03, the AMP light stays on
5) I turn the headlights on, along with the PTO, the battery volt reading is 13.50, the AMP light still stays on
6) After running for nearly 20 minutes at wide open, the AMP light stays on. I then turned the engine off and read the battery volts at 13.17
Now I'm thinking voltage regulator. What's your diagnosis?
 

85Hokie

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Here is what I have today.
Belt is tight, grounds are cleaned and reconnected. Battery cables removed, cleaned and connected
1) Battery volts before cold start up.... 13.2 V
2) I started it and let it run wide open for 5 minutes....Volts at battery were then 14.83
3) After 15 minutes of wide open running, the AMP (Battery) light came on. Volts at battery reading 15.73
4) I engaged the PTO, the volt reading at the battery is reading 16.03, the AMP light stays on
5) I turn the headlights on, along with the PTO, the battery volt reading is 13.50, the AMP light still stays on
6) After running for nearly 20 minutes at wide open, the AMP light stays on. I then turned the engine off and read the battery volts at 13.17
Now I'm thinking voltage regulator. What's your diagnosis?

This is text book of a bad ground........the rectifier is trying to charge the battery .......... 15.73 is huge!!!!

I had this exact same problem 30 years ago on a ford !!!

THEY (ford dealer) replaced the VR and problem still was there!

THe reason the output is so high is that the circuit is almost open, there is a poor ground somewhere, bet you an ice cold beer!;):)
 

lugbolt

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ZG127S-54
Oct 15, 2015
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Been there with a slew of G1800, G1900 mowers.

Usually bad connection or ground as mentioned.

The voltage regulator has a sensing circuit that tells it when to regulate the voltage. It also has a ground. So it needs good connections or it will either over charge, not charge at all, or I have seen them overheat-and grass clippings around it, cause fire, due to bad connection.

My D722 (G1900) and D662 (G1800) are both around 50VAC at the dynamo, open circuit, at full throttle. OR shoudl I say, the 1900 WAS around 50v. It is now an internally regulated alternator from a BX tractor-which bolts right on with some wiring changes.
 
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Soonerdad

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G1800
Jun 12, 2022
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Sorry for tgging on to someone elses thread but in reading this I ask ; would a bad ground cause their to be ZERO power to the key switch?
 

BruceP

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G5200H
Aug 7, 2016
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Richmond, Vermont, USA
Sorry for tgging on to someone elses thread but in reading this I ask ; would a bad ground cause their to be ZERO power to the key switch?
Please start a separate discussion regarding this TOTALLY DIFFERENT problem from what this discussion is about. (I know the answer to your question but this needs to happen in new discussion.)
 
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johndeerebones

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B7200 HST-D RC-60-72, L4200 GST MFWD with FEL
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Think of the AC wave. It is a solid up and down wave. The voltage regulator takes the up and down (basically) so if 14.5 VDC is desired needs at least 29VAC.
 

lugbolt

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ZG127S-54
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Think of the AC wave. It is a solid up and down wave. The voltage regulator takes the up and down (basically) so if 14.5 VDC is desired needs at least 29VAC.
the idea is correct but it's not the regulator that does it, it's the rectifier. It "rectifies" the dynamo's ac voltage to dc voltage that charges the battery. The rectifier is built into the regulator "box", technically called a regulator rectifer, but we shorten it to "regulator" because it's easier to say it.

also depending on which type rectifier is used, the ac voltage may not be cut completely in half, but if we think that way, it will get us in the right ballpark. Most Kubota's with external reg/rec use bridge rectifiers which mostly cut ac volt from the generator in half, but the frequency of current may be higher than with a simple half-wave rectifier.

I wish I knew more about electronics and how their hardware does what it does. It's getting too late for me to get studied up on it though :(
 

johndeerebones

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B7200 HST-D RC-60-72, L4200 GST MFWD with FEL
Aug 17, 2020
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the idea is correct but it's not the regulator that does it, it's the rectifier. It "rectifies" the dynamo's ac voltage to dc voltage that charges the battery. The rectifier is built into the regulator "box", technically called a regulator rectifer, but we shorten it to "regulator" because it's easier to say it.

also depending on which type rectifier is used, the ac voltage may not be cut completely in half, but if we think that way, it will get us in the right ballpark. Most Kubota's with external reg/rec use bridge rectifiers which mostly cut ac volt from the generator in half, but the frequency of current may be higher than with a simple half-wave rectifier.

I wish I knew more about electronics and how their hardware does what it does. It's getting too late for me to get studied up on it though :(
Ya, but most parts guys only know the regulator part. Wrenched on mowers and cars for years. Mowers and older cars typically much the same setup.
 

GreensvilleJay

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BX23-S
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re:

if 14.5 VDC is desired needs at least 29VAC.

this is assuming a single diode and some major 'losses'.

you only need 10 VAC as the input to a full wave bridge to get 14 VDC
you need somewhat less than 20 VAC into a single diode to get 14.5 VDC

You can check out he original B&S patent dating to 1973 for the basis of all 'rectifier/regulators made since then and Byte Magazine for Steve Ciarcia's great article on how to build power supplies.