# Testing ALternaor/Stator Output



## CatDiesel_762 (Jan 22, 2018)

Got questions for the techs on here,
I got a Craftsman's 24 Inch Two Stage Snow-Blower that has an built in Alternator. I got this unit used last spring and was wondering if I could add a LED light for some evening blowing.

The tag on the wires say "Red Circuit: Heated Grips, Yellow Circuit:Headlight." Which tell me that my Alternator model (951-10719) has both AC and DC current. I found a link to the The MTD Service Manual from another forum member for my engine and it says it should be 20watt/20watts Alternator. When using the specs under the Charging System Testing in that same MTD Manual I find they state the Alternator should put out 13-18 VAC (Currently 38 VAC) on the Yellow Harness and 17-26 VDC (Currently 42 VDC) on the Red. My voltage readings are almost double that 38AC/42DC volts on the yellow and red wires. I know from reading on the the Adding LED to your Snow-Blower thread that these readings can be inaccurate if not done with a load, but the manual says to test with the loads disconnected. My Snow-Blower did not come with a light or heated grips but has the lead coming from the motor. 

I guess my question is, Do I change the Alternator or try to fix the rectifier that is supposedly built into the stator? Or do I leave it and use a bridge rectifier and run LED rated for 40VDC on the yellow lead ? Did some one change the Stator when chasing a No-Start issue (since there seems to be a lot of people confusing the Magneto with the Alternator.)?


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## RedOctobyr (Mar 2, 2014)

What is the internal rectifier supposed to be doing? Making AC->DC? You are getting DC on one, so I guess I'm trying to figure out how that implies the rectifier has failed. 

Don't get me wrong,I don't know much about these. None of this is meant as sarcastic, I'm really just trying to follow the troubleshooting train. 

If it were me, I'd probably add a ~10W load and check each again. Maybe a small load drops them to a more reasonable voltage, and you could avoid needing a 40V-rated LED. You could use a small automotive bulb as an inexpensive load. 

At least for the AC output, I assumed these were fairly dumb devices, just a coil. If that were true, I wonder what could cause an excessively high output voltage.


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## Kiss4aFrog (Nov 3, 2013)

Your best bet would be to hook up a 12-14 volt 10-18 watt incandescent light to each lead one at a time and take a reading. With the engine at speed you should be around 12-16 volts.

From the looks of that part its AC/AC and you'd need to add a rectifier to either or both circuits to use LEDs. Hand warmers will work AC or DC.


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## CatDiesel_762 (Jan 22, 2018)

RedOctobyr said:


> What is the internal rectifier supposed to be doing? Making AC->DC? You are getting DC on one, so I guess I'm trying to figure out how that implies the rectifier has failed.
> 
> Don't get me wrong,I don't know much about these. None of this is meant as sarcastic, I'm really just trying to follow the troubleshooting train.
> 
> ...


That is what I was wondering if a load would change or lower the voltage at least on the DC side or even on the AC side. That is where I get stumped in finding correct parts that would work with that voltage. I do not want to waste money on LED's or grips only to have them burn out due to over-voltage. Meaning the Stator or its built in rectifier is defective and would need to be replaced which is a expense that should be corrected before connecting accessories. 

As for the built in rectifier from my investigation of the harness it is a couple of diodes in-line about 2-3 inches from the white plug. I say that it is only two because i can only see and feel two humps or cylinders under shrink tubing connecting the two red wires as a "Y" into one hidden inside the harness under the fiber heat shielding. Which I was expecting to see at least four diodes for a _clean_ VDC line. Unless the other two diodes are hidden farther into the harness. Although, It was my initial plan to put into place a full bridge rectifier just for preventive measure when installing the led lights. What I did not account for was the MTD/Craftsman accessory heated grips I also want being installed on the VDC side of the harness. I assumed they would be VAC as Kiss4afrog has mentioned.

Shawn


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## Kiss4aFrog (Nov 3, 2013)

If you can feel diodes then there are going DC on the headlight. If you wanted to get a little crazy and remove the diode(s) and instead use a rectifier bridge you increase the wattage you can use. A diode being a one way valve so to speak kills half the output. I would think there is only one in there as there wouldn't be any reason to use one on the hand warmer side. After all you need all the current you can to run the heaters.


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## RedOctobyr (Mar 2, 2014)

Shouldn't a bridge (full-wave) rectifier give at least close to the full power output, as DC? It would produce a positive DC "pulse" for each half of the original AC (keeping the positive half of the AC, and "converting" the negative half to positive). So you shouldn't immediately lose half your available power. 

Yes, hand warmers, like incandescent bulbs, are just heaters, and don't care whether they get AC or DC. The only thing that cares are LED bulbs, or onboard batteries (starter, etc) that are being charged. The only reason I can think of for them to be on one side vs the other is if the system is meant to put out different voltages for AC vs DC, and if the heaters will only function as-intended on one of those voltages. 

Before doing anything invasive or expensive, I'd connect a load, and check the voltages again. The voltages may become a non-issue once you're powering something. For giggles, I might even check each one for AC and for DC, just to see what it shows. 

On the DC side, if it's a half-wave rectifier now, you might be reading the average voltage (assuming you're using a multimeter, not an oscilloscope), the peaks might be something different. And the average would go up if you went to a bridge rectifier. 

Also, just to mention it. Let's say you wanted to add LED lights that can only take 8-15V input, and you're putting out 20V (even after adding a load). You could connect 2 identical sets of those LED lights in series, and each set would only get 10V.


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## CatDiesel_762 (Jan 22, 2018)

Kiss4aFrog said:


> Your best bet would be to hook up a 12-14 volt 10-18 watt incandescent light to each lead one at a time and take a reading. With the engine at speed you should be around 12-16 volts.
> 
> From the looks of that part its AC/AC and you'd need to add a rectifier to either or both circuits to use LEDs. Hand warmers will work AC or DC.


Your right that the system is producing AC/AC on both lines, but according to MTD Service Manual only one side is tested as a (internally rectified) VDC in the troubleshooting test. Which in turn is the side labeled for Heated Grips.The labeling has the headlight circuit as VAC which I find slightly baffling since incandescent bulbs were originally designed to be VDC friendlier. 

Could there be a short in one of the diodes on the red line that is allowing the voltages to be double instead of grounding out internally, which would effect voltages on both sides? Since there is no external ground on the unit.

Shawn


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## jonnied12 (Jan 14, 2017)

Incandescent bulbs do not care weather the voltage is AC or DC.
What kind of meter are you using to measure the voltage, is it a true RMS digital meter?
The tag on the wires say "Red Circuit: Heated Grips, Yellow Circuit: Headlight." 


Use the yellow circuit with a bridge rectifier for your headlight.
Typical headlights on these models are 20 watt 1141 automotive bulbs. 
Check the voltage with a 20 watt bulb connected to the circuit.


You want to use an 18 watt LED.
Wire it this way:
The yellow wire from the engine to one of the AC post on the bridge rectifier.
A wire from the other AC post on the bridge rectifier to engine ground.
A wire from DC+ on the bridge rectifier to the positive wire on the light.
A wire from the DC- on the bridge rectifier to the other wire on the light.
DO NOT GROUND THE DC SIDE OF THE BRIDGE RECTIFIER.
Don't forget your capacitor(s) between the bridge rectifier and light.


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## drmerdp (Feb 9, 2014)

Your voltages with no load will be abnormally high. If you want to use an 18w led pod light, then grab a roughly 18w incandescent bulb to load the circuit and take your measurement. If within spec of the lamp, wire in your bridge rectifier, capacitors, and enjoy. 

Remember that your voltages will drop at idle, and that measurement needs to be in spec as well. If your lamp is rated for 18w @ 10-32v and you have 8 volts at idle then you will likely burn up the fixture.


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## CatDiesel_762 (Jan 22, 2018)

This after noon, I went to Home Depot and pick up some cheap 12 volt Incandescence 18 watt bulbs to try and see if I could see a voltage drop. Needless to say, they did last more than 5 seconds before the element blew out on the DC circuit. I might have to try and find some of my old 1156 bulbs that i have for back up my 86 Chevy k1500. Then I will just need a old bulb socket to keep the wires from shorting out since my alligators clips were to much work trying to keep them apart.. I also picked up a 120 volt LED to try just for kicks and giggles. The small LED bulb ran great on the DC circuit (Red) and voltage barely drop from 42 volts to 41.5. When I put it over on the AC circuit it was way dim (small candle dim0 and the voltage was still maxed out at 36 volts..

To answer the question about type of multi-meter I have a Craftsman Digital Multi-meter model number 82140.. 

Shawn


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## CatDiesel_762 (Jan 22, 2018)

drmerdp said:


> Your voltages with no load will be abnormally high. If you want to use an 18w led pod light, then grab a roughly 18w incandescent bulb to load the circuit and take your measurement. If within spec of the lamp, wire in your bridge rectifier, capacitors, and enjoy.
> 
> Remember that your voltages will drop at idle, and that measurement needs to be in spec as well. If your lamp is rated for 18w @ 10-32v and you have 8 volts at idle then you will likely burn up the fixture.


I would expect the voltages to be slightly high depending on Engine RPM and lower at idle just like you said. The irony is, I am not seeing my voltages low or as rated by the engine manufacture (MTD). The voltage instead are dead on at 42 VDC and 38 VAC which is double the MTD Service Manual Spec. This is making me wonder if the Alternator has been changed and is a non stock part or maybe the diodes are bad. Although, this forum and others are showing that these Alternators tend to have dirty voltage unless externally regulated. 

Shawn


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## CatDiesel_762 (Jan 22, 2018)

jonnied12 said:


> Incandescent bulbs do not care weather the voltage is AC or DC.
> What kind of meter are you using to measure the voltage, is it a true RMS digital meter?
> The tag on the wires say "Red Circuit: Heated Grips, Yellow Circuit: Headlight."
> 
> ...


Using a rectifier is not going to be a problem. Getting the Voltages down to use an off the shelf LED bar is. My research is saying the voltage thru the rectifier will remain the same as the potential voltage before the rectifier. Such as 43 VAC into the rectifier and 43VDC out of the rectifier. The 43 volts in is the problem cause I do not want to waste money on replacing a perfectly good LED.before it was supposed to fail. I am also looking at adding heated grips to the mix. So making sure I don't cook the wires in the Alternator or heated grips would be another plus.

Shawn


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## RedOctobyr (Mar 2, 2014)

If you can't get the voltage down, you can run your items in series. If you have more identical 12V bulbs, run 3 in series. That will turn your 38V AC into effectively 12.7V AC across each bulb.

Note- this assumes all the bulbs/loads are the same wattage (resistance). If you connect a pair of 10W bulbs, and also a 5W, in series, then the voltage drop won't be the same across each, and something may get >12V.


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## drmerdp (Feb 9, 2014)

CatDiesel_762 said:


> I would expect the voltages to be slightly high depending on Engine RPM and lower at idle just like you said. The irony is, I am not seeing my voltages low or as rated by the engine manufacture (MTD). The voltage instead are dead on at 42 VDC and 38 VAC which is double the MTD Service Manual Spec. This is making me wonder if the Alternator has been changed and is a non stock part or maybe the diodes are bad. Although, this forum and others are showing that these Alternators tend to have dirty voltage unless externally regulated.
> 
> Shawn


Those numbers mean nothing unless there is a load on the circuit.

Add a load then re measure.


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