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Static test of Denso 12 V alternator

  1. May 20, 2017 #1
    I know a real test of this unit would be to mount it and run, checking DC and AC voltage, current etc. The plan is to mount it on my Super Dexta, but this would need some work as there's currently a dynamo which is starting to fail. I was given the alternator free and would like to bench test it before I bother to do the conversion. Denso A127 alternator.

    It was removed from a JCB wheel digger as not charging. So far, I have done the following tests:

    Regulator: Hooked up to bench power supply, then connected 12V bulb across brushes. Checked bulb glows as voltage rises to 12V, then goes out at 14.3V. I say the reg is OK. Plenty of brush left.

    Rectifier: Desoldered and checked the six diodes with multimeter diode test, forward and reverse. There is also a component labelled 'DIDE' which has four connections - one to each phase and one to the +ve supply to the regulator - I assume this is a diode pack for 'bootstrapping' the regulator. There is a 0.5V drop between each of the phase connections and the output to the reg.

    Rotor: Cleaned slip rings - in good nick - and checked resistance between - 3 ohms. No short to body. OK?

    Stator: Desoldered three (double) phase wires from rectifier, then separated the three pairs of wires. Checked each of the six wires had continuity to only one other. No short to case. No visible damage.

    The only thing I found 'wrong' was the slip rings being dirty - the digger had sat for a while. As far as I can see, the alternator checks out OK, pending a dynamic test on the tractor. Am I right?
  2. jcsd
  3. May 20, 2017 #2


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    Sounds like you've got it mostly covered. Other than misuse, the most common failure is the brushes. Either they are worn to the extent they don't maintain good contact with the slip rings or there is dirt in the brush holders, holding the brushes away from the slip rings. Another very remote possibility is the slip rings may be out-of-round or not concentric with the shaft. This would have the brushes floating and/or bouncing at higher speeds. In high vibration environments, winding shorts occur; but you've mostly covered that. There could be a rotor short that occurs only while spinning

    If there is a motor or generator repair shop in your area, they might be willing to do a functional test.

    Or you can do your own test; although it may be more work than the actual installation. Find a way to connect a motor to it and see if it has enough output to light up a couple headlights (or charge a mostly run down battery) and maintain 14.3V to 14.4V. Be sure to mount both it and the motor fairly securely if you try that load test, there will be a fair amount of reaction torque.

    A somewhat incomplete test would be drive it with a motor and just measure the output voltage without a load. The only advantage is a less secure mounting can be used, the disadvantage is you won't know if it will charge a battery.

    Let us know what you find.
  4. May 20, 2017 #3
    Thanks. I connected it to a battery in the proper way, with a casing earth, warning light and B+ connection, then spun it with a drill. I put my meter in series with the -ve cable, on amps. Stationary, the current drawn was about + 350 mA for the warning light. When spinning, the warning light dimmed and the current reversed to - 1.125 A. I take it this means it's putting out some charge.

    Of course, taking it to a shop or even just mounting it on the tractor would reveal if it's good or not, but this is more of an interesting exercise than a practical solution.

    I can't get my head around the stator windings - there are three phase connections, but each one is made of two wires soldered at the ends where they meet the rectifier. Does this mean it's a delta-configured stator?

    My meter also offers conductance in nanosiemens - would this be a good way to test for shorts to casing?
  5. May 20, 2017 #4

    jim hardy

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    Possibly, but the ones i've disassembled were WYE. If there's a fourth junction of at least three and probably six wires it's WYE. I'd guess it is Y wound with double strands of wire for high current capacity.

    Your tests sound thorough and proper. Nicely done.

    Good luck ! old jim.
  6. May 20, 2017 #5


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    I'm with @jim hardy on the WYE connection being on all I've played with.
    If you can supply a make and model number there is a chance to find out for sure. "Denso" same as "Nippon Denso" of Japan?

    Yes. That might detect a high resistance short (electrical leakage) that gets worse during operation, especially on the rotor. However, to cause trouble, any short would be less than, oh, maybe, 100 Ohms (10 millisiemens).

    "nanosiemens". Wow! What kind of meter is it? (make and model)

    Did you check reverse polarity also? If a diode is shorted it may show as a low value resistor and give a 0.5V drop depending on current.
    Although not likely, it could also have 6 diodes in it. Any indication of a fifth connection, such as a metal case?
  7. May 21, 2017 #6

    jim hardy

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    Typical automotive alternators i'm used to

    The diode trio powers the regulator and field .
    Old Bosch and GM and Fords in my day(70's) didnt have that wire from Ignition Switch, or that S(ense) wire.
    Bootstrap was through the "Alt Lamp on Dash"which had a low ohm (maybe ten ohms) resistor soldered across its rear terminals. Without that resistor that tiny lamp won't pass enough current to get the alternator voltage up high enough to overcome the diodes untill you wind the engine way up to about 4500 RPM.
    So lots of alternators i'm sure have got replaced because of that ten-ish ohm resistor behind dash ALT light burning open. I hope you got one of those , it'll be okay. You might look at a new brush set, for when they wear down to about 1/3 of their original length you can't get full output anymore.

    Some 60's Fords had a wire from Neutral connected to a relay that disconnects alternator from Battery+ when there's no voltage Neutral to Frame, i guess to prevent wrecking the diodes should somebody hook up jumper cables backward.

    Nowadays some have their field controlled by the ECU, a return to the simple alternator with external regulation. Others have an internal microcomputer that communicates with Mr Engine so as to not surprise him with a heavy load at an inopportune time.

    old jim
  8. May 21, 2017 #7
    There is no fourth junction, and each of the doubled wires, when separated, is continuous with a wire on a different pair. So when I label the wires 1-6, with 1&2 a pair, 3&4, 5&6:

    1 connects to 4, 2 to 6 etc.

    From what I've found about windings, this says 'delta'. There being no junction and no kind of common connection to the case seems to support this. However, I realise I'm examining a faulty unit, so it's hard to be sure.
  9. May 21, 2017 #8
    The original label says Denso A127. But there's some incest going on as that label was covered by a Bosch remanufacture label, and the upper case says Magnetti Marelli. To add to the confusion, I watched a Westronicsirl Youtube vid showing a rebuild of a Lucas A127 alternator and that was identical to my unit.

    My meter is a Fluke 87V - an eBay bargain (shop clearance and the owner didn't know what he had - boxed and nearly new for £100). I'd had enough of cheap Draper meters dying on me so I decided to get a 'meter for life'. Also much safer for working on the 240V mains we have here, and I like to repair 12V inverters for a hobby, so the true RMS, low-pass filter and capacitor check are handy.

    Yes, I checked reverse too. It appears the same as the diode trio on the schematic (thanks for that, by the way Jim). I can't see a fifth connection - the component is a flat case with only four flat conductors emerging from it. No other identifying marks other than DIDE.
    Last edited: May 21, 2017
  10. May 21, 2017 #9
    Well, the digger is from the 90's, but I never thought about looking at the warning lamp on the dash. If there is a resistor there, and it's gone, the new unit won't work either. I'll keep you posted on that!

    The brushes and slip ring are suspiciously unworn, and that combined with the Bosch reman sticker suggests they've been replaced.

    I did once reconditon the alternator on my wife's Land Rover which has a BMW engine. It had three connections, B+, L (light?) and DFM. The latter is, apparently, Digital Field Monitor and connects to the ECU, but I'm not sure if it was for monitoring or control of the field voltage.

    Had a discussion the other day about alternators and heavy loads - many say you should never jump start a modern car, but I think the problem happens when you disconnect the jump leads with both cars running - the 'good' car, having been under heavy load charging the 'bad' car's battery, suddenly has much less load and its alternator/ECU cannot respond quickly enough and produces a voltage spike.

    My solution is to shut off the good car before disconnecting the leads.
  11. May 21, 2017 #10
    Further to this, I've just bought an old Megger BM10 insulation tester. When it arrives I'll disassemble the alt again and check the stator and rotor windings.

    I often get given DC brush motors that are burning and can't do more than check the brushes, armature and field in the normal way. Hopefully this fun little gadget will give me a more positive go/no go result than an ohmmeter.
  12. May 21, 2017 #11

    jim hardy

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    I wouldn't stress over the wye-or-delta question. If there's no discoloration of windings from heat, just clean it up and give it a try.

    You know to not megger with regulator connected.......
  13. May 21, 2017 #12
    I've never meggered anything in my life! I assume it's best used on isolated windings in motors, generators, transformers, etc. How do you know what voltage to go up to?
  14. May 21, 2017 #13

    jim hardy

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    Our rule of thumb for industrial machinery was twice rated plus a thousand volts.
    On your alternator i wouldn't go above 500 volts, and that much only on a dry day. i think 200 would be plenty adequate if your tester has such a low setting.
    Again be careful, meggers are for testing insulation and they'll fry electronics.
  15. May 21, 2017 #14

    It does 50-1000V
  16. May 21, 2017 #15

    jim hardy

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    Since alternators are capable of around 200 volts on a load dump as you described above,
    i'd use that 250 volt setting. Might risk 500 if it's clean and dry.

    If you have it back together though, why not just spin it up ? My local O'Reilly's auto parts store has a test stand for customers' carry-in alternators.
  17. May 21, 2017 #16
    It's apart again - I'm a terrible fiddler.

    I've nothing to spin it up on, unless I mount it in the tractor, but that would involve making up new mounts and converting it from positive to negative earth. Pending the arrival of my megger gadget, I'll get it done and let you know. Many thanks for your help.
  18. May 21, 2017 #17

    jim hardy

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    I know the behavior !

    If it's like my 8N Ford that's just reversing the wires on battery and ammeter. And new coil & light bulbs if you're going from 6 volts to 12 .
  19. May 22, 2017 #18
    It's a Fordson Super Dexta - 12V diesel. Won't the starter motor need reversing internally?
  20. May 22, 2017 #19

    jim hardy

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    Only if it's a permanent magnet field which is unlikely.
    Reversing polarity on a wound field machine reverses both field AND armature , product of two negatives is positive.
    Starters are almost always series wound field machines.
  21. May 22, 2017 #20


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    I would never change out the coil in a case like this. Use a ballast resistor to lower the voltage so as to not damage the coil. Then have a bypass to eliminate the resistor when starting for a bit of an extra hot spark. This will definitely improve starting performance.
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