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Testing a rectifier.

Discussion in 'The Garage' started by Hutzpah, Sep 6, 2010.

  1. Hutzpah

    Hutzpah 1979 xs650

    Firstly hello to one and all.
    I'm working through my charging circuit because I'm only getting 1.5volts to
    my battery at 2500rpm and after adjusting the voltage regulator right in managed to get to 7volts max I have a 1979 xs650 Special.
    I've ordered two new bushes as they are only just above spec.

    I'm after some advice in testing the rectifier with black probe on the black wire and moving the red probe between the white wire no reading.

    Now with the red probe on the black wire and moving the black probe between the white wires I get a very fast reading more like a flash really some times quite low say 40 ohm then next around 140 ohm.
    Does this mean its not working dead?
    By the way all contacts are clean and the unit is out of the bike.
    Any advice much appreciated

  2. jayel

    jayel #9 Guru 74 TX650A

    1. First charge your battery to at least 12.5-12.7V.

    2. Find a really thin feeler gauge like 0.010" or 0.015" and hold it pointed down about a half inch away from your generator cover on the engine. Now turn the key on. If your regulator and rotor are working the feeler gauge should slap the case when the magnetic field is created in the rotor. If nothing happens or the magnetic effect seems really weak then go on to the next test.

    3. On the solid state regulator models all you need to do is locate the green wire at the regulator plug and make a jump from it to ground. That bypasses the regulator and allows full battery current to flow through the brushes out of the rotor through the green wire to ground. That causes the rotor to make a stronger magnetic field which in turn causes more current to flow in the stator. If your battery terminal charge voltage jumps up to 14.5VDC when you rev the engine then the regulator or the ground connection for the regulator is your problem.

    4. If nothing changes then it's time to check the voltage on the brown wire (It may be black on your bike) at the positive brush with the key on. It should be very close to battery voltage. The brown wire that feeds the brush gets its power from the brown wire at the key switch. Check the voltage at the three wire key switch connector while it's plugged in by probing from the backside of the connector with the key on. Again you should see the same voltage as the battery. If you do get full battery voltage there then repair the brown wire circuit between the switch and the positive brush. If not then the switch is either bad or the red wire from the battery is not passing the full current like it should. If that's the case then keep going back along the red wire, through the main fuse until you find the source of the voltage drop. No more than 0.3VDC drop is acceptable.

    5. Once you have full voltage to the positive brush re-check the charging voltage to see if you're getting 14.5 VDC or better at the battery when revved to about 3,000 rpm. If you still don't have a charge then do the feeler gauge test again. If it slaps the case your rotor and regulator are working and you can go on to stator checks. If not then pull the brushes out of their holder and use an ohm meter to test the rotor. Measure the rotor first by touching the tester leads to the brass slip rings. Then take one lead and touch anywhere on the engine that's not painted. For the first test you should see between 5 and 5.5 ohms between the slip rings. On the second test between one slip ring and the engine you should see infinity on the meter. Any reading lower than 5 ohms on the first test or less than infinity on the second test means you have a bad rotor. Replace it. If it tests good then go on to the stator checks.

    6. At the stator wire connector locate the three white wires. Use a voltmeter set on the AC scale to test the three possible connections between the white wires by probing from the backside of the connector. (The connector should be plugged together for this test) With the engine running at idle you should see about 10.5 to 11 AC volts (Not DC) on each of the three combinations of white to white that you make. If you get a very low reading on one or two legs then something is grounding your stator. If you have high readings on any of the legs (i.e. 16-18VDC) then your rectifier is bad.

    7. If you got low readings on any of the stator voltage checks then unplug the connector and use your ohm meter to check the stator windings. Check the resistance between the three fabric covered wires (stator side) on the side of the connector. On each white to white connection you should read about 0.4 to 0.5 Ohms. If you get a very low reading on all of the three combinations find the single Yellow wire connector and disconnect it. Re-check your stator resistance. If the readings are now good then the yellow wire or safety relay are shorted. If there is one or more that still read low after disconnecting the yellow then check those legs by touching one lead to ground with the other on the white wire. You should see a very high Kilo ohm or infinite reading. If you get a low resistance check the stator lead pigtail to see if it is pinched by the cases or rubbed through on the frame. If that looks ok then your stator is shorted and needs to be replaced.

    And that's about it except to say that dirty connections and worn brushes account for most of the charging system problems.
  3. Hutzpah

    Hutzpah 1979 xs650

    jayel thanks very much for that I'll print it out and have a study.

  4. pckopp

    pckopp XS650 Addict

    Here are a couple of pics from an original service manual on how to actually check the diodes in the rectifier assembly.



    Two measurements for each diode, one in each direction. Go slow, make notes. One or more bad diodes means buying a new unit.

    Good luck.
  5. Hutzpah

    Hutzpah 1979 xs650

    Oi Va Voi what language is that? and I really thought this was going to
    be easy:yikes:
    I'm pretty sure I'm doing it right by the manuel but just am wondering why it
    flashers a different reading on the same diodes and so fast.
    Thanks pckopp

  6. pckopp

    pckopp XS650 Addict

    Are you using a digital or analog meter?
    I assume you have the unit unplugged?

    Set the range on your meter to 2K or so.

    One direction should be very low - approaching zero.
    The other direction should be very high - as if you weren't touching anything.

    This is a completely static measurement. If you have a normal sort of ohmmeter and the diode assy is disconnected, the readings in either direction should remain stable. Diodes either conduct or they don't.

    When you touch the ohmmeter leads to the diode connections, the meter applies a small voltage to get some current flow through whatever you are measuring. It measures that current and shows it on the meter as resistance - ohms. The voltage will cause the diode to "turn on" or conduct in one direction - that's the very low reading, and in the other direction the diode does not conduct and that's the very high reading. That is what diodes do, conduct current in one direction only.

    If you can't just apply the leads of the meter and get a static reading, then either the diodes are bad or the meter isn't doing what you (we) expect.

    This is really very simple, there are just 6 diodes to check, so 12 measurements. Actually, as soon as you come to a diode that does not act like I describe, you can stop and order a new rectifier assembly. Diode = rectifier.

    Print out my diagrams and I bet even an auto parts store can help you test them. It really is easy.
  7. Hutzpah

    Hutzpah 1979 xs650


    "Are you using a digital or analog meter?" A: I'm using a digital.

    "I assume you have the unit unplugged?" A: yes its out of the bike.

    "Set the range on your meter to 2K or so." I get no reading at the 2k setting.

    "......the readings in either direction should remain stable." from this statement alone
    I think its not working as its all over the place with cleaned connections.

    2K ohm test

    Black ohmmeter probe to the black wire and then touching the red probe to each white wire in turn gives a reading of 1.
    Reversing probes gives the same reading 1 at 2k ohm setting.

    Same test but starting with black probe on red wire and then touching the white with the red probe gives same reading 1.
    reversing the probes e.g. red probe on black wire same results.( all tests 2k ohm setting)

    200 ohm test

    Same as above unless you touch red probe on black wire and then touch the white wires with the black you get.

    1) 46-75-144
    2) 176-72 170
    3) 174-23-85

    Or you place the black probe on the red wire and then touch the white wires you get.

    1) 172-48-85
    2) 41-48-182
    3) 46-53-133

    I hope you can understand all that but as you can see by the readings it can jump from high to low and only flashers on the multimeter for a second also the Manuel says readings should fall between 75-150 ohms.
    I'm think its best to get another rectifier it must be partly working or I wouldn't have got maximum 7 volts at the battery.
    Thanks for your help.

  8. XSLeo

    XSLeo XS650 Guru Top Contributor

    Does your meter have a diode check function? Both of mine do, it's right beside the 200 ohm scale. You might want to try that.
    I just checked my old rectifier from my 75. On the 2k scale I get zero reading one way After the reading stabilizes about .525 to .530 on the white wires. On the diode check I get zero one way and .620 to .630 the other way.
    This one is ok. The exact numbers are not as important as the high side is 10 or more times the low side.
    In your first post you mentioned adjusting the regulator and only getting 7 volts at the battery while running at 2500 rpm.
    What is your battery voltage with out it running and the key off? If less than about 12.5 or better, you need to charge the battery.
    The way this alternator works it needs an almost fully charged battery to make electricity. The battery supplies the rotor with voltage through the regulator. If the regulator reads the battery voltage at less than the preset of 14.5 volts it sends full battery voltage to the rotor. When the reg turns the rotor on, it builds a magnetic feild, this magnetic feild excites the stator which causes the stator to make electricity. When the stator makes enough to charge the battery to the preset 14.5 volts the reg turns the rotor off, the stator stops being excited, no electricty is made, once the voltage drops below the preset the cycle starts over. This happens very quickly.
    If the battery is to low it can't create as strong a magnetic feild in the rotor, low magnet = low output at the stator.
    New brushes will help. I think your rectifier maybe ok. On your model with a seperate regulator and rectifier doing the regulator bypass can tell alot about the alternator. The way jayel's step #3 describes is for the later combo reg/rec.
    On yours the regulator turns the rotor on/off by sending battery power to the brushes on the green wire, At the brushes the power goes through the rotor and out the black wire to ground. To bypass your regulator you need to jump a wire from battery positve to the green brush wire.
    On the later combo reg/rec the battery sends power to the brown wire brush, through the rotor, out the green wire brush then to the regulator and the regulator grounds the green wire.
    When you do the bypass keep you meter on the battery, this way you can see the voltage rise. If it quickly climbs up, don't let it go over 15 volts. If it climbs your alternator is ok, the regulator is bad. If it doesn't climb then there is another problem.
    If no climb I would check the rotor and stator.
    The rotor should have 5 ohms from slip ring to slip ring, infinity to ground. The stator with it unplugged from the harness, the big 6 wire plug and the yellow single plug. Check the three white wires. Each pair should have about .9 ohms with infinity to ground.
    I would check these things now while you wait for the brushes.
    Report back all your results. Once we know what you have, then we can better explain what you need to get it to charge.
  9. Hutzpah

    Hutzpah 1979 xs650

    Thanks XSLeo that was a great explanation.

    "Does your meter have a diode check function?"
    Yes thanks for bring that to my attention never new it existed finally some sort of stable test.

    Black probe to back wire red probe to whites.
    1/ 1
    2/ 1
    3/ 1

    Red probe to black wire black probe to whites

    1/ 517
    2/ 498
    3/ 505

    Red probe to red wire black probe to whites.

    1/ 1
    2/ 1
    3/ 1

    Black probe to red wire red probe to whites.

    1/ 512
    2/ 517

    Fast and easy and I was able to get a reading that didn't flash on to the screen
    I look at the manual like a bible I think I'm losing my faith.
    Hopefully I'll have time to look into some of the tests posted here this weekend.

    Thanks healers your making a blind man see.:eek:
  10. XSLeo

    XSLeo XS650 Guru Top Contributor

    Sounds like your rectifier is ok. How does everything else check out? The stator? The rotor?
    The most common thing to fail is the brushes, they wear to short. Next the rotor, years of to much heat cook the insulation and they short out.
    Dirty connections are in there some where. The metal to metal connections inside the plastic plugs get corroded. The stator and rectifier seldom go bad. The recifier, as it ages the diodes inside open and conduct both ways. Seldom all of them onften just one. This can cause a low voltage. Not as low as your getting.
    At this point I would check the rotor next, Should read 5 ohms from slip ring the slip ring. Infinity to ground. If bad the ohms will be much less, seldom too high. With the brushes out put the meter probes in the holes the brushes were in to touch the slip rings.
  11. Hutzpah

    Hutzpah 1979 xs650

    XSLeo after finally feeling happy with the test on the Voltage regulator and the rectifier
    I decided to clean all fitting and put them back in and try again as there was some serious dirt in those plugs.
    I then carried out the test again and I went from the previous 1.5 volts to 14.5 volts.

    So your comment "Dirty connections are in there some where" was very much on the ball.

    Then I went for a nice run down the coast and back then we had some quality time together, gave her a polish. She didn't say much but I know she appreciated it.

    When I install the new bushes I going to do some more testing to build up my knowledge base.

    I much appreciated your help XSLeo, pckopp and jayel:bike:

  12. XSLeo

    XSLeo XS650 Guru Top Contributor

    Thats why we are here.
  14. xjwmx

    xjwmx XS650 Guru Top Contributor


    '83 came with that kind of regulator. Take a wire and back probe into the connector at the green wire. Some people skin the wire or stick a pin through it but it's not necessary. Connect the other end to the frame or battery -. Watch the voltage and don't rev too high like that.
  15. XSLeo

    XSLeo XS650 Guru Top Contributor

    You can do the bypass as xjwmx sais or you can hook a wire to the poitive battery terminal and just touch the other end to the screw where the green wire hooks to the brush. With the bike idling and you meter hooked to the battery, when you hook the jumper wire the battery voltage should start to climb, it will climb very fast. If it goes over 15 volts unhook the jumper wire. This test proves that your stator, rotor, brushes and recifier are ok. If you don't get a voltage increase thaen checking out the wiring connectors for corrosion, and testing the stator and rotor for proper ohms is the next step.
    JayR likes this.
  16. stanleyh

    stanleyh XS650 Enthusiast

    I've had charging problem since I bought my 78 model a couple of months ago. Sure have enjoyed the help so far from the people on this site. So far I have put on a new stator, rotor, brushes, and the guy who had it before me had the same "no charge prob" so he put the solid state rectifier / regulator on it. He had 6 years and never got it to charge. We teamed up the other night and got it charging at last. But, when revving the motor over 2k rpm the voltage got too high at 16 plus volts dc at the battery. Through all the rewiring the inherited mess of connections I received with the bike, I wonder if we didn't short the his new rec/reg out? If we did how do I trouble shoot it?
  17. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    If you are seeing 16 volts DC at the battery, that means your regulator is not doing its job of regulation. In order to do its job, the regulator must receive a voltage reference signal. Cars, trucks, and motorcycles will all have very high voltages up to 16 or 17 volts, when the regulator is unable to see the reference voltage.

    You should confirm that the ground input lead to the regulator is actually giving a good ground i.e. touching bare metal. Bare metal does not include the battery box because its rubber mounted. Of course the battery + reference input to the reg also must be present.

    If the regulator has both solid inputs of ground and battery +, then the regulator is almost certainly defective.
    JayR likes this.
  18. stanleyh

    stanleyh XS650 Enthusiast

    Well, I will do that and I'll let you know what happens. Thank you very much stan
  19. stanleyh

    stanleyh XS650 Enthusiast

    Well Mr Retiredgentleman, I tried as you said to do. I added another ground wire jump from the battery to the frame. I am a master electrician so its really upsetting to feel as stupid as this bike has makes me feel. The reg rect has good positive to the blue and and the orange wire has a good ground. I still am still getting high volts at the battery. Am I like you suggested in need of a new reg rect. I guess the only reason I'm kind of asking twice is the part was supposed to be new and we changed a few wires around the other night with my friend here helping me. I have ungrounded the brushes with the nylon screws. Checked the a/c voltage out of the stator on the three white wires. Seems to be good. My red is getting 12 positive volts to the red brush when the key is on. Does it still sound like I need a new rect reg? I appreciate your help so much, thank you all stan
  20. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    With the 70 to 79 charging systems (stator/rotor), using a 70 to 79 type of regulator, there is no need to use nylon screws on the brushes. With that type, the right (inner) brush is grounded, with the brush screws directly screwed to the stator housing. The left (outer) brush, of course, is insulated from ground.

    I'm not familiar with the 70 to 79 combined rec/reg unit. Was it purchased from Mikesxs, and is the stator/rotor also from Mikexs? xsLeo had posted the colour code for the wires of the Mikexs rec/reg, so that is my only knowledge of the proper connections.

    A few questions.

    The blue wire of the rec/reg is the reference voltage input. It should be connected to the immediate load side of the ignition key. That is the same brown wire that feeds the input of the fuse panel. Are you connected to that location? If you are connected to somewhere else that has lower voltage then that would cause the regulator to raise the voltage to 16 as you have seen. It would be interesting to know what voltage you have on the blue wire, when you see 16 or more volts at the battery. In other words,if you had a large voltage drop across the main fuse and/or the ignition switch, that would cause the regulator to drive up the voltage to the battery.

    You say the orange wire has a good ground, but according to the information from xsLeo, the orange wire from the rec/reg supplies a positive voltage to the left (outer) brush.

    You also say the brush with the red wire is getting 12 volts +, whereas my information says that red wire is on the right (inner) brush which should be grounded to the stator frame. My information shows the green wire from the rec/reg, connecting to the red wire that goes to the right brush.

    Do you have any wiring instructions that came with the rec/reg and stator/rotor? I'm sensing that you or the PO have not connected the wires correctly. To many ways to go wrong without manufacturers wiring instructions.

    If my info is correct, you need to have the green wire from the rec/reg connect to the red wire of the stator (right brush) and that right brush must be grounded ..............no nylon screws. Also the orange wire from the rec/reg must connect to the green wire of the stator (left brush),and that brush must be insulated from ground.

    Here is some more reference info:

    With the engine idling at 1200 rpm, you should expect to measure 12.5 to 13 AC volts across any 2 of the white stator output phases, with the harness connector connected as normal.

    If you measure the DC voltage across the 2 brushes with engine idling, expect 12.5 to 13 volts.
    With engine reved up to 2500 to 3000 rpm, expect the voltage across the 2 bushes to decrease to 5.5 to 7 volts.
    Last edited: Nov 12, 2012

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