Topic: Rectifier+Regulator Combo Test

[cshanek]

A while back TwoTired sent me (and many others) the following procedure for diagnosing charging issues:

Charging system methodical verification checks, CB750, CB550, CB500, CB400, and CB350.

Begin with problem verification and characterization with recorded data.
A -- Fully charge a known good battery.  Let it rest for 2 hours, off the charger, and measure the battery voltage.  (Target is 12.6-12.8V.)
B -- Start the bike and measure the battery voltage at idle, 2000, 3000, 4000, and 5000 rpm.
C -- Repeat the measurements of B with lighting off.

The above tests identify charging system success, failure, or degree of "faulty".  The success voltages are listed in the Shop manual.

D -- Assuming the above indicates faulty, do check the RECTIFIER diodes with a diode tester or ohmmeter capable of testing diodes and uses more than .7 volts to make the test(s).
Of the twelve test made in D, six must read low ohms and six must read very high ohms.

E- assuming no faults were found in D,  Measure the white and green wires disconnected from the REGULATOR.  CB750s should 6.8 ohms - ish,  CB550s/350s/ and 400s should read 4.9 ohms- ish.

F- If there are no bullet holes or road rash/divots on the alternator case, the stator is probably good.  But, you can check for yellow to yellow continuity (.35 ohms) among all the wires, and that no yellow wire has continuity to the engine case.

G- Assuming no faults found in D, E, and F, measure the disconnected terminals of the REGULATOR.  The black and white terminals should measure zero ohms (subtract meter error if there is any).  Higher than Zero ohms, indicates internal contact contamination needing cleaning and attention per shop manual.

H - Assuming D, E, F, and G have not found faults. We can verify all the of the charging system minus the regulator is functioning correctly, by using a temporary jumper to connect the disconnected white wire (normally attached to the REGULATOR) and connecting the White directly to the the battery POS terminal.  Repeat the B and C tests.  However, if at any time the battery voltage rises above 15V, stop the test.  Such an indication would prove the charging system capable of maintaining a known good battery.  If this test never achieves 15V, then there is a wire/connector issue in either the ground path leading back to the battery NEG terminal, a wiring/ connector issue withe the rectifier RED path to the battery POS terminal, or you made a mistake in D through G.

I - (not used, can be confused with L)

J -  The only parts that remain to prove or expose are the REGULATOR (in active mode) and the electrical path between the battery POS terminal and the black wire that connects to the REGULATOR.

K - Lying to and starving the regulator
The regulator can only do its job correctly if it gets a proper voltage report of true battery voltage status.  The Vreg monitors the Black wire for this status.  Measuring the voltage lost between the Battery terminals and the Vreg connections identifies problems that are not really the charging system's fault.
Two connection paths must be checked, the Battery POS terminal to the Black wire connection at the VReg, and the Battery NEG terminal to the Green wire connection to the Vreg.  A volt meter can measure these losses directly by placing a probe between the two identified points, Black path and then the green path.  The numbers are summed and the error seen by the Vreg quantified.  Anything over .5V loss is cause for concern and anything over 1V is a certain issue to be corrected.  Each connector, terminal, fuse clip, or switch in the pathway can cause voltage reporting loss.

The regulator also passes the received voltage on to to the Alternator field coil to create a magnetic field within the alternator,  The voltage level determines the strength of the magnetic field and the maximum output capability of the alternator.  Therefore, starving  the Vreg of true battery voltage leads to reduced max output capability of the alternator.

L - regulator operation/verification.
  The Vreg sends voltage to the alternator field in response to measured voltage which is battery state of charge.  Any voltage at the battery of less than 13.5V sends full black wire voltage to the alternator's white wire.  The alternator output will vary with RPM, even if "told" to produce max power by the Vreg.  If the alternator has enough RPM to overcome system load, any excess power is routed to the battery which will raise the battery voltage (slowly if depleted and rapidly if nearly full).   When the battery reaches 14.5V, the regulator reduces the voltage to the alternator, reducing output strength and preventing battery overcharge.  If the battery exceeds 14.7V, the regulator clamps the alternator field coil power to zero (0V), effective shutting off the alternator.

Because, there is electrical load from the system, an alternator that is not producing power allows the battery to deplete and the voltage falls.  The Vreg responds by turning the alternator back on in accordance with battery state/ charge level.

The Vreg state changes can be monitored/verified by observing the battery voltage state, and the White wire to the alternator field.  (Two meters are handy for this.)  The "trip" voltages can be adjusted with the adjust screw, while changing engine RPM and electrical load that the bike presents to the battery/charging system to "make" the battery reach the voltage levels need for the set trip points.  IE. with load reduced (lighting off) and the engine above 2500 RPM, a charged battery will attain 14.5 V.  Anything above that and the adjust screw needs to be backed out to keep the battery safe from harm.
The shop manual outlines bench set up mechanical adjustments that should be performed on unknown or tampered units.  These should be resolved before final trip point adjust tuning.

Note that while the system is working, the Vreg can change states rapidly before your very eyes., changing 5 times or more while you blink.  Therefore, you may have to mentally average values measured on the White wire if your selected meter doesn't do that for you.

• What are the 12 tests in D? The rectifier has 3 yellows, a green and a red running to the loom. I am testing both directions from each of the yellows to both the green and red?
• Assuming everything in D checks out, I am just measuring resistance across white/green regulator wires to make sure they are at or near 6.8 ohms? No other testing needed for regulator?

[scottly]

What exactly is your problem with the charging system? Lloyd's trouble shooting procedure tends to cover every single component in order, when the problem can usually be zeroed in on in a few steps.
BTW, that info on testing the regulator does not apply to your combo unit.   

[cshanek]

It isn't charging. I suspect the rectifier since I was working on an issue that was blowing the 15a fuse regularly, and was using a dual filament test bulb in its place, but Id like to know for sure before purchasing a replacement.

I've checked voltage across battery terminals when charged, @ idle, @4000 and 6000 RPMs.

[bryanj]

OK, the twelve tests i can only think of 6 and they are,
With multimeter set to ohms (low scale)
+ve probe on red lead of rectifier then -ve to each of the three yellows in turn
-ve probe on red lead of rectifier then +ve to each of three yellows in turn

One set of readings should be very high ohms, probably no reading, the other set very low readings.

This would tell you if the rectifier part is dead.

With it all connected and ignition on you should get battery voltage accross the white and green wires, or whatever wires connect to the white and green of the field coil, if you do and it doesnt charge you have a connection fault somewhere if you dont it wont charge and you may have a connection fault or a duff reg

[scottly]

OK, the twelve tests i can only think of 6 and they are,
With multimeter set to ohms (low scale)
+ve probe on red lead of rectifier then -ve to each of the three yellows in turn
-ve probe on red lead of rectifier then +ve to each of three yellows in turn

One set of readings should be very high ohms, probably no reading, the other set very low readings.

The other 6 tests are to the green lead of the rectifier, and if using a digital meter should be done with the diode test setting, which has a symbol that looks like -l<-. A good diode will read about .5 in one direction, and nothing when the meter probes are reversed.
 

[scottly]

It isn't charging. I suspect the rectifier since I was working on an issue that was blowing the 15a fuse regularly, and was using a dual filament test bulb in its place, but Id like to know for sure before purchasing a replacement.

I've checked voltage across battery terminals when charged, @ idle, @4000 and 6000 RPMs.

Have you fixed the short that was blowing the fuses?
What voltages did you measure
What bike is this?

[Bodi]

There are 6 diodes inside the 3-phase bridge rectifier and each needs to be tested in both directions.
One end of each internal diode goes to either the positive (red) or negative (green) output wires.
The other end of each diode goes to a yellow wire.
Each yellow wire connects to two diodes, one going to red and one going to green: the diode "directions" (loose diodes all have a stripe, this indicates end to connect for positive output) are such that when the yellow wire voltage is above the red wire voltage, power will flow to red; the other diode lets power flow to green when the yellow wire voltage is below the green wire voltage.
3 phase power is a bit hard to visualize but the voltages between yellow wires cycles up and down as the rotor spins. Power may be drawn from any of the three to either or both of the other two which changes as the voltage cycles from the rotor magnetism spinning in the stator.
So you have 12 diode tests to do. Put one meter lead on red and test Ohms or use diode test to each of the yellows. All three should read the same: either low Ohms (or a typical diode voltage (around 0.5V)) or high/infinite ohms (or whatever your meter displays with leads loose in diode test mode) - and all three readings must be the same (diode test voltage can vary a bit). Then change the meter leads, put the other one on red. Repeat the 3 tests. All must again be the same but the reading must be opposite from the previous test.
Now do it all over with the meter lead on green.
Bad readings: if a diode reads high ohms or "no diode" the rectifier will lose about 1/3 of its output power (this is a guess and it may be 1/6 but still a drop for sure). Still usable for getting some alternator power in an emergency or while awaiting a replacement. If a diode reads low ohms or 0V do not use the rectifier, this causes high stator coil current and may overheat and damage it.
Note that there is no magic in the stock rectifier. Mouser, Digikey, Ebay, whatever, all have monolithic 3 phase bridges that will work super well. At a fraction of the cost for an OEM one. You just have to wire it up: yellow wires go to the "~" terminals in any order... + to red and - to green. You want something like 25A @ 100V PIV - over spec but safer than one with lower specs.
You can even wire up your own with 6 loose diodes, but that's a pain to do and a pain to cool. The monolithic one should have a cooling solution, I've just clipped the wires off a burned out stock one (to use hooking up the new one) then drilled and tapped holes in it to screw down the monolithic replacement. Pretty ugly but I didn't have any other heat sink. Just a chunk of 1/4" aluminum plate cut to fit where the original rectifier was works well. It doesn't make enough heat to need a finned heatsink, but it should have something.

[cshanek]

Alright .... as requested, some background info before I get into test results.

• This is a 76 CB750
• I am measuring 12.2- ish volts across battery terminals regardless of RPMs.
• Battery is somewhat new
• The short, which was occurring in the right hand switch to the handlebar, has been fixed
• My rectifier + regulator combo is aftermarket, purchased from vintage CB750 (https://www.vintagecb750.com/parts/480/24-2109.jpg)

Test results:

Battery
   Voltage = 12.2 V
   Voltage @ 2000 RPMs = 12.18v
   Voltage @ 4000 RPMs = 12.18v
   Voltage @ 6000 RPMs = 12.17v

Rectifier
   Diode
      + on Red + all 3 yellows = OL
      -  on Red + all 3 yellows approx 3.9v
      + on green + all 3 yellows approx 3.9v
      -  on green + all 3 yellows = OL
   Ohms
      Same as above but units different (approx .44 M Ohms?)

Regulator
   11.5v reading across Green+Black (when battery is @ 12.2v)
   0v reading across Green+White (when battery is @ 12.2v
     
       

[Bodi]

2 things:

- 3.9V, if accurate, is way too high a forward voltage drop across a diode.
- with 11.5V going into the regulator it should be putting max voltage onto the field coil (white and green). This can't actually be equal to the input voltage as there will be some voltage drop across the switching transistor, even a FET has some voltage drop but a junction transistor would be around 0.5V. You measured it with the input power connected, yes? I would expect ~11V to the field coil in this case.

I have some doubt about your diode test meter. No rectifier diode type used now has a 3.9V forward voltage drop. Do you have any loose diodes to check the meter on?
And the regulator is super suspect if it isn't powering the field coil with such a low input voltage.
You also have a slightly high system voltage drop - I like to see 0.5V maximum. If the headlight was off, that's definitely too high. This usually comes from dodgy fuseholders, bullet connectors, and switch contacts.

[cshanek]

2 things:

- 3.9V, if accurate, is way too high a forward voltage drop across a diode.
- with 11.5V going into the regulator it should be putting max voltage onto the field coil (white and green). This can't actually be equal to the input voltage as there will be some voltage drop across the switching transistor, even a FET has some voltage drop but a junction transistor would be around 0.5V. You measured it with the input power connected, yes? I would expect ~11V to the field coil in this case.

I have some doubt about your diode test meter. No rectifier diode type used now has a 3.9V forward voltage drop. Do you have any loose diodes to check the meter on?
And the regulator is super suspect if it isn't powering the field coil with such a low input voltage.
You also have a slightly high system voltage drop - I like to see 0.5V maximum. If the headlight was off, that's definitely too high. This usually comes from dodgy fuseholders, bullet connectors, and switch contacts.

- Don't know what to say about the 3.9v drop. That is what I was getting with a new multi-meter. I do not have any loose diodes to test.
- I am reading NOTHING across green-white. The 11.5v was across green-black.
-- The headlamp was on when I took these readings, no easy way to turn it off unless it is off when key is in position 2.

[scottly]

Don't worry about the measured voltage drop for the diode test: the results showed all 6 diodes conduct in one direction and not in the other, so the rectifier is not the problem.
If you read 11.5 volts on the black regulator input wire, and zero volts on the white regulator output wire, the regulator is faulty. You can TEMPORARILY jump the white field coil wire to the battery plus terminal. This will turn the charging system full on, and will drain the battery if not disconnected whenever the engine is not running. Put the jumper on, start the engine, and see if the battery voltage slowly starts to climb. If it does, that proves the rest of the charging system is working. 

[cshanek]

Don't worry about the measured voltage drop for the diode test: the results showed all 6 diodes conduct in one direction and not in the other, so the rectifier is not the problem.
If you read 11.5 volts on the black regulator input wire, and zero volts on the white regulator output wire, the regulator is faulty. You can TEMPORARILY jump the white field coil wire to the battery plus terminal. This will turn the charging system full on, and will drain the battery if not disconnected whenever the engine is not running. Put the jumper on, start the engine, and see if the battery voltage slowly starts to climb. If it does, that proves the rest of the charging system is working.

Yup, that fixed it. Jumped white to battery positive and fired her up and voltage started climbing immediately even at idle. Ordering a new unit.