Topic: Testing a stator

[SurfinBird]

1978 CB750F SS

After ignoring my electrical problem for a while I heard an interesting idea to test the stator. Just for a bit of history, no charge at the battery, not going over 12v at any rpm. I have a new battery, replaced the rectifier, replaced the regulator tested continuity on the stator, all pass.

A friend suggested it's the stator, i said it tested good and they either work or don't (as i've been told on this forum). He suggested starting it up and disconnecting the battery to see it if still runs. His justification is if the stator is good it'll keep running. What do you think?

My thoughts in general is that I either needed to replace the wiring harness (a shop suggested that) or replace the stator. With them being about the same price I didn't want to do one and it turn out to be the other and be out 2x's the money. If this is a viable test it seems like a good way to narrow it down.

[eigenvector]

I will claim ignorance about the Honda's specifically, but if the stator is anything like the one in my Tiger 1050 - you can test the output voltage from each of the 3 legs.  They should all be the same.  If the stator is bad, one leg will be low - indicative of a short.

Should be plenty of google hits on testing a stator.

[Bill/BentON Racing]

First, what was/is your electrical problem? There's tests as you said, what test did you do? If stator no good, it won't keep running with battery disconnected. Still have to keep it revved to stay running, not the best test, but does tell you whether it's putting out or not. Check the 3 yellows for voltage etc. Also check when first cranked and after running til hot. G'luck, Bill Saw one the other day, all tested great, but still battery wouldn't stay charged. Battery box I happened to touch with leads..WTF? 12 volts, huh? Yep positive wire from battery to solenoid( not fused) had worn through the insulation on bottom of cable! Was contacting battery box top! Haha, took me a little while to figure that one out.

[SurfinBird]

Thanks guys.
I used TwoTired's extensive checklist and everything checked out numbers wise. I checked continuity on the stator (all good) but not voltage. I'm a kook with electrical, so pardon this question. Do I just fire up the bike with the stator disconnected (to the rect/reg) and check voltage on the 3 yellow wires?

[05c50]

The stator provides AC voltage to the rectifier where it is converted to DC voltage, so the test will need to be done with an AC volt meter. Voltage between any two yellow wires should raise to about 70 volts when the engine speed is raised to somewhere about 3,000 rpm. Test all pairs of yellow wires. The results should be the same.

................Paul 

[eigenvector]

In answer to your question - yes.  Just unplug the connection, hook up an AC meter (most multimeters should have this setting) and test between the prongs.

1 to 2
2 to 3
1 to 3

I'd pull any fuses you don't need first - reduce the draw on the system so your battery lasts longer while you have the stator unplugged.

I wouldn't worry about specific voltages, just focus on whether they're all the same.  The higher the revs the higher the voltage.

[TwoTired]

1978 CB750F SS

A friend suggested it's the stator, i said it tested good and they either work or don't (as i've been told on this forum). He suggested starting it up and disconnecting the battery to see it if still runs. His justification is if the stator is good it'll keep running. What do you think?

I think your friend doesn't know much about SOHC4 charging systems.
At idle, the alternator makes about 1/3 of it best output wattage at 5000 RPM.
The bike will only keep running without a battery if the bike consumes less power than what the alternator can make at present RPM.  If bike uses 100 watts and the alternator is making 80 watts, the ignition will not make spark and the engine dies without a battery attached.

My thoughts in general is that I either needed to replace the wiring harness (a shop suggested that) or replace the stator. With them being about the same price I didn't want to do one and it turn out to be the other and be out 2x's the money. If this is a viable test it seems like a good way to narrow it down.

Sounds like you're more interested in finding the problem via part substitution than checking/verifying the function of the components you have.  Why would you replace an entire wire harness, if there is only one or two connections/routes that are faulty?  And, if replacement still make sense to you, why don't you replace the entire bike because it doesn't charge its battery in a manner you expect?

Want to learn?
The SOHC4 stator can't make output power unless there is a magnetic field within the alternator at the rotor.  The magnetic field is created by the field coil, also located in the alternator housing.  So, this is where you must begin the verification.  Apply full battery power to the White and green wires running into the alternator.  If you have replaced the stock regulator with an electronic one, then make sure it is disconnected from the white/green wires.
With the battery directly powering the field coil, and the rotor spinning, the stator will generate AC voltage.  This should then be rectified to DC via the rectifier and as long as the bike's electrical load is less than alternator generation, the battery voltage should rise.  It may do this very slowly, if the battery is not at full charge, so this test is best done with a fully charged battery accomplished by an off bike battery charger.
Report to us what voltage you see at the battery, at 1000, 2000, 3000, 4000, and 5000 RPM.
repeat this test with the head light fuse removed.  If you notice the battery voltage going above 15V discontinue the test, as there is nothing wrong with the alternator, or rectifier.

Normally, with the regulator included in the circuit, the white green voltage would be reduced when the battery goes above 14.5V.  The regulator's action reduces the magnetic field inside the the alternator which reduces the power generated in the stator.  In this way, the "regulator" controls system voltage by simply keeping the battery at full charge whenever the alternator has the capacity to supply what the bike needs plus some extra to top up the battery.


You now owe us numbers to analyze.

[scottly]

Here's a low tech test of a 750 regulator/field circuit: with the key on, engine not running, there should be a strong magnetic field at the alternator cover. If so, then suspect the stator or rectifier, or their related wiring.

[SurfinBird]

Battery is reading 12.5v before I tried this

Scottly
I have a weak magnetic field, it could hold the small 10mm wrench and nothing else.

TwoTired
My regulator is stock

Apply full battery power to the White and green wires running into the alternator.

Do you mean to run the bike and check volts at the battery at the suggested revs? In previous test I haven't been able to make more than 12.2 volts at any rpm. I haven't tried checking voltage with the headlight fuse removed.

[TwoTired]

Battery is reading 12.5v before I tried this

Okay, that is only sightly down, not quite full.

TwoTired
My regulator is stock

Apply full battery power to the White and green wires running into the alternator.

Do you mean to run the bike and check volts at the battery at the suggested revs? In previous test I haven't been able to make more than 12.2 volts at any rpm. I haven't tried checking voltage with the headlight fuse removed.
[/quote]

You disconnect the white wire terminal from the regulator, then use a separate wire (used as a jumper) to connect the free end of the white wire directly to the battery POS terminal.  Then run the Volts/vs RPM test with, and then again without, the headlight fuse installed.