Topic: Bad Stator/Field Coil or both?

[CBdave76]

My 1976 CB750A is a battery powered machine.  I found this out the hard way, after pushing it back home a mile.  Fortunately, it died 2.5 miles from my home.  A neighbor with a rope helped me the last 1.5 miles.  I recharged the battery overnight and it shows 12.85 VDC.  I kickstarted the bike and watched the voltmeter on the battery.  At 1000 rpm (the head light is off) 12.05 VDC, at 2000 rpm 12.15 VDC, at 3000 rpm 12.25 VDC. I checked the rectifier and the diodes respond correctly to the polarity swap.  I checked the voltage regulator, gaps OK, even used wire the jumper the White and Black and get no change to voltage on battery.  Checked the resistance on the stator, .5 ohms on all three Yellow wires.  Checked the field coil, only reads 4.5 ohms, (Manual said it should be 7.2ohms).  Rigged the bike to run with the stator and field coil plug disconnected from where it comes out of the case (The connector that was tucked between air cleaner and battery box). I read 10 VAC on one pair of Yellow wires and it goes to 35 VAC at about 2500 rpms.  The other Yellow wire reads 3 VAC and does not change.  I tried all of the Yellow wires in different combinations and only one pair give me VAC that change with the rpm.
 
Question, should I replace both components as a set?  Has anyone had bad experience with old used (pre 1975) electrical parts not lasting very long.  Thanks for any info.  David

[tlbranth]

I'm still on the original alternator on my 1970. But it only has 40,000 or so miles on it.

[TwoTired]

Stators rarely go bad on their own. It usually takes some sort of physical injury to do that (crash).
I'd recheck connections before doing a full condemnation.

Did you check all six diodes in the rectifier?  Or, only the red to green polarity test?  If some of the diodes are open, those associated phases won't generate correctly from the stator.

Your field coil test failure shows the most likely culprit.  The field coil makes the magnetic field that the stator requires in order to make power.  The low reading translates to a weak magnetic and extra power drawn from the battery to energize that weak field.

If a used field coil measures the correct resistance, it should fix your charging issues, provided the wires, connectors, regulator, and rectifier are performing their jobs correctly.

Cheers,

[CBdave76]

I checked the diodes on the rectifier following the service manual instructions.  Green to each of the yellow wires, change polarity, Red/White to each of the yellow wires, change polarity.  I found continuity in one direction only on each test.  I tested at the plug connection to the rectifier.  I ran the bike with the field coil connected, and the stator wires free.  That's where I found VAC on only one pair of Yellow wires, and only 35 volt max at 2500 rpm.  The service manual and Clymer doesn't state what the AC output directly from the stator should be.  I was expecting to see AC volts across any combination of the Yellow wires I picked.  I only found AC volts on one pair.  Could a bad field coil only power one coil of the stator?  I thought both were bad because I have only one AC output and its also weak.
I search the forum for field coil and stator problems and didn't see a dynamic test of the AC output, just resistance and DCV output. 
Thanks David

[gane]

Dave, I just went to the library and found, Well... not much.. But, experiance offers these.  1st, simple check of field core is,  hang a light guage feeler guage about a 1/4" from stator cover and turn on ign. sw. if feeler is drawn to cover, field core is working. 2nd, stator output is measured from stator wires, (3 yellow) to ground  (not to each other) independantly, typical  readings are 10-20 VAC per leg at idle and 50-80 VAC per leg @ 4 grand. heat . age .vibration can cause stator failure,  also, poor harness routing can allow chain to eat wires. hope these help.g

[CBdave76]

I checked the field coil with the feeler gauge, it shows magnetic power.  I tested the 3 Yellows wire to ground and get the following: 3 Vac to 10, 2 Vac to 10, 10 Vac to 30.  The first numbers are at idle.  The second numbers are at 2000 rpm.  I took the readings from the plug connection that goes to the rectifier. 
I plan to check the wires to the stator where they pass through the engine casing.  It looks like a tight area to work in.  If I have to pull the wires through the casing to replace the stator, I wonder if I should replace the field coil as well.  I'd hate to have to mess with the wiring in that area another year down the road if the original field coil goes bad later.
David.

[TwoTired]

I already told you your field coil or the wiring to it is bad.  Can't say for certain that's the only issue.  Replace that first and see what the stator outputs are.

[CB750F2]

David, the field coil certainly looks suspect but I would check it again using another digital multimeter or check your meter by using it to test a resistor of known value. Standard resistor values around the same value as the field coils are 5.6 ohms,6.8 ohms, 8.2 ohms or 10ohms. On the manual transmission models there are stator connections behind the front sprocket cover. If you have them on your model they would be worth checking - you may have two of the yellow phases shorted together or going to ground.  Pat

[CBdave76]

I checked the connections that hide in the starter area.  They were tight and clean with no breaks or burns, but they unplugged when I pulled them out to where I could see them.  I had to pull a feed wire through the maze and strip the sleeve from the harness in order to pull it back through to where I can easily get to the individual wire plugs on the stator side.  I ordered parts off E-bay.  There was a stator / field coil combination that was tested and priced a little less than the two parts separately.  I'm replacing both, even if the stator is good.  I prefer not working on the same problem twice, (replacing one at a time) and I want peace of mind knowing that I won't be pushing the bike home again soon.  I am also going to add a voltmeter to the fairing so I can have some warning if things go bad again.  While I'm at it, I plan to check, clean, and add dielectric grease to all the main connections, and maybe replace the fuse holder to blade type fuses.  I haven't had any fuses blow, but the holder does not have a cover.
Thanks for all the replies.  I appreciate all the advice and help with my problem.
David