Topic: Stator Question

[The Gurg]

CB750K5 33k miles. My first rebuild. Valve cover is back on, working on side covers.

Field coil and stator coil are both reading .2 ohms. Haven’t used a multimeter a ton, I believe it should be set to the lowest ohm setting which is 200 ohms on my meter.

The shop manual says the stator coil should be .2 ohms and the field coil should be 7.2 ohms. So I’m assuming my field coil needs to be replaced.

The bike had charging issues while it last together. The mechanic at the time thought it was likely a phantom drain rather than an alternator issue. I know he checked the voltage while running with the cover off.

I did get a stainless steel hex kit which requires anti-seize. Could see the anti-seize stopping electrical connection. Don’t think that would create an issue with the field coil ohm reading, but thought it was worth mentioning just in case.

See photo.

[Bodi]

The coil is rugged enough that I suspect your meter more than the coil. Digital multimeters are often wildly inaccurate at low resistance.
Try connecting the field coil directly to a battery, if it really is 0.2 Ohms you will get sparks and the wires heating up rather quickly - no sparking and everything reasonably cool after a minute means way over 0.2 Ohms. Field coil current with 12V should be around 2 amps if the coil is OK, your meter is supposed to measure up to 10A DC so measure it - AFTER you test with coil direct to battery to avoid blowing the meter fuse (or meter) if the current is the the 60A expected with 0.2A Ohms. It's best to connect to a battery - you'll just blow fuses and maybe damage the regulator or harness wiring if you connect it to the regulator (if it really is 0.2 Ohms).

Anti seize is not the problem. All coils are isolated from ground, the only connections are via the wires.

[The Gurg]

Thanks for responding Bodi!

Just tested the Cb750 starter by pulling my car battery (I need to buy a new motorcycle battery) and using jumper cables to connect. If I did the same thing with the field coil, how would I test amps? Connect one end of the jumper cable to the field coil and the other end to the multimeter, then touch the other multimeter lead to the other field coil contact? Need some wires with alligator clips.

Is there a multimeter I can buy that would be the right tool? I have a few things I’m waiting on, so wouldn’t mind ordering a better multimeter.

[Bodi]

Connecting the field coil directly to a battery directly or with jumpers - no harness connection - is pretty easy. One wire to battery "-", the other to battery "+".
That's essentially how the regulator powers the coil, so it won't explode or anything.
If it seems reasonably OK, ie no major spark when connecting it, no melting wires, and not getting super hot within a minute... the coil is almost certainly OK. There should be about 20W dissipated in the coil if all is well, and that will definitely heat it up but not to burning hot for several minutes. If it is actually 0.2 Ohms the current will be around 60A (assuming your battery can deliver that), enough to heat the wires and melt their insulation in a few seconds.
To measure current plug the meter's black test lead into the left black socket marked MAX 10A 250V with the red plug in the center red socket. Turn the knob to 10A. Connect one coil wire to battery "-". Connect one meter probe to the other coil wire, touch the other meter probe to battery "+". The meter should read the coil current amps. Polarity does not matter, the meter should read negative amps one way around but the number is the same and that's your coil current. If you connect the meter leads to battery "-" and "+" (DON"T!) in "10A" ... the meter's internal fuse will blow instantly.
You can calculate the coil resistance with the current reading and the battery voltage with coil connected, via Ohm's Law. Take great care to switch the meter to volts before measuring voltage after measuring current!
I'm not sure about your meter, but I would swap the black lead to the right side as well before trying a voltage measurement. I just blew the fuse on a Fluke 75 as it has its current shunt directly between the 10A socket and the black one.
Just divide the volts by the amps to get actual coil resistance. 12V and 1A is 1 Ohm resistance. 12V and 2A is 0.5 Ohm. 12V and 0.5 A is 2 Ohms, etc.
There will be an inductive kickback from the coil when voltage is disconnected, not dangerous but it will shock you if you are touching both the battery side and the coil side of the connection then.

[The Gurg]

Bodi-
Spent some time trying to use Ohms law but couldn’t get my multimeter to read amps. Wound up calling an electrical engineer friend to make sure I was following the instructions you gave correctly. I was, so he suggested I borrow his nicer multimeter. Got that today. Measured the field coil at 7.2 Ohms. And the stator measured between .3 and .5 Ohms depending on which pair of wires I tested.

I tested the Ohms of the multimeter leads together (.1 Ohms) and the values above have that value subtracted out.

.2 Ohms is the spec for the stator. From what I’m reading on forums seems like less than 1 Ohm might be ok. Thoughts?

[bryanj]

Those are fine

[The Gurg]

Thanks Bodi and BryanJ!