Topic: 76 CB550 K2 - Overheated field coil?

[xhevi]

hello all.

Last fall I got a 76 CB 550 K2 Candy Garnet with around 12000 miles, all stock other than the 4 to 1 exhaust and it was rejeted.

This forum has been a tremendous help. Thank you all.

I took it all apart starting in January 2019 and i did not get to finish and ride till late November. After rebuilt, i rode it a few times around the block at low revs and low speed. YES, the battery died a few times and i blamed on low revs. I remember measuring around 14V at 5000 rmp so i assumed the charging was ok and i parked it for the winter.

As i am getting ready for the spring i am going over the electric system. The bike is not running and i am testing the alternator off the bike. All coils are dry, however there was a bad smell when i removed the cover.

Here is what i have done so far: 

1) LED tail light and blinkers. - all work well
2) LED headlight bulb in a new Hella H4 reflector. - works well.
3) cleaning up all connections - second time around as already did it last year
4) testing the stator coils: there is no continuity from any yellow to the cover and all yellow to yellow readings are 0.4 to 0.6 ohm, my voltmeter displays only the 1/10ths  digit so that 0.4 could be .35 rounded up.
5) testing the field coil reads 3.8 ohm thats about 20% less than 4.6 ohm that listed on the repair manual.

Here is an old post that i found that relates to my issue.

http://forums.sohc4.net/index.php?action=post;quote=2049525;topic=176203.0;last_msg=2049612

Quoting TwoTired: "Field coils can have failures if they are overheated, which usually lowers the resistance value, leading to poor magnetic field creation and lower stator output capability."

Smell + low reading makes me believe that my field coil did overheat and now is not within specs. Yes, i understand that riding it at low speeds might have caused the bike to overheat. I did not have any leaks so i hope nothing warped.

questions:

1) How bad is 3.8 ohms vs 4.6 ohms?
2) Whats the solution to a bad Field coil? repair? replace?
3) What else might have gotten messed up if the bike did actually overheat.
I have not gotten to the voltage regulator or the rectifier yet.

[69cb750]

1) How bad is 3.8 ohms vs 4.6 ohms?
2) Whats the solution to a bad Field coil? repair? replace?
3) What else might have gotten messed up if the bike did actually overheat.
I have not gotten to the voltage regulator or the rectifier yet.

Battery voltage is the best test, 14 volts @4000 rpm is good.
The alternator cover often has a burnt smell even when everything is ok.
It used to bother me too, I thought failure was near, alternator kept chargeing.

[bryanj]

Unless you have a meter that cost $1000's it is just about impossible to get an accurate reading on low ohms.

Also i have been working on the fours since they came out in the UK(71 for 750 72 for 500) and i have NEVER seen an electrically duff field or stator. I HAVE seen physicaly damaged coils after an accident, a stator on one i bought had been dropped and a varnished wire broken, simply soldered it back together and insulated it for it to work for years.

What i am basically saying is refit it and forget it problems are 95% connections and 5%rectifier/regulator( unless an electronic one has been fitted then 60%reg/rec)

[xhevi]

That's what I wanna hear.
Alternator is back on. And moving on to vreg and rectifier. Well have to find out when it's running.

However. Just in case my field coil needs work. Any recommendations where and how to get it fixed, how much? 

[bryanj]

Used from ebay

[xhevi]

Thank you all