1978 CB750 Alternator Resistance

[PortlandThomas]

Hi there,

First, I'm new here, first post. So my apologies if I don't do it properly.

I have a battery issue and I'm trying to troubleshoot it. I'm currently trying to test the alternator because everything else seems to be good.
On my shop book, it says that "the rated resistance value [between the two lead wires of the Field Coil] is 7.2Ω.
On my multimeter it shows 6.8Ω.

Same for the Strator Coil. The rated resistance value [between two the three terminals wires] is 0.2Ω. And my multimeter shows 0.6Ω.

On each cases, it sounds close enough to me but I have actually no idea.
Do you guys think it mean that I have to replace the alternator?

Tks a lot.

 

[amc49]

No 2 voltmeters on the planet read exactly the same and close enough in my book. You also need to check for any winding grounded to other things like the steel core.

Load test the battery at the proper CCA, you'd be amazed how many people chase phantoms assuming the battery is good when it's not. I have trouble believing it and have seen it thousands of times, just no reason for it in this day of 'illuminated' people. Starting with all the ones that think a 12 volt battery is good if it has 12 volts. DEAD if at that, needs 12.3 or more, a new one has around 12.8.

 

[PortlandThomas]

Tks for your answer. My battery shows 12.6 when it just being charged and now it's been sitting on my bike for two weeks and it still shows 12.3
Do you think that the battery is good. Or not necessarily ?

 

[amc49]

Could be, they commonly drop off more than that over that length of time if bad. I look at them every 24 hrs. for several days, they drop in 24 hrs. or so and then pretty much stay at the new lower number after that if good. A good battery actually will overcharge to say 13+ volts, then drop back to a real number, that's 'surface charge', or charge that has not dissipated into the plates evenly yet. Why you let them sit at least like 4 hours to get the real number.

The load test is the last word there, batteries can seem to be good yet spit their guts out as soon as you load them hard. Seen it more times than I can count.

 

[PortlandThomas]

Alright, tks a lot. Great help.
Other (probably dumb) question: Is that normal that neither my rotor nor the coils (field and stator) of my alternator are magnetic. The coils are not plugged FYI. Even when I plug it (but don't run the bike obviously because my alternator and gear box are open).
Can that kind of thing can lose their magnetism?

 

[amc49]

Only permanent magnet alts have magnetism while engine stopped, the SOHC has a field coil that makes magnetism when the engine is fired up, the core that fits inside the rotor itself. Like most all car alts. The field can be varied stronger or weaker as needed for charging output, permags can't do that as the magnet has a certain amount of attraction and that's it. They overcharge at higher rpm and the norm is to bleed some of that to ground to control output.

A permag CAN lose it's magnetism, a sudden impact just right can take it away instantly but not common.

 

[PortlandThomas]

I have a SOHC so it should be alright. At least for that point.

Now I've checked the output from the 3 starter coil wires while my engine was running. Like this video is telling me to do: https://youtu.be/44hLjPBObYw?t=12m10s And it shows around 20 to 25 volts on every of the 3 wires. Apparently this guy says that it has to be around 75 volts for each of the wires. I know we don't have the same bike but it seems really far away from my 20/25 volts. FYI, I can't find in my shop manual how much it should be.

Do you have an idea?

 

[amc49]

Lemme guess, you were at idle right? Engine revved up makes more volts. A/C though and useless to you, you need D/C and why it gets rectified and knocked down in amount. Any alt on the planet can make up to 75-100 volts unregulated of A/C, you just can't use it, too high. Need a limiter that ensures the same volts regardless of engine speed, ergo, the regulator, it increases or decreases the field current to change charge output by making the magnet stronger or weaker. The diode plate changes the A/C to D/C. It can be incorporated inside regulator or a separate part depending on system design.

You won't find that in the OEM shop manual, worthless as the stator windings checking out OK automatically lock the running volts in place, the volts amount comes from the number of turns in stator and the amount of field current making the magnet.

 

[PortlandThomas]

Never mind, I've done the test again and it shows 0 or 0.2.
I'm guessing that it might be my rectifier/regulator. I think the regulator doesn't send juice to the alternator. Then the alternator can't send juice back to the rectifier.
See this photo to know what my regulator/rectifier looks like:


I tested the rectifier (If I'm not wrong: yellows green red wires that go to the plug) with my multimeter and it seems to be alright. However, I don't know how to properly test the regulator. (green black white) And I can't find a proper tutorial or something. Driving me nuts

Would you have an advice?

 

[amc49]

Can't say on regulator, many commonly swap only with known good one to test for sure. Too many ways they can pass and still not work right.

Test more than once for sure, you can get false results on that stuff all the time if not dead careful.................you're having volts then not points at that easily. Or points at a problem. Only you know as you are the tester.

 

[PortlandThomas]

I got a new battery, a new regulator/rectifier and I tested the alternator output (3 yellow wires) again and again and it shows no voltage at all.
Do you know how i could check if the alternator is still grounded correctly? I'm really surprised that it shows 0 and not even a small random number.
Also it might help or not but I took the neutral switch and the oil pressure switch cables off. Do you think it might influence the "grounding"?

 

[madmtnmotors]

Here are very detailed troubleshooting methods (scroll down to the charging system section):

http://www.hondachopper.com/garage/...ng_system/troubleshooting_charging_system.pdf


Good info here too:

http://forums.sohc4.net/index.php/topic,1012.msg8345.html#msg8345