Topic: cb750a alternator.

[maliveline]

Hey I just bought a 1976 cb750a... My battery is losing power when I ride my bike around because its not getting recharged. I was going to pull the alternator out and take it to a parts store and have some guy test it. Anyway I unbolted the alternator cover which was held on by a bunch of allen cap screws. Everything seemed to be coming apart easily. Then I realized that the wires run deep inside the engine and i can't get them unplugged. can some help me figure out how to access them?

[MCRider]

Hey I just bought a 1976 cb750a... My battery is losing power when I ride my bike around because its not getting recharged. I was going to pull the alternator out and take it to a parts store and have some guy test it. Anyway I unbolted the alternator cover which was held on by a bunch of allen cap screws. Everything seemed to be coming apart easily. Then I realized that the wires run deep inside the engine and i can't get them unplugged. can some help me figure out how to access them?

You'll need to get a manual, downloadable from this sight, and/or a parts diagram available at motogrid.com and similar. The CB750 alternator. Ultimatley the wires unplug under the sprocket cover I think. You can do a static test (continuity) at that point without removing the alternator itself. Button everything up and start it to test it's output while operating. No need to take it anywhere. The manuals will describe the testing procedure, or someone else here who knows more than me.

[maliveline]

ok well i've down  loaded manuals and I own one and it talks briefly about disassembling the alternator but no where does it say anything about how the wires from the strator coil and the field coil run inside of the crank case and it doesn't say anywhere about disconnecting those wires. also forget about testing. I would like to know how to do this for future reference that way I know if I actually have to replace my alternator.


actually I did read somewhere about removing the shifter cover to get to these wires. does anyone have a write up for this procedure or can someone with experience give me some input?

[HondaMan]

Just remove the shifter cover. The connectors are inside there, at the top. You'll need a new shifter cover gasket afterwards. Z1 Enterprises offers individual gaskets now, made from one of my own engines!

The stator ohms values are the same for all the CB750 engines. The field coil on the "A" engines is about 0.5 to 0.9 ohms less resistance than on the other 750 bikes. The ohms values are shown in the manuals. Check between the Yellow wires for pole-to-pole, 2 at a time (they might look bleached white by now), and there are just 2 field coil wires to measure between. It's pretty simple.

The most often culprit: pitted contacts in the voltage regulator. These alternators are almost bulletproof. So, try jumpering between the Black and Wite wires on the regulator and see if that improves the charge.

This all said: far more likely than the alternator or regulator: the rectifiers on these bikes can get damaged if they are charged with a 10 amp or higher battery charger. Check the rectifier FIRST. Search for "check rectifier", as it has been posted here about 20 times. 

[Don R]

It's not really one part like on a car. It's built in pieces.

[Alan F.]

Testing things like these on your own with the help of a good manual is pretty much the norm when you want to own an old bike and keep it running. Thats why so many bikes got put into sheds and barns over the years waiting for a more caring, hands-on owner to fix something simple that had sidelined the bike for so long.

You can do this, we can help. Good to meet you.
Alan

[Alan F.]

The stator ohms values are the same for all the CB750 engines. The field coil on the "A" engines is about 0.5 to 0.9 ohms less resistance than on the other 750 bikes. The ohms values are shown in the manuals. Check between the Yellow wires for pole-to-pole, 2 at a time (they might look bleached white by now), and there are just 2 field coil wires to measure between. It's pretty simple.

I'm asking this without looking at the Ohms values in the manual so please forgive any obvious ignorance, but does the above mean that converting a 750 alternator into a 290W 750A alternator requires just a different field coil? I have more reading to do maybe.
Thanks HM
Alan

[HondaMan]

The stator ohms values are the same for all the CB750 engines. The field coil on the "A" engines is about 0.5 to 0.9 ohms less resistance than on the other 750 bikes. The ohms values are shown in the manuals. Check between the Yellow wires for pole-to-pole, 2 at a time (they might look bleached white by now), and there are just 2 field coil wires to measure between. It's pretty simple.

I'm asking this without looking at the Ohms values in the manual so please forgive any obvious ignorance, but does the above mean that converting a 750 alternator into a 290W 750A alternator requires just a different field coil? I have more reading to do maybe.
Thanks HM
Alan

Exactly. The only difference between the 'normal' ones and the "A" units was a field coil that drew more current, making more magnetic field as the result. The tradeoff is that at idle, the headlight tends to dim as the field goes into full-drain mode, but it also "catches up" quicker after you ride off again. Hence the bigger battery in the "A" bikes: at idle in gear, they draw extra power this way and also because of that high-idle solenoid that helps keep oil PSI up with an extra 150 RPM or so. If ridden like regular bikes (i.e. go to Neutral when stopping), the dimming effect doesn't show up.

[Alan F.]

  Thanks Mark, Happy Holidays.
Alan

[MoMo]

[This all said: far more likely than the alternator or regulator: the rectifiers on these bikes can get damaged if they are charged with a 10 amp or higher battery charger. Check the rectifier FIRST. Search for "check rectifier", as it has been posted here about 20 times. 
[/quote]


Rectifier first-in the almost 20 years I was a mechanic I think I saw one bad alternator, most any bike with a charging problem had a bad rectifier, and there had to be dozens of them, Larry

[maliveline]

ok this  is turning into a major pain in the ass. so heres where I am. I have a low grade volt meter. I think it will measure amps and resistance as well (ohms).

So I was reading more in the manual and I'm starting to figure things out. basically If I pull my alternator cover off and the shifter cover I can unplug the wires comming from my stator coil and my field coil.  From what it looks like in my manual I can just take the 2 test wires from my multimeter and stick them in to the wire harness comming off of my stator coil and my field coil and check for the ohms values. so the ohms value on my stator coil is like .2 and the
and the ohms value on my field coil is 7.2

so i'm a little confused because I know there are 3 wires coming from my stator coil and I only have 2 test wires on my multimeter, so i guess i just check 2 connections and then check the 3rd one? Also I'm guessing its the same way for my field coil but I haven't got a good look at how many wires are coming off my field coil. So anyway I haven't done any of these tests yet and I will exlain why.

Heres where it gets fun. After I started this post I became convinced that I must have a bad rectifier. So I scraped my alternator cover and put a new gasket on it and bolted everything back together without checking anything LOL.

So then I built a rectifier exactly like the one from a post off this site where the guy used to radio shack rectifiers and wired them together so it would work for a 3 phase A/C current. heres a link to the post   http://forums.sohc4.net/index.php?topic=19155.0

So then I put everything back together with my new rectifier and sure enough... my battery is still not getting charged while my bike runs... So I guess my question is
assuming that I wired up my new rectifier correctly does this mean that I either have a bad alternator or a bad voltage regulator?
And I'm pretty confident I would be able to test my alternator with the low grade multimeter I have but I am at a total loss on how to test a voltage regulator
and I am having a hell of a time understanding the information about voltage regulators from the manuals online that i have seen so far...
I just want my bike to work




    

[05c50]

Don't know if this will help, but it might  http://home.earthlink.net/~trinomial/SOHCcharging.html

......Paul

[maliveline]

well I unplugged the rectifier and I checked the 3 A/C wires comming from the stator coil for continuity and I was getting a measurement of like .7 ohms so I'm pretty sure my stator coil is good and i didn't even have to take my alternator cover off again which is nice. I was reading something in that link you posted Paul and I found something interesting.


" The first troubleshooting step is to bypass the regulator. However, before bypassing the regulator, make sure the field coil isn't shorted, because the bypass test might otherwise result in some rather dramatic electrical arcing. Set your test meter to Ohms X 1 and measure the field coil's resistance. Under 2 ohms is a short. Over 7 Ohms is an open. Both are bad. If you replace the field coil, skip ahead to Step Three to make sure the regulator isn’t bad also. Otherwise continue with the regulator bypass. How the bypass is performed depends on whether your system has the rectifier and regulator as separate parts or together in one part. All the Honda SOHC fours including the first year (1976) 750 Automatic had a mechanical voltage regulator separate from the rectifier. Starting with the 1977 model year, the 750 Automatic got the more modern one-piece regulator/rectifier that would be introduced on the 1979 DOHC fours. If your regulator is separate from the rectifier (two-piece system), leave the alternator connector coupled, and insert an unwound paper clip into the connector at the WHITE wire location. Run a jumper wire from the paper clip to the battery's POSITIVE terminal. This bypasses the regulator. If you have a '77 or '78 750 automatic that has the regulator and rectifier in the same housing (one-piece system), put the paper clip and jumper in the same place, but in this case connect the other end of the jumper wire to the battery's NEGATIVE terminal. In either case, with this temporary connection repeat the charge amps test, but this time, DO NOT EXCEED 2000 RPM (to keep from overloading the system during the test). If the alternator starts charging (and it wasn't before), the regulator is the problem. Service (if the regulator is mechanical) or replace (if the later electronic) the regulator and skip ahead to Step Three. "

Well I have a '76 750 automatic and i'm thinking about trying to bypass the regulator. On my bike the regulator and the rectifier are not one piece. So I was reading that part about hooking a paper clip into the connecter at the white wire location. well the white wire is comming from my alternator so i was wondering If i can just run a jumper wire from my positive battery terminal to the white wire that hooks up to my voltage regulator to by pass it. anyway hope to hear some more tips thanks...


***EDITING UPDATE***

Im just now realizing that hondaman covered all this in an earlier reply on this thread.  So hondaman I guess I am kinda confused. can you explain how to do that jumpering procedure between the black and white wires on the voltage regulator. Also is there anyway to check for continuity on the field coil without having to take my alternator cover off again?

[HondaMan]

well I unplugged the rectifier and I checked the 3 A/C wires comming from the stator coil for continuity and I was getting a measurement of like .7 ohms so I'm pretty sure my stator coil is good and i didn't even have to take my alternator cover off again which is nice.

Im just now realizing that hondaman covered all this in an earlier reply on this thread.  So hondaman I guess I am kinda confused. can you explain how to do that jumpering procedure between the black and white wires on the voltage regulator. Also is there anyway to check for continuity on the field coil without having to take my alternator cover off again?

If you have the separate voltage regulator, it will have the 3 wires on it, Black, White, and Green. To do the bypass test on this type of regulator, all you need do is to jumper the White and Black together, which simulates the High Charge mode inside (it's just a fancy relay inside that cover). You can do it with a paper clip, as described above: I do it with a clip lead, like the ones from Radio Shack, with a little alligator clip on each end.

The field coil is also connected to that big white plastic connector at the back of the engine, and is also the same wires as the White and Green at these mechanical voltage regulators. So, just unplug the White from the regulator for the Ohms Test: connect one ohms probe to the engine case and the other to the White wire that is going into the big plastic connector. That's the same circuit, except now you are going through the big connector (not a bad way to check it, too!). 

Many times, this big plastic connector (8 circuits, of which 7 are used) gets really corroded from the soap in the high-pressure carwashes we all use. It might be as simple as a bad connection in that connector. Usually, pulling the connector apart and plugging it back in will straighten things up, at least for a while. I take these apart on every bike I service, though, and scrape the male blades clean with a tiny flat-blade screwdriver. It's cheap insurance...I have seen the wires corroded right off of these connectors, which necessitates a new one (www.vintageconnections.com has them for about $3).

[maliveline]

So basically I tested all the 3 A/c wires in the wiring harness that plugs into the rectifier for continuity and I was getting about .7 ohms to .9 ohms depending how I hooked up the tester wires switching from black to red...

Then I plugged my tester wires into the green and white wires that plug into my voltage regulator and i was getting a reading of .3 ohms to .5 ohms depending on how I hooked up the test wires again switching around black and red. Then I hooked a test wire up to the white wire and touched the other test wire to the alternator cover and I got .3 ohms as well.

I also wired that black and white wires together on my voltage regulator and checked the volts on my battery and it still wasn't getting charged.

So with all this info could someone please help me or possibly tell me what is wrong with my bike

[maliveline]

it says the ohms on my field coil is suppose to be 7.2 in the shop manual. you guys think I have a bad field coil?

[TwoTired]

To test the Green white wires, they must be disconnected from the Vreg.

Check your meter error.  Place the two probes together.  Note the reading.  This value must be subtracted from all ohm readings for that particular meter.  (I don't know if your meter is auto ranging, lots of different meters out there.)

If your ohm reading is still low, then you have to isolate or bypass the wires between Vreg and alternator field coil, test the field coil resistance at the field coil.  If the resistance is still too low as close to the coils as you can get, then the field coil is faulty or a connection to it has a problem that must be isolated from the test.

[maliveline]

i just sent my bike to a shop screw it lol

[andy750]

No patience huh?

I went through the same thing with a CB750K4 and K2. Both not recharging the battery as you describe. The K4 was the rectifier - $50 from Oregon Motorcycles. The K2 was the green alternator wire - it had frayed beneath the gear cover. When wires get replaced it will be fixed.

I hope your shop is as cheap. Owning an old bike means that you need to learn to work on it or have deep pockets to pay the shop costs. The 750 is one of the easier machines to work on - and I say that as an incompetent mechanic . Learn through trial and error...

good luck
Andy

[maliveline]

I gave it the old college try and it just ended up pissing me off for about 3 days. I just sent it to the shop so they could diagnose the problem. Because I pretty much tested everything I could and wasn't comming up with anything and I dont really have a lot of confidence in my piece of #$%* ohm meter to tell me if my stator or my field coil is bad. I would hate to order a new field coil off the internet and hook it up to my bike and it still doesn't work.

[TwoTired]

You can buy pretty good Digital multi-meters these days for well under $25.  I've seen them at Harbor freight for about $2, on occasion.

[maliveline]

well the shop guys just called me and informed me that my stator is not working and they can't help me because that part is not available anymore.
does anyone know a good place to buy one of those besides ebay? has anyone had any experience with http://www.hondapartsnation.com
at all? seems like I could probably buy one from these guys. it only costed me like 40 bucks to get it figured out so oh well.

[TwoTired]

Damn rare that a stator goes bad.  Did it get attacked by road rash or a madman with a hatchet?

They don't destroy themselves even with a directly shorted output.

Bad field coil I can believe.  "Stator problem" makes the eyebrow raise inquisitively.

[MoMo]

Damn rare that a stator goes bad.  Did it get attacked by road rash or a madman with a hatchet?

They don't destroy themselves even with a directly shorted output.

Bad field coil I can believe.  "Stator problem" makes the eyebrow raise inquisitively.

Never saw one go bad either.

[maliveline]

well the guy checked it and said there was no current comming from the stator to the rectifier and he said it was a bad stator so w/e