cb750 rectifier

[ogre]

hi all on my 1970 cb750 just bought new battery before connecting it checked to see if i was getting a drain connected positive terminal and put meter between earth on battery and earth lead off the bike not connected got 12v its a new wiring loom disconnected rectifier no volts so took the sleeving off the wires of rectifier must of got hot in the past and melted so seperated the wires connected it back up getting 12v again am i right in thinkin the rectifier is shortening between the red and green through the rectifier or travelling straight thru the diodes in it
any help would be appreciated
thanks

[LesterPiglet]

It's hard to understand what you have done there without putting a few full-stops in. I'm not sure what you have connected to what.
If you have the + terminal of the battery connected and your - not,  then put your tester between the - battery terminal and the earth lead you will read 12v.

[ogre]

sos for the grammar.yes i have the negative disconnected ignition off and meter between negative on the battery and earth strap to me it should read 0 volts. i did this test a few years back on my 750 supermagna cos everytime i come to the bike the battery was flat .i found i was gettin about 5 volts reading on the meter finally traced it to the starter solenoid it was shorting inside it.

[Lobo]

Don't think you're performing this test correctly. As you described it, I'd expect zero volts reading with the ignition switch OFF (because open circuit)...  & 12v with switch ON, because you're now reading the potential diff across the battery.

What you should be doing is setting your meter scale to AMPS (10A range), and connecting it as you describe... ie in between the battery +ve & red lead, OR .... -ve battery & earth strap. (Ie in series).

The following all engine not running... and assuming all switches, INCLUDING the kill switch, in good working order. Any current drain less than 50mA is basically OK, tho' zero is the goal.

Remember any non-standard mods... eg alarm fitted, side stand warning switch, lights perm ON, ... whatever... ensure these circuits are disabled, eg alarm fuse out, side stand up, bulbs removed... what have you.

With the ignition switch OFF, you should have a zero Amp drain (assuming you've no alarm, etc) If you have got a significant reading its likely your ignition switch is faulty... try disconnecting it at its loom plug.

With the ignition switch ON, there will now be a 'drain', according to which circuits are 'ON'.

With ign ON and all switches OFF (including the kill switch) only the alternator field coil should be energised, and this will be drawing about 1.6Amps. This is normal.

If you now disconnect the Alternator loom plug (Yellow/Yellow/Yellow/White/Green) you will isolate the field coil, and should now see a '0 Amp' drain.

If still registering something then there's a current leak somewhere... perhaps indeed signifying a defective rectifier.... in which case now disconnect this loom plug.

If still registering a current drain try now isolating the voltage regulator .... simply detach the black lead.

If still registering a current drain there's a problem outside of the charging system, and you'll have to start disconnecting more items to 'zero' that ammeter...

Hope this makes sense / is of use.

Simon

[ogre]

thanks for the reply but with the ignition off and my meter between the earth of the bike and battery earth i'm getting 12v am i right in thinking something is giving the ground side a 12v supply which it shouldnt.it was an auto electrician who told me this a few years ago. could be doing it wrong but funnily enough about 12 months ago our fork lift truck kept draining the battery over a couple of days i did the same test on it sure enough got a 10v reading across the disconnected earth.the flt guy couldnt find the fault we ended up putting a kill switch from the battery cured the problem sos for goin on a bit. 

[ogre]

hi disconnected rectifier and connected a wire from my liesure battery to the red wire of the rectifier and connected my meter to the green [earth] wire of the rectifier and to the negative of the battery and getting 12v is this normal.yet when i test for infinity or continuity between the red and green wire with no power to rectifier no reading. 

[Lobo]

Hi again.

Think we're talking crossed wires here (hah!).  Slightly confused, but until someone with proper quals / genuine Honda experience (= Steve / Brian / Oddjob etc) step in I would expect a zero V reading with a voltmeter in series & ignition OFF. If you're getting a 12v (or 10v) reading it really does suggest a 'leak' somewhere. Did you try disconnecting the ignition switch & redoing the test?

As for the rectifier test; no battery required. On your meter there is likely a diode setting... a triangle on its side with a line across its bottom icon. Connect the red & green wires to your meter ... one way open circuit, the other way (ie meter connections swapped) should give a reading of abouts 0.45 if I remember. This would indicate a healthy rectifier.

But honestly, you really need to use the ammeter setting to verify current drain, and troubleshoot the source as I suggested above.

[ogre]

hi this is the meter i am using as you can see the ammeter part only reads ac i think so hae to go off the volts.did try it but no reading.with the positive connected to the battery on the bike though the red and white connects straight to the rectifier permanently. so the test i did with the leisure battery shows the 12v is coming from the green wire on the rectifier just dont know if it normal.   

[ogre]

dont know why some of my last post letters are showing in red?

[K2-K6]

Have a look at "how to measure amperage" on Wiki-How as there is a small vid to demonstrate on an 18v drill with dc supply that may help you.

[ogre]

thanks for that basically the same as i have tried with no reading  maybe with the bike being 40 yrs old its wired slightly different to the modern ones i have tested.on the wiring diagram of the rectifier it looks like the voltage shouldn't go past the diode one way but aloud to go back up the red wire to the battery to charge it. 

[Lobo]

Ok... the official CB750 page on testing rectifiers.

Your meter, whilst good for other things is not really suited to automotive trouble shooting (specifically DC) ... can you bludge a better one off a mate for a while?

And K2s suggestion of Wikipedia is great... have you tried diode (etc) testing there?

Good luck

[ogre]

yes think ur right about the meter with being in engineering the tools i have acquired over the years have tended to be more industrial the test meter for example.i have got a little cheap one lying around somewhere.