Help Needed

[benlawlor85]

Hi there,
i have been very stupid, on sunday i changed the timing chain on my R65 mono,
on putting it back together i messed up abit so had to take it apart again and in "trying to do things faster" i didnt disconnect the battery, yeah i know very stupid etc etc.
The diode board arked against the engine casing :-(
when i came to start the bike the Gen light no longer comes on!
any ideas what this could be?
or have i messed up the Diode board?
Kind regards
Ben Lawlor

[Rob Valdez 79 R65]

Shorting out the diode board can cause failure of one or more diodes.

Some of the diodes can be replaced, I have heard.  I have not repaired one, myself.

If you do go this route, remember that diodes need a heat-sink on the wire that is being soldered, as it is a semi-conductor, and these can be damaged by the heat of the soldering iron.

[NC Steve]

All I know about this is that if you forget to disconnect the negative terminal, and slip with an 8mm wrench, thus grounding the battery between the terminal and the bike's frame, the wrench will glow cherry red and bend up just like a banana.   Your battery will also be deader'n a doornail afterwards too, so I'd expect the same from the diode board, which is much less sturdy.

Now, please don't ask me how I know such things....

[Rob Valdez 79 R65]

All I know about this is that if you forget to disconnect the negative terminal, and slip with an 8mm wrench, thus grounding the battery between the terminal and the bike's frame, the wrench will glow cherry red and bend up just like a banana.   Your battery will also be deader'n a doornail afterwards too, so I'd expect the same from the diode board, which is much less sturdy.

Now, please don't ask me how I know such things....

No pictures???    

[Ed Miller]

snip
Now, please don't ask me how I know such things....

No pictures???     [/quote]

...guess what happened to his camera.

Just kidding Steve!

[airhead]

There are only a couple of places on the diode board that can damage the diodes if shorted to ground with the battery connected. The large top plate of the board is the ground so won't do anything. The large plate to the bottom is connected to the battery direct and doesn't pass current through any diodes, so no problems there. Shorting some of the soldered leads on the board can cause problems.
You say the ignition light doesn't light up. This isn't a function of the diode board, so if I were you I'd check the connections again. More specifically the blue wire behind the diode board, and those to the alternator rotor. There is a very good chance there is nothing wrong with the diode board.


Bill..........................;-)

[NC Steve]

snip
Now, please don't ask me how I know such things....

No pictures???    

...guess what happened to his camera.

Just kidding Steve!

[/quote]

No, actually I was so enraptured with the fireworks display, and trying to make it end without 3rd degree burns or electrocution, that I completely forgot about the camera!  

Guess I could have taken a pic of myself 2 days later though, as I walked into Batteries Plus for a new Odyssey gel cell.  
$$$

[nhmaf]

Nope, he said the Gen light doesn't come on, and that generally means that one or more of several possible diodes took the hit.
There are some other smaller diodes on the board which handle the low current charge/Gen light circuit, along with the
large power diodes for the 3 phase current rectification.   The Gen/light circuit is generally handled by the smaller diodes,
so it *might* be that the large rectifiers are OK, or one or more of them could have taken the hit, too.
IF you have a digital multimeter with a diode test function and a  soldering iron and are handy with electrical stuff, we
can help you figure out which needs replacing, though this also means that you are able to buy the replacement diodes....
I gotta dig through my stash of info to see what the options are as I don't recall offhand and am too lazy to go out in
the thunderstorms we're having to pull the front cover on the bike...

You can buy new BMW diode boards for a bit over $110 US, I think, and there is one for sale now on ebay off an 83 R100 that will probably
work, too.

Many people in the US go with the ThunderChild aftermarket replacement board - it drops right on and costs less than the BMW part:
http://tinyurl.com/46fh7r

Unfortunately I think I recall that you are in Britain ?   If so, they probably have a hefty VA Tax on the Thunderchild part..

[benlawlor85]

Hi All,
Thanks for the funny and the serious replies,
i have checked all the contacts over and over again! to no avail!
so im guessing its FCUKed i think i shorted the contact on the left which goes down to the stator!
Im OK with electrics but to be honest cant be bothered so im guessing a new one wudnt be a BAD idea anyway!?
my only other question is could it be anything else?
from what i can see on the wireing diagram i have thats about all....!?
about £50 inc P&P for a board so a bargin considering a litre of fuel is £1.25!!!!!!
and thanks again guys! on the short circuit note i did that to my old MG midget once tighting up the positve batter terminal and hit the body with the spanner! never seen a spanner move to fast  smashed the garage window and shatter the spanner! lol
Ben

P.S am trying to locate a thunderchild one in the UK, any ideas?

[nhmaf]

I am assuming that you do have other lights working on the bike, so that you know at least that:
1) The battery is still OK
2) The fuses aren't blown

It is possible, but somewhat unlikely, that your voltage regulator is blown and failed in an "open" state, such that Gen light
won't come on when simply turning ignition on but not starting the motor - in that instance the light is illuminated because
the battery voltage is causing current to flow through the light to the regulator, and the regulator provides a path to ground
(as well as to the rotor).  If the regulator has no ground at all, this will prevent the light from lighting in this case.

Does the light come on when you just turn the key to the on position ?

[benlawlor85]

Hi,
the bike starts fine and but the light NEVER comes on!
the voltage across the battery is a constant 12ish Volts
i recall that this should be 14ish when the motor is running!?
Thanks for the help!
Ben

[Justin B.]

Ben, I think the generator light is connected to the alternator through one of the small diodes on the diode board which I remember as being fairly easy to replace and easily sourced.  If you have a meter you can test the small ones easily as I don't remember any circuit interaction once the board is unplugged.  I think a suitable sub. would be something like a 1N4001 or 1N4002 rectifier diode.  What the generator light does is supply a "tickler" voltage through the diode to the rotor to turn it into an electromagnet.  Once the alternator is supplying juice the voltage differential across the bulb goes away and the light turns off and I think the diode is to keep any juice from flowing "backwards" through the circuit and running the battery down when things aren't running.  

This is all speculation on my part as it's been over 20 years since I tore the circuit apart and attempted to figure out how it worked but something in the back of my mind tells me this is close.  I'm just rambling again so it would be safe to ignore me - as most knowledgeable folks around here do already...

"Normal" voltage would be probably anything above about 13.5 volts, I think my '95 puts out about 13.9 V at about 4k RPM and that's with a new rotor, brushes, and regulator.

As far as anything else?  If the rotor is open ( as well as bad brushes ) the light will not come on.  

[Rob Valdez 79 R65]

Ben, I think the generator light is connected to the alternator through one of the small diodes on the diode board which I remember as being fairly easy to replace and easily sourced.  If you have a meter you can test the small ones easily as I don't remember any circuit interaction once the board is unplugged.  I think a suitable sub. would be something like a 1N4001 or 1N4002 rectifier diode.  What the generator light does is supply a "tickler" voltage through the diode to the rotor to turn it into an electromagnet.  Once the alternator is supplying juice the voltage differential across the bulb goes away and the light turns off and I think the diode is to keep any juice from flowing "backwards" through the circuit and running the battery down when things aren't running.  

This is all speculation on my part as it's been over 20 years since I tore the circuit apart and attempted to figure out how it worked but something in the back of my mind tells me this is close.  I'm just rambling again so it would be safe to ignore me - as most knowledgeable folks around here do already...

I'm sorry, Justin, did you say something?

[nhmaf]

Yes, if all is as it should be in good condition one should see 13.5 - 14.2 Volts when the engine is running at least 3000 RPM.
Below this, and it will be somewhere in the 12-13V range.

Justin is correct - a simple 1N4001 or preferrably 1N4002 rectifier from Radio Shack could handle the duty of the small diodes (I don't
recall what is in the diode board at the moment, but these would serve as viable substitute).. The large diodes however, you would have
to get from an electronics wholesaler or through a place that rebuilds automotive alternators I think that these are 1N3660 or 1n3661
equivalent power rectifiers.

One thing that you may want to check before you go much further, is to just try to verify that the GEN light bulb, is actually OK.
For the moment, we've been assuming that it is OK and can pass current, but if somehow it is no longer passing current
after all these electrical shenanigans occurred, it can send you barking up the wrong tree.  You can check this by using your
voltmeter, and with the key in the ON position (but don't start the engine) you should be able to see pretty close to 12V at the
D+ terminal of the regulator (if you see 12V on any of the terminals of the regulator - if you can get to them - then the light bulb
is most likely fine).
[EDIT: After thinking this over, I realise that IF the diode board is still connected and IF one or more diodes have failed in a shorted
 condition then the only way to properly perform the above test of the GEN light continuity is to do it with diode board cables disconnected]

It is possible, but the odds are against you that the diode board is OK and it is just the voltage regulator that is dead in a very specific
way that could cause this.  If you have a buddy that will loan you his regulator, all you would have to do is put in in place of yours and
just turn the key on (no need to start engine) and see if the GEN light does come on at that point.    IF that is the case, then
you *might* still have an issue with the diode board, or not, but you definitely had a problem with the regulator.  

IF changing the regulator doesn't make the light come on just by turning the ignition to the ON position, then you do have at least
(1) dead diode (possibly more) in your diode board that are behaving as short circuits.

So, if you can borrow or find a proper regulator cheaply, it *might* enable you to do a bit more diagnoses and possibly eliminate
the diode board as a failed unit, but I don't think that the odds of that being true are very high.    It might not be a bad idea to have
a spare voltage regulator around anyhow, I guess.

Good luck with finding a thunderchild unit over there - I hope that you are successful.   They have a good reputation over here and many
airhead owners use their board in place of the BMW unit whenever something like this happens.

If you are interested in knowing a bit more about how the charging system works, this person has a bit on his website that is a
simplified, but basically correct explanation of the charging system, diode board, etc. functions with a diagram.
http://tinyurl.com/5bppnj

[airhead]

Here is the charging system!!
As you can see, the charge light will illuminate even without the diode board, IF all connections are ok to the rotor and the regulator is ok.
 With the diode board connected and the engine running, once the voltage reaches greater then 12.5v, the light will go out as the potential to the regulator connected side of the light, D+,  equals the 12.5 volts of the battery + terminal. Essentially, the voltage to each side of the lamp is >+12v so there is no potential difference between the lamp terminals.
The only time the lamp can't light with the ignition on and engine not running is if one of the larger B+ diodes to the right is shorted allowing +12v to flow through it and one of the now forward biased smaller diode on D+. The larger diodes normally will fail open circuit (though not always) if asked to carry too much current, as in a short.
I would still suspect something other than the diode board.



Bill............................;-)