I started getting odd electrical faults and traced it to the alternator. I dropped the alternator into a repair shop and was told the parts to repair it are not readily available but the guy would have a go at it for $300. Anyway to cut a long story short I left it with him along with some instructions on where to insert it. I picked up a used one on Ebay and I seem to have the same problem. Can anyone tell me if their bike still runs if they disconnect the battery while the engine is running? I have put my meter on it and don't seem to be getting a charge.

Yes, with a good alternator the bike should still run without a battery.  From what I've read, a lot of race bikes don't have a battery to save on weight.

Do NOT disconnect the battery with the bike running or you WILL blow the ECU computer the ICU computer and myriad other parts like any light bulb that is turned on and quite likely any diodes in the system.
Electrical components that use a 12v supply to run don't like 18-20volts and that is what happens when you disconnect the battery.
Race bikes that have no battery are designed to run without a battery.

To test if you are getting output from the alternator measure the voltage across the battery at 2000rpm it should be between 13.8 and 14.2 volts.
The voltage will drop to 12.2 volts or maybe a bit lower when at idle.

By the way did the auto lecky do as you suggested?  You would be able to tell by the way he walks.

RicK G wrote:Do NOT disconnect the battery with the bike running or you WILL blow the ECU computer the ICU computer and myriad other parts like any light bulb that is turned on and quite likely any diodes in the system.
Electrical components that use a 12v supply to run don't like 18-20volts and that is what happens when you disconnect the battery.
Race bikes that have no battery are designed to run without a battery.

To test if you are getting output from the alternator measure the voltage across the battery at 2000rpm it should be between 13.8 and 14.2 volts.
The voltage will drop to 12.2 volts or maybe a bit lower when at idle.

By the way did the auto lecky do as you suggested?  You would be able to tell by the way he walks.

Bugger! I already did disconnect it with the engine running and it died instantly. Hope nothing blew up!

I will check the voltage tomorrow.

Telling the auto lecky to shove it is starting to look like a bad idea!

Reconnect the battery and try to start it. I am thinking that if the alternator you bought also wasn't working you may be lucky.
Don't try repeatedly to start it will either start immediately or not at all so don't waste your time.
If you do have to fix and or replace the components I reckon you should send duck the bill or he should send you the parts for free.
You should be able to come to a forum like this one and get the right advice or if they know beans about it then say nothing.

duck wrote:Yes, with a good alternator the bike should still run without a battery.  From what I've read, a lot of race bikes don't have a battery to save on weight.

yes that is right duck ...they run a full discharge only system ....usually a li-ion battery at nearly 1/4 the size of a lead acid battery ...and In the ducktail .

there is no charging involved as ...usually they have removed any charging stuff ... alternators as well as they tend to blow up with the high revs with standard gearing and cause too much load to be considered as beneficial In anything under a few hours of running

I think ben kingham has removed starter, alternator , and all the sprag clutch assemblies et al and saves a little over 10 kilos for those components alone  ...or something like that

I concur with rick .....do not run any alternator without a battery as there is no stable voltage reference so it just ramps up after each load change (read-  high - low beam change ..blinker function etc ) until such time as the diodes in the alternator die (rated for 50 odd volts piv )...long after it has killed everything electronic in the system ...( perhaps it should be explained that bmw does have lots of protection in most sub systems  ...ie old school current limit resistors and zenner diodes before the main controlling and conditioning circuits (instruments and stuff ).... BUT there is no protection from over voltage in any of the output switching devices ...so the igmp switchers in the ignition module just curl up and go "fail " ...likewise the efi outputs.)

bigsax  we have seen this before and every auto sparky has unknown issues when it comes to motor bikes ....but if you describe it as a vw alternator ....there is no issue at all ever (and it isn't so different at all )  you will find that they do in fact carry spares at almost every auto electrician in the universe  you could possibly find suitable for our poor old unloved bmw machines

hope that helps

RicK G wrote:Reconnect the battery and try to start it. I am thinking that if the alternator you bought also wasn't working you may be lucky.
Don't try repeatedly to start it will either start immediately or not at all so don't waste your time.
If you do have to fix and or replace the components I reckon you should send duck the bill or he should send you the parts for free.
You should be able to come to a forum like this one and get the right advice or if they know beans about it then say nothing.

Well after several drinking sessions I forced myself to look at the old girl again.

I reconnected the battery and lucky for me she started first turn!

It seems the secondhand alternator I purchased on Ebay is totally dead. The seller was most apologetic and has agreed to refund me.

I purchased another secondhand unit, again on Ebay and I am currently waiting for it to arrive. Unfortunately I will be away for five months chasing the sun ( we are called grey nomads in Australia, I think you Americans call us snow birds or geese) so unless it turns up in the next 5 days it will have to wait until I get back.

Thanks for all the advice Rick and Charlie

Wow that is good news to hear. I wouldn't be buying any lotto tickets for a while all the good luck has been used up.
Where are you heading this year that whole coast gets clogged with the grey nomads.

RicK G wrote:Wow that is good news to hear. I wouldn't be buying any lotto tickets for a while all the good luck has been used up.
Where are you heading this year that whole coast gets clogged with the grey nomads.

We spend the winter in Broome, we have a holiday house there. For those that don't know anything about Broome, it is tropical and the weather from now to October is the dry season. 30 degrees of sunshine every day, best beach in the world (Cable Beach) great fishing, good golf course (well playable but at least it has one). 

I rode a 86 LT up a few years ago and sold it to a local guy. I will probably pay him a visit and beg a ride while I am there.

PS. Don't tell anyone about Broome paradise, we try to keep it a secret 

Having a house there is great. we went in with the caravan and the van parks were a nightmare. They were packed in so close we couldn't even use the annex. We ended up at Exmouth for a couple of months and that was a nice underrated spot.

RicK G wrote:Having a house there is great. we went in with the caravan and the van parks were a nightmare. They were packed in so close we couldn't even use the annex. We ended up at Exmouth for a couple of months and that was a nice underrated spot.

Yep, the secret is out!  You need to book a caravan site a year in advance and go on a waitlist!

The tourist mobs are busy turning it into WA version of the Gold Coast with no idea of where that leads.

Okay really starting to regret telling the auto lecky to ***** off now.

Did not bench test the original alternator, just tested (or so I thought) on the bike and no charge.

Purchased a secondhand unit and got the same result. Purchased another secondhand unit with the same result.

I have one guy hopping mad because I returned an apparently good working alternator to him, one auto lecky that has more than likely tossed my original alternator and a dead bike.

I am at a total loss to know what to do next.

The positive and earth leads are good, the battery is good, yet when I run the bike and test the voltage at the battery it is like there is no alternator attached at all.

Anyone with any idea what is going on at all?

Does the charge light come on when you turn the ignition and kill switch on?
The charge light must come on to give the alternator enough power to get going.
There is a red wire from the Alternator to the battery, replace it with a temp connection, the original may be broken.
Is the Alternator actually turning?

When you fired up the bike did you rev it to 2,000rpm? The alternator does not kick in if you fire it up and go straight to idle......I think you get a zero reading if you do that. Just check you have an alternator light coming on and going out.

Yep the alternator is turning, I actually thought of the pulley spline stripping or something. But no spinning fine.

The battery light comes on when you turn the ignition on and does actually go out when you rev it to about 3000 rpm but there is no change in the current at the battery and (as i found out) if the battery is disconnected while running the engine dies immediately.

Whatever you do, don't ever disconnect the battery while its running or try to run it without the battery.

I would expect the red light to remain on if the alternator was faulty. RickG mentioned a wiring issue and it seems  to point to something like that. Run the temporary wire as he suggested and check the reading with a meter. Do not disconnect the battery as a test because if its charging the disconnection will fry the electrics- a damaged wire to the positive terminal would allow the red light go out but show zero at the battery.

The only thing I can think of is if the red wire from the alternator to the battery is open circuit. That is the wire with the big spade terminal in the 2 pin plug in the connector at the alternator to the positive terminal on the battery.  The positive connector should be 3 red wires all going into the one connector.
If that was broken it would explain the charge light going out without charge going into the battery.

On my K the connector to the battery terminal is crimped on to the charging wire from the alternator...On the LT that connection came loose and you could turn the connector on the cable meaning there was a bad electrical contact. I had to squeeze it up to make it good. Constant removal and refitting could cause an internal break in that cable.