As part of putting ‘Heidi’ back on the road after her 4 year rest, she has been stripped down and rebuilt (engine removed from frame level of strip down). Amongst other things, she was treated to a new 30AH Motobatt AGM battery and I replaced my alternator with a shiny spare one I had.

Battery fully charged before connection…a bit of cranking to get fuel though (tank, fuel pump, injectors etc had all been off during the refurb) and engine fired up. Voltmeter showed Alternator kicking out 13V+ and 14V+ when revs increased to 2000rpm. All looked good and she was running sweetly on a short test ride. ABS brain successfully reset.

Imagine my surprise the following day when went to start again and there was nothing doing. Checked battery volts - it had dropped to 6.5V. Bugger! Battery disconnected and recharged. Happily, it held 12.9V when disconnected. Hopefully no damage done.

So…electrical investigation ensued. What was flattening the brand new battery so quickly, with ignition off?

First things first, a forum search…some good testing advice found and bought myself one of these gizmos from Amazon…(thanks for the suggestion Side Car Paul & Arby)



Tested each fuse position in turn. With ignition off, the only current draw was max 0.02A which I associated with the clock (20mA is a bit high though). I have a few separately fused auxiliary circuits I have added (only live when ignition on) - things like gps, fan override and rear fog light. They all showed 0A draw.

Disconnected battery positive terminal and connected my Fluke 179 test meter between battery and positive wires…first on 10A range and then on 400mA range. Fluke showed 0.01A…however, missed the clue that the clock was off and totally puzzled how 0.02/0.01A could be draining a 30Ah battery in 12h. Discovered later that both the 11A and 400mA fuses in the Fluke had blown, hence no current flow, no clock indication and a spurious 0.01A reading.

Much pawing over the forum tech page electrical diagrams, particularly the battery flat version which shows the permanently live (unfused) components. ABS doesn’t seem to be shown, but RickG comment in one of my searches suggested that the ABS brain can develop a fault that cause battery drain. I believe in easy first. ABS Brain disconnected and, with fully recharged battery connected, I waited and monitored battery voltage which continued to steadily fall over next 6 hours. Incidentally, RickG had said his fault was associated with an ABS fault #7 tasting was hard to clear. Mine had had a fault #4 (rear sensor) which I had replaced and successfully cleared the fault.

Side fairing and fuel tank off. Next check was the relays…had one of those welded on and drawing current? All relays checked out ok. I can just get to the relays by lifting the rear of the tank, but it is awkward, so easier to pop the tank off.

More pawing over the electrical diagrams and scratching of my head. More checking of currents with plug in ammeter and Fluke. This is when I realised the clock wasn’t working with the Fluke in circuit. Checked Fluke fuses - both blown. Spares ordered from Amazon (£11 for two fuses  ) and battery recharged while waiting for the next day delivery.

While waiting, cleaned up ignition switch and checked with Fluke (only the ammeter function wasn’t working). No problem with ignition switch. 

Starting to think my new motobatt was the problem…charges ok but lacking capacity. At an eye watering £145 (motorworks) it was going to go back, if that was the case. Easy way to check for sure. New battery removed from Heidi and swapped with Gretel’s older motobatt. Both batteries connected and went out for a long walk in the Spring sunshine, feeling quite confident that I had sussed out the problem.

9.5 miles and a few hours later…voltmeter on Gretel (new battery) - no voltage drop from initial readings. Voltmeter on Heidi (older battery) - voltage had dropped. Agggghhhhhh!!!!!! So, diagnosis is the new motobatt is not defective and there is definely a fault somewhere on Heidi’s circuits.  Put the new motobatt back on Heidi and let Gretel have her old, but fully functional, battery back.

More pawing over the electrical circuit diagrams. New fuses in my Fluke and put it into circuit as ammeter. Aha moment, detected a current draw of 2A (2A drain on a 30Ah battery…ties in with battery discharging in about 12h). 

Started rummaging in the nest of wires and plugs under the tank. Earth connections rechecked. All wires disconnected in turn (there are a lot of them) while watching the ammeter. Nothing found. After market ‘Oxford’ heated grips do have a permanently live supply to their control unit (only draws current when turned on) and no impact on ammeter reading when unplugged.

Final check…unplugged alternator. Bingo - ammeter reading dropped to 0.01A. Plugged back in, 2A, disconnected 0.01A. Found the issue at last! Checked voltage at alternator plug. Red wire 12.8V Blue wire 0V (ignition off), both 12.8V with ignition on. Removed alternator voltage regulator and replaced with one from old alternator. Rechecked with ammeter…still 2A…so fault lies within the alternator itself. Reassembled old alternator, with its original voltage regulator. Refitted to Heidi, plugged in and checked current with ammeter…0.01A current draw. Fuel tank back on (temporarily), engine fired up, alternator outputting correct voltages.

Recorded battery voltage with everything connected. 12h later, rechecked and voltage has remained at 12.8V. Now to put the bike back together and get her MoT’d. What a saga…particularly as there was no need for me to change the alternator in the first place…it just seemed like a good idea as I had a shiny (second hand ) spare and why not while the bike was in any pieces? Now I know why not…the old adage. “If it ain’t broken, don’t fix it!” 

It was a long slog getting to the root of the problem. As with all things electrical, the investigation needs to be methodical and the test equipment needs to be working! Moral of that story - check all indication (e.g. clock, in this case). Unless you’re lucky, it takes a lot of time to find the fault. In slower time, I’m going to pull the defective alternator apart and find out why. 

Once again, the K100 forum and its search function, proved invaluable to me. Hopefully, this post will help someone else in the future.

Do you have a 33amp or a 50amp alternator in there? If you have a shot 50amp one, just for the hell of it you can upgrade it to 60amps using an R/GS or similar alternator because getting an R/GS alternator from ebay willl probably be cheaper than getting the 50amp one rewound. Knock the front mounting plate off the R/GS alternator and replace it with the K mounting plate. The only mod you need to do is to insert a 1.0mm thick M5 washer between the K mounting plate and the R/GS casing to prevent the front bearing from binding when you tighten the bolts to pull the two halves together.

Mind you, getting the mounting plates off the bearings can be a bit of a b*st*rd. I've upgraded two 50amp alternators this way - one that was dead and one that was very much alive .

Thanks Dai - mine are all 33A versions.

I dismantled the suspect one at lunchtime. Getting the shock carrier and fan off was hard work…well in excess of the 44Nm torque (my 110Nm torque wrench wouldn’t touch it). It was then quite a lot more effort to get carrier off the shaft. Immediately suspected previous foul play when I found there was no key in the key way…previous person probably lost it and glued carrier to shaft, backed up by torquing the pants off the securing nut.

Lots of crap and gunge inside (no frogs!) and found issues with one of diodes and stator winding. No idea of the provenance of the alternator, it came as a spare (along with a lot of spares) with Gretel. Looked great on the outside…but looks can be deceiving. Consigned to my scrap metal bin. 

I have another spare (from donor bike), if needed in the future.

Interestingly, Heidi’s original ( which is back on her now) had a capacitor bolted onto the end cover and plugged into the spade terminal (near the main supply and exciter supply plug). Neither Haynes nor Clymer manuals show or describe this capacitor. It is shown in both k100 tech page articles on alternators. Neither my donor bike or shiny (now scrapped) spare alternators had this capacitor fitted. I definitely did not take it off the donor spare.

 Is it standard, an upgrade or a old model?

Those capacitors were often used as noise filters to reduce noise in radios.  I could be that you alternator may have come off of a police bike.

Point-Seven-five wrote:Those capacitors were often used as noise filters to reduce noise in radios.  I could be that you alternator may have come off of a police bike.

Ah…that makes a lot of sense. Thank you. I wasn’t even thinking along those lines.

My bikes are LTs which, originally (but long since removed by previous owners) had radio/cassette players installed inconveniently in the left hand fairing glove box. It is likely that the capacitor was included as original fit to reduce interference with the radio.

 I’m now having flash backs to fitting a radio/cassette to one of my sister’s cars back in the very early ‘80s and having to fit ‘suppressors’ to her HT leads to stop interference. 

My donor bike (also an LT) alternator doesn’t have the capacitor, but was a higher mileage bike with the radio/cassette already long gone when I got her and may have had the alternator changed at some point by a previous owner.

The regulator is a known issue and seems to be happening more.

You can get them really really cheap from tills.de and I put a few of them in the shed as they are so easy to change.

I the event of a parasitic drain of any kind I now figure this should be one of the first checks for the cause.