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Discussion Starter #1
There are many things that make the K75 & K100 radically different motorcycles that they are. aside from the horizontal inline engine, is its charging system.

You have to give attention to the selection of a new battery for your bike. 12v 12Ah battery is not adequate for proper function. If such a battery is installed in your bike, there will be false warning lights, Plausible ABS malfunction, Voltage regulator damage and possible damage to the I.C.U. or the F.C.U. (L.E. Jetronic). For bikes made 1983 - 86 BMW specifies 20Ah from 1987 to 1996 25Ah is specified. If a Optimal battery cannot be found a 18Ah battery will work and will sustain proper function. A K100LT with the addition of optional equipment may requite a 30Ah battery. None the less the batteries BMW specifies are a high Ah they are still 12.v

If you are using a Typical Lead acid battery. With your hydrometer the specific gravity of the electrolyte should be 1.28 @ 20°C (68°F). Remember when servicing a battery only add more electrolyte if there has been spill or overflow. If you Just want to top off the batter use only Distilled water.

The alternators in these bikes are rebuildable and are by far cheaper to rebuild. places such as IRV Seaver BMW in SFO or GO AZ in PHX command as much as $300 to $900 and higher for a alternator. Rather if you need a new alternator or just parts you will need the VIN number or the old alternator present at the parts counter because there are many different alternators for these bikes and they are not all equal.

If your rebuilding you alternator this is what you will need to know. Its three phase EMS (Electromagnet System), Meaning there are three coils of wire and nine diodes. The rotor will have 2 slip rings and 2 brushes. The whole system begins charging at 900 - 1,000 RPM with a out put of 13.7v - 14.5v (DC voltage). With your alternator dissembled. The slip rings on the rotor should be no less then 26.8mm (1.055'') new should be 27.8mm (1.094'') and should be no more out of round then .03mm (.001''). The brushes should protrude no less then 5mm (.196'') new 10mm (.393''). The two sliprings / exciter windings should offer no more resistance then 3.6Ω to 4.4Ω. The stator windings should offer a resistance .252Ω to .308Ω.

As stated above the charging system starts charging at 900 - 1,000 RPM. This is also the range you want you bike to idle with out the fast idle.

Thanks again MC forum readers and BMW riders i hope you find this information useful.
 

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..command as much as $300 to $900 and higher for a alternator....
lol...BMW... There must be SOMETHING amazingly superior about these bikes that makes BMW afficionados willing to tolerate ...this.

Getting rolling here, there's no need (in this century) to use a hygrometer on a lead acid battery. Battery voltage directly reflects state of charge, the EXACT same thing a hygrometer shows you. Hygrometers were something your great grandfather used because he didn't OWN a voltmeter. For lead-acid:

12.6V and above...fully charged.
12.3 50% discharged
12.1 90% discharged.

for Lithium, etc. Google the appropriate #'s. And remember, the CHARGING system on your bike was most likely DESIGNED with lead-acid in mind before you pop for a LiFeP04...at 4x the price.

The only conceivable time ANYONE would add acid (electrolyte) to a battery would be if you spilled the entire contents. Otherwise just add water and put it on a charger, lead sulfate will be reduced to lead plus sulfuric acid. In other words, it will make its own electrolyte.

As an electrical engineer with fuel injection ECM design experience I SERIOUSLY DOUBT that installing a 12AH battery, versus an 18 or 20 is going to lead to many of the malfunctions you list, except possibly an ABS light.

On what basis do you make this claim? BMW NOTAM?

A larger battery is needed A) For starting on cold mornings, when a 20AH battery may only deliver 10AH, and B) To keep the voltage from sagging during extended periods of idling when the alternator indeed cannot keep up with demand. Perhaps on a freezing morning, bike at low idle, the ABS pump might not be able to get enough current to operate properly with "only" a 12AH battery.

Or if the low/high idle is mis-adjusted below where the alternator provides any significant output...

As for "Voltage regulator damage and possible damage to the I.C.U. or the F.C.U..." from using a too-small battery? I seriously doubt it. Seriously.

Battery is like a swimming pool. A larger Amp-hour battery is simply a larger swimming pool. It takes longer to drain (voltage to go down) than a smaller one. That's all. (FWIW, an OLD battery is an even smaller pool with rust/scale in the pipes that make it fill and drain even slower)

As far as alternator rebuilding realistically there are two ways to go. One is to pay a professional to rebuild it for you. The other is to hope you can get away with JUST installing new brushes and do it yourself.

Correctly diagnosing what's wrong with your charging system is the first step. Diodes are dirt cheap, and if you CORRECTLY diagnose diodes are all it needs, because it's only putting out 1/3 or 2/3 of it's rated output, you can save a lot of $ and effort over a rebuild. And no, there's no need to pay BMW $10 apiece for diodes you can get for a buck from any rebuilder/ebay/Digikey...

Never "throw" parts at a bike to try and fix a charging problem. Either diagnose it yourself or pay a qualified tech a 1/2 hour of labor to properly diagnose it before buying/rebuilding ANYTHING, alternator, regulator, ... you'll save far more than his $50 in labor.
 

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Discussion Starter #3
The information above is actual BMW technical data, Not numbers I've acquired in the process of maintaining and repairing my own machine and others. Yes I have seen an 12Ah battery in a K75 / K100, this is why you don't go to the local auto parts and get a battery or one they say fits. In the machines I've seen with a 12Ah battery. One the ABS was giving false warning light, Another the fuel pump failed Before 17,000 mi. In another instance both hall sensors and coolant temperature sensor failed. Best of all Fuel and Ignition failed together I.C.U and F.C.U with the combination of an 12Ah battery.

As far as using a current tester to check a battery that will only tells you the battery voltage. not the condition of the battery electrolyte. the series of K bike I've mention in my two post Lead acid batteries most prevalent, sealed or dry cell. AGM batteries were still military technology at the time. Although I've never seen a technical bulletin I have heard BMW technicians discourage the use of a AGM battery because of electrical problems and charging issues.

Its is cheaper to rebuild the alternator in this bike. Aside from $300 to $900 for a new alternator from the BMW dealer, most any junkyard wants there about $150 if there not out for a scam and you'll get stuck with a part from a bike thats been left to rot in the weather, beemer boneyard you're lucky if can find one in stock. Replacing the rotor and stator windings would be close to $300, although the more common repair replacing a voltage regulator and brush pack less than $150. the diodes BMW doesn't sell in fact ive never seen any of the motorcycle manufactures sell the individual diodes for a stator or alternator.

Throwing parts at a problem, not good practice. But to have technical data available rather then just judgement of Burnt, Bent or Broke makes the work effort more effective. $50 for a half hour labor sounds like a hack garage with no factory support. last BMW dealer I checked with $130/hr.
 

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Very interesting, hope I don't have to rebuild the alternator, but nice to know I could do it if I had to. My bike's battery has been sitting for several years. Can I resurrect the battery, or should I just pony up and buy a new one?
 

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Discussion Starter #5
After a battery sets for a while sulfatination occurs. There is a way to fix this but its dangerous and theres Hazmat to dispose of. I would get a new battery, dont just get one from batteries plus or the autoparts store. If the wrong battery is installed not only will it hurt its electronics but it can cause the ABS to malfunction. Spend the extra money and buy a bmw or bosch battery. Your bike will be happy.
 
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