50 Amp Alternator Upgrade Kit

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50 Amp Alternator Upgrade Kit

This is a common upgrade for K bikes and relatively easy to do. It takes roughly an hour or so and the kits I have include everything you need: a working low mile 50A alternator (over $1,100 new from BMW!), heavy duty electrical cable, terminal and complete easy to follow instructions. You don't need to be an electrical whiz to do this.

The original K bikes came with a 32 Amp alternator. This is more than adequate for running the bike, heated grips, etcetera but once you start upgrading to a higher wattage headlight (don't do this unless you install headlight relays first), using a heated vest or jacket liner or running auxiliary fog or driving lights the alternator can become overtaxed and the accessories will start draining your battery. In 1993, BMW started upgrading K bikes to a 50 Amp alternator which provides ample extra electrical power for adding "unlimited" electrical accessories.

For example, my 1993 K1100LT came from the factory with a 32 amp alternator. After performing this upgrade I'm now able to run a higher wattage headlight bulb, a pair of 70 watt fog lights, an MP3 CD head unit, a heated jacket liner, heated boot insoles, and heated grips with no problems. If I tried running all of that with my old alternator my battery would be dead before I could ride a few dozen miles.

The table below shows which bikes came from the factory with a 32A alternator and which bikes came with a 50A alternator:

K Bike Model	32 Amp Alternator	50 Amp Alternator
K75	Model year 93 and earlier	Model year 94 and later
K75C	All model years
K75S	Model year 93 and earlier	Model year 94 and later
K75RT	Model year 93 and earlier	Model year 94 and later
K100	All model years
K100RS	All model years
K100RT	All model years
K100LT	All model years
K1	All model years
K100RS(4 Valve)	All model years
K1100LT	Model year 93 and earlier	Model year 94 and later
K1100RS		All model years

Alternator Identification: It's easy to tell if you currently have a 32 or 50 amp alternator by visual inspection. The back of the alternator can be seen right in front of the coolant reservoir:

The back end of the 32 amp alternator is metal. The back end of the 50 amp alternator has a round black plastic cover which, if it hasn't worn off, will have "50" on it.

If you'd like a 50 amp alternator, you can email me by clicking this link:

mailto:smithduck@gmail.com?subject=K Bike Parts: 50A Alternator Upgrade

IMPORTANT - Coolant Reservoir: If you have an early K75 or K100 that has a clear hose with hose clamps on the side of the coolant reservoir then you will also need to replace the coolant reservoir as the 50 amp alternator is slightly longer than the 32 amp one.

Here are the three types of coolant reservoir used in K bikes from 1983 to 1997:

Type 1 "Early"	Type 2	Type 3 "Late"

This will not work with the 50A alternator and will need to be replaced with a Type 2 or Type 3.	This will work with the 50A alternator.	This will work with the 50A alternator.
The clear hose sticks out too far to fit in conjunction with a 50A alternator.	This will fit pretty well but it needs a little nudge.	This is the reservoir that BMW redesigned to work with the 50A alternator. BMW moved the spout from the front to the rear.

NOTE: You may want to consider installing new rubber bushings that cushion the alternator drive when upgrading your alternator. The BMW part number for them is 12311464877. You’ll need three of them and they cost about $19 each new from BMW.

Even though the 32A and 50A have different driving "cups" at the front they both use the same rubber bushings so when upgrading you can re-use the old ones from the 32A alternator with the 50A alternator.

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