Can anyone explain why my battery tender will not charge the battery through the accessory port on a 1991 K1, but it will on an identical port on my 2002 K1200. I have checked the accessory port and it is live and have made sure the terminals within the port are contacting the charger. Any ideas? Thanks

Does the accessory port supply power when the ignition is on/off? Have you tested the port? Your question is so vague that no one could answer without looking at it.

Hey BobT, there is always power to the accessory port, and yes, I have checked that it is live. I have even removed it from the bike and put the port into the circuit for my K1200 and it works perfectly so the problem may lie in how BMW set up the accessory port in '91. Maybe it is unable to take a battery tender? An option is to simply disconnect the wiring at the back of the port and fabricate my own fused wiring direct to battery.

If that port is working fine and supplying power with the ignition off then it is connected to the battery. There is no reason why the battery should not charge unless your charger/tender is not working.

We're talking about two different bikes with two different batteries.  Maybe the battery in the 91 is bad and won't take a charge.

kioolt wrote:We're talking about two different bikes with two different batteries.  Maybe the battery in the 91 is bad and won't take a charge.

If that was the case then it would not start the bike!

I can feel your pain.

The accessory port on my K75RT ignition coil cover has been giving me fits for the past couple years.  In order to get my charger to go into the charge mode I had to clean the wiring connection on the back of the port.  Even then, I still have to fiddle around with the charger's plug wiggling it around in the port until it makes contact.  The problem is not in the plug, because it works flawlessly in the accessory ports on my other two bikes.

Part of the problem may be corrosion in the port, cleaning mine helped, but didn't completely cure the problem. 

Another issue I have noticed is that my batteries do not discharge enough between charging to reliably bring my charger out of float mode and into charge mode.  I have found it necessary in some cases to turn on the ignition and headlight and even to bump the starter to lower the terminal voltage enough to enter charge mode.

BobT wrote:

kioolt wrote:We're talking about two different bikes with two different batteries.  Maybe the battery in the 91 is bad and won't take a charge.

If that was the case then it would not start the bike!

I don't see anywhere that the original poster said it will start the bike.

kioolt wrote:

BobT wrote:

kioolt wrote:We're talking about two different bikes with two different batteries.  Maybe the battery in the 91 is bad and won't take a charge.

If that was the case then it would not start the bike!

I don't see anywhere that the original poster said it will start the bike.

And the topic is not "my bike won't start" either.

There's just not enough information in the original post to know for sure of anything.

kioolt wrote:We're talking about two different bikes with two different batteries.  Maybe the battery in the 91 is bad and won't take a charge.

is it the same battery tender? these have been known to fail.
test, test, test

Yes the bike will start the bike. Yes the battery is well charged. Yes the battery is not very old - two years max. Yes the battery tender acknowledges there is a connection regardless of whether charging is required by illuminating a green light. On the K1200 a connection is illuminated. On the K100 it is not.
Thank you Point-Seven-five for some constructive feedback.

Thank you Point-Seven-five, I explored the back of the port further and found there was slight movement of the negative spade terminal where it attaches to the body of the port. A good tug and out it came, so after a good clean with a small file I pushed it back and secured it with some solder and all is good. Charging beautifully. 
Thanks again

On your charger, does the green light tell that it is connected to the battery or that the mains power is on? 

On my charger, the green indicates mains power is on, red indicates charging, and when the light goes back to green charging is complete and the charger is in float mode.  The only way I could tell for sure my charger is connected to the battery is to get it to go into charge mode.

Another thing, cleaning the terminal with a file removes the protective plating, and will allow more corrosion in the future.  Get some dielectric grease and put it on the terminal.

In the future, it would be a good idea to get a can of DeOxit D5 cleaner and use that.  That stuff has fixed more electrical problems on my bikes than anything else in my tool chest.

Cheers 7-5 thanks for the advice. The charger I have shows no illuminated light unless there is a good connection to the battery via whatever method the owner chooses. So, the charger can be plugged in and turned on at the wall, but no light on until connected to the motorcycle. Once connected it will show half charge and later, full charge.