Since I discover a lot of information along the way, I would recommend reading the entire instructable before attempting to do anything shown here.
Doing anything to, or even looking at an old computer could cause it to stop working. Please do not attempt anything unless you are aware of the possible results:
A computer that is still broken, broken even worse than before, fire, smoke, etc.
I am now (painfully) aware that similar work on the DS 1287 has been done before, but since I couldn't readily find the information (because I was searching for IBM 55SX error codes and CMOS problems), I am presenting my own version, and then some observations to help you do a cleaner, safer job.
Step 1: The discovery, and intro to problems
I've been riding this around in the van, in Florida, until last week. I had to finally clean it out (there may be an instructable on that too) because a water leak was growing a mold colony in the carpet.
Anyway , I grabbed this machine out and stuck it in the garage. After a few days there, I had some free time and decided to see if it even powered up.
To my surprise, it did, so I grabbed a monitor, mouse and keyboard and hooked them up.
I was getting errors 161 and 162, and then a crude picture basically telling me something was not OK.
I went online and found out that the codes are:
161 System Options Not Set-(Run SETUP); Dead battery
162 System Options Not Set-(Run SETUP);CMOS checksum/configuration error
I never had an IBM back in the day. I was a commodore guy, so this was my first trip down PS2 memory lane.
I learned that I needed a reference disk, which I downloaded at:
IBM PS/2 Model 55SX
There are also several other places to get this disk.
After getting the disk running, an IBM program took me through some steps, then asked to restart the system.
After the restart, the problem errors repeated. I left the computer on for a couple of hours (hoping to get a charge into the CMOS battery), then tried it again with the same results.
I did more online reading, and discovered that the CMOS battery was not only dead, but obsolete as well. They offer an updated replacement from the company that made the originals, but it is not guaranteed to work.
Link updated 10/4/2012
Dead link: www.maxim-ic.com/appnotes.cfm/an_pk/503
Replacing the DS 1287
After reading this, I decided to look around to see if I could find which pins connected to the battery.
The pinout guides are all pretty bad, and even the best ones that I could find didn't have the battery input terminals labeled.
This leads me to my next course of action.