I have checked the test pin voltages for my keyboard and they are as follows:

pin 4 pin 1 -0.00
pin 4 pin 2 -3.87
pin 4 pin 3 -4.37
pin 4 pin 5 -0.01
pin 4 pin 6 -3.87

Could anyone advise if these values are within limits and, if not, is it an indication of a problem with the system board?

Thanking you.

Considering that keyboards are about $10 I quite certain that people rarely go through the trouble to test the voltage outputs, especially in the old PS2 formats since most are USB now.
But I did find this: IBM Support - Troubleshooting Mouse and keyboard issues (http://www-304.ibm.com/jct01004c/systems/support/supportsite.wss/docdisplay?brandind=5000008&lndocid=MIGR-40293) (scroll down to the table)