P1698 check engine and speedo jumping

Hello everyone. I purchased my first xj a couple months back. I have encountered a issue. My check engine light came on the other day while driving and my speedometer started to jump to 100mph. I replaced the speed sensor because I thought that may have been the problem but there has been no change. Any assistance would be great. It is a 98 4.0 sport. Thanks

thanks for all the help. I replaced the alternator and it fixed all my problems.