When I start my '98 XK8 the motor runs rough, Ck Engine Light on, no reading on Temp gauge, Volt Gauge not reading. Dash reads Battery not charging, Restricted Performance. Just replaced Alt and reads 14.2 at Battery. Battery cks out with 1,000 CCA's, Fully charged.

 

Various aftermarket alternators often cause issues with these cars. Always best to have your factory alternator refurbished....

 

It was acting the same before and after the Alt replacement.

 

Just curious, don't you have temp on dash?, or just not on OBD2 data?
I have same issue except temp gauge works well, just not on OBD2. (only with ELM327)
Changed voltage regulator inside alternator but without error message going away..
My Icarsoft LR2 does not connect with my XKR-yr2000...(can't see ISO9141, think car is causing this...)

 

It doesn't read anything on the dash but does on my iCarsoft i930. It reads all over the board. From 1.2v to 14.4v. Depending on RPM.

 

I have found the problem with the temp gauge not working...Loose connection with Temp Sensor. A spray in male connection and reinstalled.