That's great Paul, thanks so much

 

Coolant temperature sensor for Marelli ignition ecu on 1992 5.3 is located on A bank water rail near thermostat. Sensor resistance varies with temperature:
10℃ resistance 4.4Kohms
20℃ res 2.5K
30℃ res 1.6K
40℃ res 1.2K
50℃ res 840 ohms
60℃ res 630 ohms
70℃ res 470 ohms
80℃ res 320 ohms
90℃ res 210 ohms
This sensor is used by ignition ecu to trim ignition timing based on engine coolant temperature.

Engine coolant sensor for Lucas fuel ecu (26cu on 92 model year 5.3) is situated on B bank thermostat water rail.
-10℃ res 9.2 Kohms
0℃ red 5.9 Kohms
10℃ res 5.9 Kohms
20℃ res 2.5 Kohms
30℃ res 1.7 Kohms
40℃ res 1.18 Kohms
50℃ res 840 ohms
60℃ res 600 ohms
70℃ res 435 ohms
80℃ res 325 ohms
90℃ res 250 ohms
100℃ res 190 ohms

This sensor is used by fuel ecu to provide temperature based mixture enrichment during starting and warm up phase by increasing the injector 'on' duration.

 

I had similar issues when I first bought my 92 XJS V12. I was throwing codes periodically and had issues starting the car. I spent a lot of time going to each ground point in the car, disassembling them, cleaning and reassembling with dielectric grease. Then I installed an Optima battery. These two thing cleared up all of the codes as i had consistent voltage and the car started most of the time.

But, it still did not start occasionally. I was talking with Jim at Jim's Jag Shop in Dallas and he said to check the starter relay. It is located on the passenger side of the car under the hood protected by a black case. The starter relay is the one with the white wire with a red stripe on it. I took that relay out, cleaned the socket and the contacts, added dielectric grease, and the car has started every time since.

 

Thanks dutkowsky. Changing the relay solved my starting issues as well actually. Crazy it was something so small after months of trying. I am still getting one code, but I am hoping to track it down soon. Thanks again!