Classic/Antique Car Repair: Electrical System XJ6 1986, correct polarity, injection computer

Question
Car ran perfectly, let it sit for two months. Removed battery to get it charged, reset battery in box the way it came out, no keys in ignition, no light switches on, connected ground first, the got a big spark on the + pole...  Connected pole. Car cranked but wouldn't start. Noticed the whole dash board (with exception of oil pressure light) is out!   Are 1986 Jags STILL positive ground? How do I trace the problem, what fuses, relays and breakers? Is their some sort of reset button? Where do I start the diagnosis? Stymied.

Answer
Hi Mike,
86 jags never were positive ground. Is that how you connected it? Double check to see that the battery was not charged up backwards. If the battery was stone dead it can be charged backwards. Spark probably came from alternator if connected as positive ground or if battery was charged backwards. If the polarity was wrong either by connecting the battery up backwards or the battery was charged backwards it probably destroyed the diode pack on the firewall and probably the alternator. If this was the case, and you are lucky it may not have fried the ECU (injection computer) or the ignition module.

Take it one step at a time. Check the battery with a volt meter to see if it has the correct polarity and that it is connected up correctly (Negative Ground). I can walk you through all the tests if you want, all you need is a volt meter and a test light. You can contact me at longez8@bellsouth.net
HMF