Jaguar Repair: Jaguar xjs V12 wont turn over., jaguar xjs, battery gauge

Question
Hi Howard, I just received a Jag for free. The previous owner had given up on it after replacing the alternator. At first it had a problem starting, everything was checked twice, the best suggestion the mechanic could give was to replace the alternator, then after it was replaced the problem continued until it eventually stopped starting all together. Now when I turn the key I hear a little click, the battery gauge goes up about halfway (new battery) and I can hear a hissing sound, not sure where from. I've heard about other people having this same problem and they concluded that it was the ignition coil. I on the other hand don't believe them. I'd just like your opinion on this. Thanks.

Answer
Hi Steve,
I learned working in Jag dealerships not to try to diagnose by symptoms because like yours it only narrows the possible causes down to only a thousand or more. All diagnosis must be done by testing.
Your first test is to put a volt meter across the battery and read battery voltage (12.5 to 13v) second is to have someone try to start it while you watch the voltmeter. It must not drop below 9v if the starter is turning the engine.

If it does not turn the engine (like yours) Do voltage drop tests of the main cables. (put the black test lead of the Voltmeter on the negative post of the battery (not on the cable end) and put the positive test lead on the engine head or a clean metal part on either head. Then have someone try to start the engine and read the volt meter. (set on a low volt scale) It should read less than .5v. Now connect the positive test lead on the positive post (not cable end) of the battery and the negative test lead on the head and hit the starter again and read the volt meter.
Let me know the results of these tests and I will give you the next step.
Howard