93"xj6 starting problems

Not quite clear on the problem. It seems to be that you have a new battery, but you can't start (goes flat very quickly). Then you can jump start easily, so there's not a problem turning over the engine (ie not an unusually heavy drain from the starter motor). If I've understood this all right then there can only be 3 causes IMHO.

For test, turn on interior lights. Does trying to start cause the lights to dim more than a bit? If not, either the starter motor has gone open circuit, or the connecting wires are loose.. Note: the accessories are de-energised momentarily when you operate the starter, but the interior lights should remain on (on most cars - is this different, other owners?). Is there any click from the starter motor relay? If not, it may be that or the wiring to it.

If lights do go dim, either your new battery is faulty (not unheard of). Try with another battery.

Or might be an unexpected quiescent drain, which you seemed to suspect. Best way of verifying/measuring that is to put an analogue ammeter in series with the negative battery lead (NOT when trying to start or you destroy it and/or its leads!). Digital not so good as there will probably be a pulsing on the line. Not sure about your model but an XJ40 is OK with about 50-60 mA, and earlier models take about 20 mA. May be a bit more when an alarm is fitted. Do not measure with door open as you will be measuring interior and puddle lamps etc. I had a XJ6 Mk 2 once with this problem. Turned out to be the boot light switch not opening properly. Boot light stayed on when boot was closed (and was of course invisible).

Let us know how you go.

Sorry. On re-reading that this morning, saw it was rather confused. Re-summarising:

If lights dim a lot but starter won't turn when trying to start - battery nearly flat (faulty battery or high quiescent current drain. Slight possibility jammed starter.

If lights don't dim at all and no click of relay - faulty starter relay, connections, starter (in that order).

If relay clicks but no dimming of lights - faulty starter main cable or connections.

In all tests, remember starters take a LOT of current (hundreds of amps) when turning, and this is the ultimate test of a battery. BTW, never try to measure this current.

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