1997 XJ6L Well, I have read a couple similar problems to this. Thanks for Listening. Bought the car with the disclosure of a Parasitic drain "from the BPM". The car had a bypass installed on the neg. battery terminal to turn off the car every night. I quickly learned that if you do not use the cut off every time you turn off the car it wont start until you do cut the power for 10 min or so. So i replaced the BPM and things were miraculous. Left the batt. connected over night, started every time the next day, (it used to think a door was ajar and wouldn't auto-lock) that problem resolved, too. Well 2 days later I am back to orig. issues. let the car sit a couple hours and it would not start. Lights and all are good, I highly doubt it's a battery issue.

Question time. Does a BPM actually drain a batt? or a component operated by the BPM running through it. Secondary as I say I don't think it's just a battery issue. could something be shorting out the BPMs? will an OBD tell you these little things? Things were great for 36 hours with the new unit. And (vaguely) lastly, what should I dooooo??? lol thanks

 

There is a linear switch in the ignition switch that is activated when the key is inserted.
The excellent Steve Sparrow has documented the problems the linear switch caused for him in the jag-lovers.org forum (JagFORUM Logon). Bear in mind that his work here is on a UK spec car which has the added complexity of a security transponder chip in the ignition key that I believe the US spec cars don't have.

To summarise what Steve found - the contacts that are activated when the key is inserted were stuck so that the ECU relay behind the headlights was active even with the key out of the ignition (this also led to running down the battery prematurely). Because this part of the ignition system was not turning off, the ECU didn't know the engine had been shut down, so it failed the engine start sequence, disabling the engine. (Your method of removing and restoring +12V forces the ECU to reset along with all the other modules so that the ECU does know that the engine has been shut down).

A clue to this problem is that you also can't lock the car using the key fob.
Steve found -
When the engine is disabled in this way (at least by this ignition switch lateral movement fault) the doors and boot cannot be locked with the fob for the same reasons that they cannot be locked if the boot lid or doors are actually left open. The locking procedure only works when the ECU relay is off (not powered) but won't work
if the ECU relay is powered when a request to lock the doors is made by the fob. In this case, you will hear a refusal 'peep' from the security horn. Removal of the ignition barrel to free up the igntion switch is done by putting the key in and turning it to position 'I', and then pushing in the little detent pin on top of the outer barrel with a
bent pointed tool. The lock barrel can then be pulled out, exposing the laterally moving part of the ignition switch. Naturally the column shrouding will have to be removed to do this. An alternative method would be to remove the ignition switch itself and free it up or fit a new one.

I think Parker has posted pictures and more information on the ignition switch if you search for his posts on this.
Steve's explanation sounds similar to your problem - battery drain and locking issues. Not an obvious solution but I hope it helps.