Hi. Got a 2.7D S-Type, 2006. I can start the car OK and drive 15-20 miles, no problem. If I switch off, and try to start again within 5-30 minutes, I get the "gearbox / parkbrake fault" and can take me ages to clear it with the reset trick. (the trick where you unplug the 2 cables at the back-right above the battery area doesn't seem to work for me e.g. )

I know this is a common issue relating to the battery, but...

* Alternator reconditioned - charges 14.3-14.8v during driving
* Battery replaced 3 months ago (Willard 638, 12v/90a = biggest we can get here) and tested OK. (after getting the new battery it was fine for about 1-2 months before the error kept reoccurring)
* When car stops, typically battery goes to 12.3v, drops to 12.1v (lowest 11.9) within a few minutes
* When car starts e.g. next day, battery sits around 12.0-12.1v but starts OK
* Checked battery connections, no dirt or spots
* It has an aftermarket frontal PDC device with a wire going into the 10amp fuse in the front; I disconnected it for a few days to test if it's that device draining the battery, but it doesn't seem to be.

What else can I try? Aside from perhaps buying a battery charger to charge it externally, or perhaps clear ECU error codes? (or take it on a road trip, as I generally only do town driving albeit about 20 miles each way with a stop halfway x5 days a week) Thanks in advance!

 

Sounds like something is drawing far more current when off than it should so do the tests for quiescent current.

Bear in mind a suffering battery does not always recover, though hopefully it is ok.

 

The charging voltage looks good.

Your next day prestart voltage is way too low, even though the starter itself seems to work just fine. Unfortunately, on these cars you can't judge battery health by starter behavior. Forum experience shows you want to see a minimum of 12.6v before starting. If not, the faults you are seeing are a common response with low prestart voltage. I get it, 12.1 versus 12.6 doesn't seem like a big difference, but many people have found otherwise.

As JagV8 suggested, check for something draining the battery overnight. Details here:

https://www.jaguarforums.com/forum/s...-drain-123535/

Note how the problem went away for a while with a new battery. I wonder if the overnight drain has been killing your new battery. Not just discharging it, but wearing down the overall health so it can't recover. Don't just throw another battery at it, but do check first for an overnight drain.

If still hesitant to accept low prestart voltage as the problem, hook up an automatic battery each night for about a week. Sure, that's a nuisance to do, but it's free and will provide conclusive results. If the faults are gone after beginning each day with a fully charged battery, then you know you're on the right path.

More details about this common problem:

https://www.jaguarforums.com/forum/s...attery-193787/