Hello all,

I'm looking for peace of mind or an explanation/next action on this, please!

I have just picked up a MY2000 XJR. On the long list of things to do is sort the heated seats out in the front - neither work.

All 4 bimetal thermostats had failed, I got the passenger side working fine but the driver side still wouldn't warm. No power getting to the foam.

Under the seat, the power loom had snapped at the crimp of the plug/socket between the body loom and the foam wire.

Remade the connection, excitedly moved the seat back, jumped in, ignition on and met with a very dim dash and many faults rolling in the display - all the ones you love to see, gearbox fault, DSC, rear lights check...

Oh no!

Plug in the OBDII reader to check voltage - battery is at 7.5v and the car will not turn over (of course!), confirmed battery level with a multimeter at the engine bay power bolt.

This is very weird, the battery has been fine in the few weeks I have had it, the reconnecting of an existing system shouldn't do this? (I appreciate batteries can seem fine and drop off quickly, I have not proven/tested the battery yet)

I jumped the car off from another and it started fine, all but the check rear lights faults had gone. Power cycled the car with no jumpers on, all faults clear and battery voltage back up. In the space of maybe 2 minutes going from 7v to a strong start - why? Car runs at just over 14v.

I have a feeling I should have disconnected the battery before making the connection again, but it was an unpowered circuit at the time (the seat was off).

There was a code on the ECU which did not show on the dash after this, I forget the code but linked to Comms with VCM lost. It was cleared and has not come back. On the second and 3rd start, Trac unavailable flashed up, no codes stored. Battery going, do you think? I'm just amazed at the 12-7-12 voltage behaviour. If it hadn't dropped so far I'd have already bought another battery, I just can't understand why it dropped when it did and by so much. Please help me understand!

Why do you think this battery level dropped while doing this? I'm asking more for if this is the precursor/indicator to another issue.

Thank you

 

I have seen the same behavior with a bad battery. One minute fine then the voltage drops and car will not start plus loads of error messages. Replaced the battery and all was fine

 

Be worth a check of the battery earth lead & the mega fuses in the boot, both are known to corrode & give issues. won't take long & doesn't cost anything before buying a battery.

I wouldn't be surprised if it is the battery though.