V12 underpowered when starting

[Jaguar_Enthusiast]

Would be interested to hear from others whether the following behaviour is normal or merits further investigation.

My XJ Series 2 V12 injection starts up just fine but lacks power until the battery charge indicator gauge has sprung up to the upper end of the normal range. If I try and drive away without the voltage reading this high, the car is very sluggish and will occasionally misfire.

I can typically make the charge indicator get up to the level needed to achieve normal engine power after starting by revving to >2000rpm.

Is this just something I need to get used to or a possible sign of a fault?

Thanks for ideas.

[mike020150]

The simplest explanation could be a defective battery. If you've ever seen a V12 running with a cell down on a battery, you'll know that it runs very rough due to over-fuelling. The lower voltage obviously causes much confusion in the engine ECU. Mike.

[Jaguar_Enthusiast]

Thanks for this guidance regarding the battery.

A new battery (just to check) didn't solve the problem which was eventually traced to a broken connector to the alternator which meant that the exciter was not operating properly at start up.

However this has revealed a further problem - the alternator is simply not putting out enough power for the engine to run smoothly when the headlights are on!

Has anyone out there had success in identifying a supplier for a replacement alternator for an injected V12 please? Looking for >80A output.

Thank you!

[mike020150]

It's good to hear you found the break in the wiring. Presumably, the problem now is that the car runs OK from cold without the headlamps but badly when they are illuminated?

The standard alternator puts out a maximum of 66 amps which should be plenty for most circumstances. After all, lights should account for no more than 12/13 amps on top of the ignition and fuelling circuits which are low amperage consumers.

Is it possible the alternator is either an incorrect unit or is not producing all the power (amps) it should do? Alternatively, is something else causing a fault or an exceptionally high current flow (eg the heater/aircon, radio/stereo etc)?

It shouldn't be necessary to upgrade the standard alternator unless there is a non standard current drain affecting the car. I would be tempted to start from scratch and check all the car's earthing points before using an ammeter to check the current draws of each additional component as you turn them on one by one. Mike.