The battery of my jaguar is 12.7 when turned off. After turning it on, it drops to 12 and dies after running it for a while. We have changed out the battery and checked the alternator twice and it is fine. I don't know what the problem is. Any ideas?

 

Well, you seem to have done the obvious first things, like replace battery and check alternator. What checks were done on the alternator ? What do you mean by "dies", what is dying, engine or battery ?

I can only suggest you take the car to a qualified auto electrician, or a Jaguar expert.

 

First of all, turn the engine on and let the engine run. Then remove the battery cable to see if the alternator is charging or not. For efficient operation, alternator should generate enough voltage to maintain the engine speed idle. If the car die as soon as the battery is remove then, U have definately a wire problem.

 

I would not remove the battery cable with the engine running .It could cause a voltage spike and damage modules and computers. It should read 13.5 to 14.5 aprox when running.
Mike

 

Bjag is wright 13-14.5 volts,it sounds like the alternator! Take it out and get it bench tested to make sure its ok.Double check all positive and negative connections remove and inspect. Good luck.And never play with the connections when the car is runnig or with the ignition on.