Battery died on me on Friday morning. Have replaced with 658 as per Jaguar specification. On testing the output voltage I'm getting 19.4 volts. The alternator has obviously fried the old battery. Could this be an ECM or alternator problem?

 

Sounds like a voltage regulator issue to me; this component is located within the alternator. What model (I am assuming an S-type), production year and engine type is your car?

 

hi
looks like alternator failed
alternator has 2 charging states, low and high.
but 19 volts is way too high
cheers

 

This recent thread may be of interest to you, with a battery killed by severe overcharging:

https://www.jaguarforums.com/forum/s...ut-out-228338/

The fix was a new battery and alternator.

Just curious, where are you taking the voltage reading? Directly on the battery posts? Meter test leads clipped to the battery terminal clamps, not the actual posts? Somewhere else, such as the alternator itself or the front power distribution box? I ask because if you have a marginal connection somewhere between the alternator and battery, the alternator output may "run away" to some extent.

 

Alternator overcharging... just had the same issue. You cannot rule anything else out until you remove an alternator that is charging to that voltage.

 

When shopping for a new alternator, make sure the internal control board is an OEM unit. Most aftermarket alternators have a generic control board, and although it will adequately regulate the voltage, it doesn't communicate properly with the rest of the car. The result is the little red battery light on the dash will not work. The light is always illuminated or not at all, and so you won't have any warning of future charging problems.