Electrical issue - Jaguar XK 2005 (pre production model)

[Karlos]

Hello,

Can anyone help me, please?

I have an electrical issue with my Jaguar XK 4.2 V8.

Long story short, I started to experience some electrical gremlins but wasn't overly worried. One afternoon, when returning to the car, it wouldn’t start, the lights were flashing, and the key got locked in the insert. I tried to jump-start the car, but I think I did this wrong. The RAC came and said I had blown my 400-amp mega fuse, probably trying to jump-start it, but I don't think this was the original problem. The car has since been towed and the fuse replaced, but I am still experiencing some electrical issues with the car not being able to start unless the battery is disconnected and reconnected several times. After the incidents and watching a YouTuber "Life on Cars," I recently checked and had to splice the earth cable under the drivers wheel arch.

The RAC man and the garage said there's nothing wrong with the battery, but when I put a multimeter on it, it read 11.92V. Does this warrant a new battery?

I would be grateful for any help.

Many Thanks

[N6 JMX]

Hi Karl, your battery is a key element of your Modern XK and its ability to start the car (or not as in your case) is no longer a pointer to whether the battery is fit for further service. All modern cars, especially Jaguars, need the battery to be in top condition to allow the multitude of electronics onboard to function correctly.

Typical signs of a failing battery are spurious warning lights and apparent electrical gremlins. In truth, I and many others, consider the battery in a Modern XK to be a consumable item and changed at circa 5 years old. In some cases with low usage and regular connection to a battery conditioner when the car is not being used, the battery life can be extended.

In your case, unless you know the history of the battery and can confirm it to be less than 5 years old, I would suggest it be changed at the earliest opportunity. Ensure you select the replacement to be of the correct power and dimensions - the battery must fit and be secured into the battery tray, otherwise it is an MoT failure. At a resting voltage below 12 volts, it would seem to be past its prime.

Also, as you have discovered, earth connections are also important and I would suggest you check any and all that you can locate around the car.

Let us know how you get on.

N6 JMX

[Karlos]

Thanks, Jim. I changed the battery this morning, but the problem still seems to persist. I managed to get it to start after disconnecting and reconnecting the battery about 10 times. I'll try changing the fuse and its holder again in case they are loose or damaged from previous attempts. I don't know what the problem is; it's driving me crazy. How many earth connections are there to check, I wonder?

[N6 JMX]

Hi Karl, that is strange. Have you managed to get a code reader to see if there are any stored codes held in the "brain"? Incidentally, when you disconnect the battery, touch the negative cable to the positive terminal / cable which will discharge any residual capacitance in the system. I can't understand why the multiple disconnect and reconnect of the battery ultimately encourages it to start.

In terms of the not starting, even a modern car only needs 3 things to start, air, fuel and a spark. Assuming you have a decent spark as you indicate it runs okay once started, it might be time to look at the fuel supply. Additionally, air leaks on the inlet side can create a problem - try a smoke test to see if there are any leaks.

Good luck.

N6 JMX

[Karlos]

Thanks Jim, I'll give it all a try 😊and keep you posted sir.

[Karlos]

Hi Jim,

Think I found the problem. Although it was the 400 amp fuse which I needed to change, the issue around the intermittent starting i think was down to the middle fuse being lose too. The back locking nut wasn't tight even when the front one was. Seems to be OK for now.