When the car wouldn't start one Saturday morning, the RAC
man exchanged the battery, it started. Later, I took the
wife to Ikea, when we came out it wouldn't start. After 20
mins sitting, it started OK.5 days later, power failed at 75
mph on the M4.
RAC came, he took off earth, cleaned it up, it didn't start
at first, but 15 mins later it did fire up. Local Jag
specialist has twice put her on diagnosis but found nothing
as she is working ok at time of checking, he say's when she
fails again, bring her in on tow truck to try analysis then.
Now she is on the drive, as I cannot rely on her, one day
she starts, another day she wont. At all occasions all dash
lights come on as normal. There are dashboard alerts at
times of failure; ABS not working, Park brake not working,
DSC not enabled.
The garage don't want it any more as they cannot see fault,
can anyone offer help?

 

So it has a recent brand new battery?
And of correct (very hefty) spec?

Please be 100% sure as you will spend a ton of time/money if you are unsure but go looking elsewhere

 

That will be the first step.

Second step, post an intro in the new members section.

 

Yes, checked the battery, its 100ah/900amp, is this OK for an S type 2.5 ltr petrol engine on a 52 plate?

 

Checked? See #2.

 

Yes, that's plenty for an S-Type.

 

Hi Clackie,
Check the ground point in the boot along with the main power fuses(small black box).
Then go around and check all ground points are clean, dry and tight.

Check also the starter motor connections.

Then do some simple checks on the Battery, volts with engine off and volts while running.

Always pays to thoroughly check the basics first before opening your wallet.

Hope this helps.

 

Thanks chaps I have a little homework to do looking for these earth fixings, but wouldn't all electrics failed if the earthing was iffie?

 

No. Starting the car is the largest electrical draw the car will ever see. So everything else might work fine until you attempt to draw a huge amp load when starting the car.

One test is to turn the headlights on and then crank the car. If the lights go out or get very dim the battery is probably weak. If the battery is good then suspect a bad connection. I have seen battery cables that were corroded down inside the jacket of the cable. With the battery in the trunk it stays much cleaner so hopefully you won’t run into that!

Of course it could be the starter has gone bad and is drawing way more amps than it should but that is a rare failure compared to a weak or under size battery. This is easy to check by taking the starter to a parts store and let them bench test it.

Also consider checking what the voltage drop is from each ground compared to the negative post on the battery. It takes some fairly long test leads but is really the only way to measure your ground resistance.

Since the S Type has the battery in the trunk it is a long way from the negative of the battery to the negative of the starter. As a temporary check I have run a jumper cable directly from the negative battery post straight to the starter ground to eliminate the ground as a problem.
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My local garage has now agreed and has checked and tightened the engine earth which he said, was not good. So I'll not run it to Devon this weekend but will trial it for a FEW WEEKS, (HOPEFULLY)