Hi,

I have a starting question and hopefully someone can help please.

On a few occasions my 5.3 V12 has been reluctant to fire up. The battery is fairly new and it spins over quickly but doesn't fire but if I connect a booster pack it will fire up straight away.

It is not a fuel issue but something to do with the ignition. Feels like it is a poor spark problem and connecting up the booster is giving it a enough of a lick to get it going.

Any ideas ?

Thanks
Richard

 

Out of curiosity how long have you had the car and did this just start? Maybe you have already done this and worked out lots, I dunno, but my first suggestions would be to start cleaning up grounds in the engine bay... The ones to the left and right of the rad and on both sides of the fire wall. Also, check the connector, it's a strange looking double plastic surround, that goes over the positive battery cable AT the battery. Make sure the wiring coming into and out of that is clean and the connections are sound. Check the tabs on the "main" relay and it's connection. It's one of the 2 or 3 above and to the right (when looking towards the front of the car) of the battery in the boot.

Make sure BOTH you battery posts AND the cable clamps that bite down on them are clean and corrosion free.

These kinds of things.
That's just me.
It's time SUPER we'll spent.

 

Hi,

I have had the car for a few years but recently I did quite a lot of work on it last year during lockdown including a 5 speed manual conversion (which is awesome by the way ) so I was trying to work out if there was something I had done that has caused the problem.

The only thing I could think of was that I wired in a remote unlocking for the doors into the alarm multi pin in the boot (by the aerial). My car did not have the alarm when new but the wiring is there for it.

The starting issue seemed to be after this but I can't see how a couple of wires into the central locking would cause an issue ? I do hook into the power on the multiplug as well.....
I might try disconnecting the remote locking to see if the starting issue goes away.

Cheers
Richard

 

Good idea.

I always try: and get any car back to "as made" when chasing battery drain, or starting issues etc etc.

Aftermarket stuff just seems to run rampant in any car, not just Jags.

The fact you have remembered a change in what the car does after a particular event is going to save the day, and your stress levels.

 

Have you tested the car while off for battery drain?

How old are the amps/Ign coils?

 

Man; would I do a super cleaning on the battery terminals and clamps. Been around for over 50 years with these things. Its My first guess.

 

Lots of ways to approach this.

Personally, I'd begin by checking battery voltage....at the battery itself.....with a voltmeter. Now check voltage at the "+" junction posts on the firewall. These posts supply the entire car. The voltage should be the same, or virtually the same, as the reading taken right at the battery. If not, there's the stepping-off point where your journey begins.

Cheers
DD

 

When you mentioned the gearbox change, one thought that came to me was are there enough ground straps? Maybe there is a problem completing the circuit on the earth side?

 

Where are you connecting the jumpers? If it is to the battery, check the cable fittings for tightness and cleanliness. Do you have the original clamps or aftermarket?

 

Hi All,

Thank you for all your comments.

I am connecting the booster to the leads on the battery.

So far I have;

1. Disconnected the remote locking - so it was not taking any power - No difference
2. Cleaned the battery terminals and clamps - No difference
3. Cleaned the earth post behind the brake booster - No difference
4. Cleaned the round multi plug that takes the ignition feed to the coils - No difference
5. I used some jump leads as extra engine to earth straps - No difference

But what I have just found is that at the plug (point 4 above) if I check the ignition power feed wire (white wire) with the ignition on it is 12V but cranking this drops to 9.7V which I don't think is enough?
I then connected the booster and I am getting 11.8V on the white wire.

I wonder if the new Bosch battery (its a an S4 004 - 60Ah) I have is failing under load? I will try another battery

 

That battery is not big enough. You need 700 CCA on a V12. It also helps to use the key'd igntion coil feed to trigger a 15 amp fused relay triggered from the coil positive feed, so the relay power comes direct from the firewall post and then feeds the coil positive, the OEM keyed feed being used solely to trigger the relay power side. This gives a far more reliable 12v feed to the coils. This is a Grant Francis "Great XJS Prophet of the Southern Cross" tip.
Also, check and clean the earths to the right of the battery on the chassis rail.

 

Thanks Greg.
The 60Ah was the recommended type but I can get a more powerful battery.

The additional feed to the coil is a good idea. Is there a post with some instructions on what to do ?

Thanks
Richard

 

Wonder if the engine ground is bad. Try running a ground from the engine to the chassis

 

See sketch for relay to get Battery voltage to + side of ignition coil. (The red wire from ign. switch to post 86 on relay) could be the wire currently at 9.7 volts going to + terminal of coil. Mount relay close to coil and use # 12 wire from + Battery to post 30 on relay and relay post 87 to + terminal on ign. coil... (inline fuse holder and fuse 15 amp fuse). Rout #14-16 ground wire back to where the -battery cable mounts to frame.
Rgds
David

 

Richard,
Further to David's diagram above, on my UK spec RHD V12, I sited the relay on the RHS inner wing just near where the bracer strut bolts on. I ran the power feed from the RHS firewall post, inside a protective silicone tube to the relay (to 30 in the diagram). I used a relay with a fuse as part of it, which simplifies the wiring. Something like this with a 15 amp fuse in it:

Then I took the 12v feed to the coil off the coil (check it switches on and off with the ignition key) and ran an extension wire down the strut to the relay, where it was used as the input to 86 in David's diagram (ie the relay trigger). Then I ran a wire back to the coil up the strut from 87. The earth went to a bolt on the wing suitably cleaned up to bare metal.

 

Also bear in mind the electrical section of the ignition switch is in dire need of a clean. Age related work.

These attachments will assist.

 

All sorted. I have plumbed in a relay to power the coil. Pretty easy to do and took about an hour.

I took the power feed from the firewall post

I cut the white wire coming out of the multiplug and this was the ignition feed into the relay with the relay output then going back into the white wire.

I mounted the relay next to the fuel pump and ac relays under the cover where there was a handy earth point.

The V12 now starts on the button and seems to idle more smoothly too!

Thanks all for the help.

Here is the multi plug

And this is the relay cover behind the brake reservoir

 

You have effectively by passed the original high resistance in the system. After a long drive, you can reach to the back of ignition switch, and if it is warm to hot, then cleaning the contacts as Grant has outlined, is something you should do when you have a few wet spring days. Well done on the relay installation.
Rgds
David

 

Hi All,

I thought I would just update you. I still have an issue but it turns out it is the battery which is failing and only putting out 10.8 volts so even with the relay it is not getting 12v to the coils.
The battery is only a year old so I will get it replaced and I will go for a better battery.

The only problem is that on the 5.3 V12 the battery tray and the clamp that goes around the top of the battery to hold it in place only fit a battery that is
242mm in length
175mm Wide
the height doesn't matter so much - the Bosch is 175mm high
For the length I could go to 260mm but that would be the maximum.

The 6.0 V12 has a different battery tray and clamp as most 70+ amp batteries are 280mm+ in length.

So I can't get a 70 amp battery without modifying the tray.

I can get an optima battery - https://www.tayna.co.uk/car-batteries/optima/8003-251/
It is only 55 amp but it has a CCA of 815 amp which is far higher than the Bosch that is on it which is 540 amp (the 75 amp hour Bosh only has a CCA of 680)

So what do people think? Should I go for the 75amp hour battery with lower CCA and modify the tray or the optima (which is twice the price) that will fit straight in ?

Thanks

 

I got so fed up with this problem that I removed the tray and modified it. Actually, if you have a welder, it was not hard to do. A few pics attached. The OEm fixings were kept as standard.

OEM and modified compared

Cut-out on modified to avoid fouling tank strap


Extended outwards too, new steel edge welded in

Installed

plenty of room for a D15 sized Varta