Jaguar XJS Parasitic Battery Drain Explained

Jaguar XJS – Parasitic Battery Drain – Final Resolution.

There we are! Google Analytics tells me that this topic will go crazy… This time you don’t need a string and upholstery skills. Why your Jaguar XJS is draining battery so quickly? There are hundreds of theories across the Internet telling you what you should check as a first second eleventh thing, how you should play with your fuse boxes etc. The most interesting ones are made by the true MG fans and lovers: “because it is a sport car and it was never meant to be used as a daily drive”. Parasitic battery drain explained. Don’t follow it. Especially if the theory belongs to the people changing their Rover K-series engine’s head gasket on the way to the church (see THIS LINK for independent clarification). Frankly the MG brand is owned by Chinese and after 100 years - starts to prosper.

So, the assumption is – you have tried everything. Multi-meter installed in the XJS’s circuit, all fuses wobbling from puling out and sticking in, time after time every weekend.

You can disconnect your alternator’s positive wire with eyes closed, while having a cigarette and without disconnecting battery. You know more about your central locking system and window thermal cut-out relay - than the person who designed it. You’ve checked and isolated frayed electric wires on both sides of the bulkhead behind upper part of rear sofa. Still struggling.
You have measured the battery drain with multi-meter before doing what’s below and you should know how to measure it without blowing fast-fuse in your measuring device.

Without serving a bull any second longer – I am showing you photos below. This really should be your starting point in complicated drain investigation, not the end, but hey – it’s all for fun…

Dig into UK version’s passenger footwell, where your wet ECU is located. You will recognise the location by specific smell of classic Jaguar, also known as the Damp. Don’t use that term while in conversation with British Jaguar owners – it’s like insulting their XJS, bathroom or the whole house…

Unbolt the ECU cover from the top – proceed with caution – there may be some metal left in the sea of pure iron oxide.

Now, pull out the silver relay. That’s not clear – correction: pull out the metal one regardless of the colour left on it. If there is no metal left – pull out the with exposed internals.

Clip in your amp meter in the battery circuit. Better? Almost NONE.

Now the truth that hurts (both, Bosch and Jaguar). Relay with 4 terminals is absolute shit and prone to faults by design or materials used (those years only, non-jaguar cars – maybe).

Relay number 0 332 014 406. Throw it away. It was functioning right for non-British customers - and you will find PLENTY of replacements and brands on the market just type the number above. Even so – I recommend the one with 5 terminals. Reason unknown and as per feedback given from old-ish, grumpy German automotive quality engineer that thinks he can teach everyone how to play Squash: “British companies? We were sending everything and they’ve accepted everything”

And yes, the main relay on the photo came from my XJS and it is actually DRY and not corroded. I was extremely lucky with my Jag, an exception that proves the rule.

Get relay from the picture to the right – new, any brand or recovered during post-mortem training on the graveyard (starting from pre-lift FROM VIN 179736).

Relay number: 0 332 014 113 - 12Volt, 30Amps

Naturally - I went for the new one. Those are approximately £12 each. Just mentioning as I have heard about wealthy XJS owners changing oil filter without changing actual oil in the engine. Relay on the photo was taken from my other complete Jaguar packed in the garden shed (shed made of flexible rubber). And yes. It's corroded. Website purpose only.

Install new-ish 5-terminal relay and check the reading on your multi-meter again. Should be well below previously measured value. No other option if you already checked the bits from my assumption above. The resulting value equals to drain of car stereo, trip computer (or clock) and the alarm – if any installed.

As you are always vigilant, prepared –and creative willing to prove my theory - you will have one or two spare relays indicated above in your XJS owner’s collection. If not and trying to test what's above prior to spending sandwich-worth noney - I bet your old-messed-up relay went out in pieces... XJS won’t fly for a while and it’s your fault. Be prepared next time.

Job done. now go there, go to every Jaguar Forum and cut short every know-it-all hater with your knowledge.