XJS 3.6 Manual Cabriolet - died on me

Hello All, I have posted this before ,but after lockdown and no use for a couple of months,I took the XJS out for a run today - started perfectly and ran for about 6 km and then started dying on me. When she dies,it is Immediate with no spluttering - just cuts out - ,which makes me think that it is electrical and NOT a fuel problem. It then Started immediatelt and I managed to get home.I changed the fuel filter ( luckily I have been importing and selling jaguar spares for the last 30 odd years here in South Africa and have just about everything I need in stock !! ) .After changing the filter I went on another run and she died completely and would not restart. I got my son to tow me home and went out for lunch !! When I got home i checked the following - Plenty of petrol flowing and a gr8 spark at the plug I removed ,and she started Perfectly and ran well - I have not taken her out for another run,but what could be the problem ? Next I will go for another run and check the above when it won’t start . Rgds

Just a guess but have you checked that the injectors are flowing as they should?

You will probably need to describe many details of your car, as there aren’t many here with experience with the 3.6. There’s far more experience with the 3.6 over on the XJ40 forum, but unfortunately that may not help – the XJ40 came with a different EFI system than the earlier XJ-S 3.6.

Tell us: Do you have a Lucas ignition system with an AB 14 amplifier?

Oh stop it Kirbert… :slight_smile:

There are only few of 3.6 owners here - because they have no issues and there are no reasons for them to seek advice here. !EDITED! *applicable ONLY to Lucas 9CU version.


Which year of XJS do you have? Have you got something called “trip computer”?

It is a 1984 and has an AB 14 fitted above the RH side of the radiator cover,and yues I do have a trip computor fitted.


My 3.6 did that. I forget which one, but there are two fuel pump relays in the trunk (boot). One of them was bad. Replaced it and Bob’s my uncle!

I had the same problem on my car, it turned out to be the ECU. I made
some test gear & tracked it down to a tiny capacitor costing about 5P!

Jim XJSC 1984 3.6L

… ehm… mmm… ehh. Lucas 8CU unit. Saddam Hussein of your XJS model. No remorse, no regrets. No MAF sensor either. Advanced engine constraint with primitive controls.
Have a look here:

As Jim and Helen mentioned above - instant death is caused by relay tripping off due to capacitor. Usually - easy to diagnose, just check fuel pump with multimeter after engine’s stall (with ignition still on) or better - disconnect the pump and connect directly to the battery (with inline fuse if you’re concerned) and let it run. No death = ECU.

From the information above I assume it’s a sudden death - is it not???
None of the sensors will instantly shut down the engine (unless thrown into the inlet).

I recently learned all about electrolytic capacitors and have now repaired two different DVR’s in my home. Electrolytic capacitors have a flat aluminum disk on the top, usually with some creases in it. They go bad with age, and when they go bad that aluminum disk gets domed. Open box, look for domes on top of caps, replace all found. On one, I replaced with caps with a higher voltage rating – I dunno if that does any good, but I figured it couldn’t hurt.

If you have any other serious issues with that 8CU EFI ECU, you’ll probably need to contact Roger Bywater at AJ6 Engineering.

Meanwhile, a fuel pump relay could definitely cause your issues. So could a malfunctioning AB 14 ignition amp. The popular way to check the ignition amp is to carry a cooler with some ice on a ride. If the car craps out, put some ice right on top of the amp. If it starts working again, replace the GM HEI module inside the amp. It’s cheap. Some insist upon genuine Delco HEI modules while others claim to have had as much trouble with the Delco as with the aftermarket. Whatever, be aware that a new one might malfunction right out of the box, which can really confuse your diagnosic process. And don’t forget to use some heat sink compound when installing.

Oh, another thing: Reportedly the new HEI amps are greatly improved over the originals, so even if yours is OK getting a new one might be a good idea. For one thing, the newer ones are less prone to getting fried if you start the engine with a spark plug wire disconnected.

… and don’t forget to wash your hands after fiddling with ignition amp. Beryllium heat sink can cause you look pale permanently (especially if heat sink plate is already oxidised). No cheating.

Well, yeah, beryllium is that nasty, but you cannot possibly get exposed to it unless you saw that HEI module open.

Kirbert, I’m glad you’re still alive. Honestly!

Beryllium - is not inside - It’s the HEAT SINK plate to which amplifier module is mounted to (lacquered black to prevent oxy creation).
It was later on replaced with standard Aluminium.

Beryllium heat sinks are top tech, used in aerospace and defence only - due to EHS risks involved. It also found the way in to our beloved XJS model - whoich is nice accent compared to the stock price, however can be deadly.
I saw the ebay adverts from Lithuania - guy was selling those polished with sandpaper as he thought it’s aluminium…

Where did you hear that? Because it’s simply not true.


Haven’t heard that, rather experienced it.

In aerospace those things are causing a lot of risks, hence you will find plenty of instructions and identification guides in the places under EASA control.

My Jag came with heat sink damaged (probably by someone trying to pull it off by force). So with oxidation in chipped places it looks like something with:

  1. Beryllium Oxide content
  2. Aluminium Nitride content

By the oxy colour I would say it’s the first one.
If that’s the second one - it makes no difference as both can be fatal if inhaled at single exposure.

Can attach photo tomorrow - mine is from end of 89’ - maybe those were different across the XJS evolution.

What’s the fuss?
Does it make any difference? Those cars are having asbestos and all sort of crap inside. I may be wrong, we need just one volunteer to drill it or grind it to find out… :wink: I can test it precisely if you wish - aluNitride will react with phosphoric acid, BerOxy won’t…

Thanks to all who took the time to re[ply to my problem - I have quite a few AB 14 in my place for 6 cylinder cars - are they ALL the same or is it dependent on the serial number that is on the casing ?


I think they’re all the same. If you’ve got 'em laying around, might as well try one.

The saga continues - I changed the fuel relay and noticed that one of the wires from the AB 14 to the coil was down to a single strand of the wire - I fixed that and the car started perfectly and I ran it in the garage until normal running temp was reached.It did not cut out ! I left it for a while and it now won’t start - the fuel pump is working, but when I took one of the plugs out ,I noticed that the spark seemed quite weak ( coil or AB 14 ? )

Gerald .

That is one common failure mode, another is leakage from below which may not be obvious. Also, old electrolytics can just dry out without external signs, and you need to remove and test them to know for sure. I once rebuilt a friends Quad amp which had gone crackly. First clue was that it rattled because the ancient, plastic cased, caps had exploded. Replacing caps is an easy and cheap fix/insurance policy for a lot of old electronic kit.

If I was going to the effort of removing one, I think I’d go ahead and install a new one. The darn things are cheap.

Is there any way to make capacitors that don’t deteriorate with age?

Good plan, yes. Cheap for most common values you’ll find on regular consumer products.

Sure, there are lots of other types of capacitors. Polyester, polypropylene, polystyrene, tantalum, ceramic. Each have there own advantages, and different price/performance/reliability trade offs. My pre amp only has pladtic film caps. It’s hardly ever been switched off, and works perfectly after 30 years. Not cheap though. Electrolytics, provide a good range of values for a reasonable price. There was a period where electrolytics were pretty bad https://en.wikipedia.org/wiki/Capacitor_plague. It’s better now, but they are still number one suspects when electronic circuits are acting up. Dry joints and bad connections are next on the list.