When the Alternator Belt Snapped on my XJS V12

It hit the Crank Position Sensor which was underneath, which then instantly turned off the Engine and had I been in the process of overtaking something at the time

Then very bad things could have happened, as I could have either been Killed or Injured or involved in a major pile up, it really doesn't bear thinking about or else maybe it does?

As when you buy a Jaguar, Over-Engineering seems to come as Standard on such a high quality Car including the Jaguar XJS which I think we may agree was one of the finest Sports Cars ever made

Although quite often a Challenge to Fix but Oh! so well worthwhile

Though after my (Freak) Breakdown, which shredded the Alternator Belt that caused all the problems, I could not help wondering if they may have missed a trick

As the Alternator Belt on the XJS does look a little bit wimpy, when compared to the one on my Merc, which looks like it was built like a 'Brick Outhouse'

And my little Merc is used as my daily driver and general 'work horse' with her little 4 cylinder ultra reliable engine and only has one drive belt, to drive everything

Instead of that demure looking beast of a powerhouse, underneath the Bonnet/Hood of an XJS V12

So something inside is starting to wish that maybe they should have copied the Merc with a wider and much stronger drive belt, as I can only imagine the strain that belt has to go through when the Alternator is under a Full Load

And that is without taking into account, the extra load that would be on it if you had converted the Car to Electric Fans

And so I would really love to know what you think



The Belt that goes on the Merc is the one on the right



The Shredded Alternator Belt with its New Replacement for the V12 Jaguar XJS

 

Nope.

Belts should be changed every 5 to 7 years, or 50000kms for "V" belts and 100000kms for the newer flat style, WHICHEVER comes first.

Belts snap/shred/do dumb shiit, its a motoring acceptance. Most :"enthusiast type" owners, give the belts/hoses/etc a look over when they do the oil/coolant etc checks. We V12 guys/girls also look at the fuel hoses, WONDER why?????

Many brands of cars have things that get whacked when a belt does the unthinkable. We get them at work on a daily basis, keeps the till ringing. Our poxy Commodore V6 cars take out the Crank Sensor, and the bracket, and the engine mini loom when a belt snaps, comes off etc, and there a thousands on the road still. Its accepted, forget the maintaining, and ya going to get smacked.

The newer cars with the one belt that does everything, have a series of small idler/tensioner pullies, and they should be replaced at each 2nd belt change. Who does that?????, not many, but when one of them suckers lets go, it usually vents via the radiator and a/c condensor, and all that grief for a $40 pulley, OOPS.

THEN

The lack of use our beloved V12 'sget, NO idea why, is a KILLER. Belts get HOT, then sit for months COLD, in a pre-tentioned state, and then away they go again some months later, etc etc, and then the "brown stuff" hits the fan, and a belt lets go. Beer o'clock down here, and a shady tree.

That's why I always suggest a rarely used car, NO matter the brand, gets waaaaay more attention to the little things than a daily driver.

Spouses S Type V6. Now 6 years in the fleet, done about 30000kms in that 6 years, but that belt is getting changed this weekend, and its worse to do than the V12 four, BUT, if that sucker lets go, many little huge $$$ items will be toasted on its way out of the engine bay. Its a Dayco brand belt on there since I did it last 6 years ago, and probably just fine, but its past my 5 year time frame., and $54, is cheap. I estimate about 12 stubbies for the job, we will see.

 

Hi Grant

Yes! I'm sure you're right and lack of use could be one of the main problems, so since a full set of belts arrived today to go on 'Cherry Blossom'

I'll have a little practice on my Rubic Cube and then change them ASAP

 

You have basically answered your own question on why the Mercedes belt is more chunky. It does more.

 

OB
That shredded belt pic in your post does not look like a V belt to me. Neither does the new belt look like a V belt. Are you sure you have the correct part, or does your car, being later than mine, have flat belts in flat ridged pulleys?
If it is a flat belt, and the car has no automatic tensioners (ie it has the threaded rod tensioners that my engine with V belts has got) then correct belt tensioning is far more vital than with a V belt. Too loose they slip, too tight and they go BANG, as you may have found out.
Confess now, how old was the belt that let go?
Greg

 

Hi Greg

All the Belts are 'V' Belts which have the (Tractor Style Notching) to give them better grip, as opposed to the (Smooth Style) ones in your Photo

All except for the Alternator Belt which looks more like a Flat Belt, with a 'Multi 'V' Profile, which is almost identical to the Serpentine Belt (Profile) on my Merc

Maybe they are different on the Lucas Cars, as I don't know

But this Multi 'V' Belt is the same as the one that I put on my other XJS and so I was not at all surprised when I ordered and received another one just like it

 

Hi Greg

There was nothing about a Belt Change mentioned in the Jaguar Service History, so it could well have been the Original Belt from when the Car was New for all I know, although She has done only 60,000 Miles and 'Cherry' has done only 25,000 Miles from New

All the Belts have Screw Adjusters and I get what you say about the importance of getting the Belt Tension right, especially the Flat Serpentine (Style) Belt that drives the Alternator

So I will put them on and Tension them up to what I think feels right and then get the Garage to Check them when I take 'Cherry' for an MOT

But having the CPS go down without any Warning, was a terrifying experience that I wouldn't recommend, so I will never be driving another XJS unless I have Changed ALL the Belts!

As that is one of the things you can do to tip the odds in your favour

 

Well done Alex.

Beer time mate, that Rubiks will drive you to drink, fair dinkum.

I had similar on one of my long trips, low flying, as you do, and a small bird was not quick enough, and got sucked up the LH cold air intake. That put the brakes on the mighty V12,.

NOT as savage as that sensor cutting out, but down to 6 cylinders in a heartbeat, made my pacemaker skip a beat also. Took 4 hours, in 45c temps on the side of the Highway, to dig that sucker out. It got about 2 feet up the tube and jammed up, but bits of bird made it to the air cleaner.

All good fun.

 

Hi Grant

While I know that you and Greg have read the 'Book and got the 'T' Shirt' having the Sensor cut out like that, takes excitement to another level and came very close to getting me an entry in 'The Darwin Awards Manual' but you have to be dead to do that!

So hopefully my experience may serve as a Warning to others of what can happen to you, if you don't change those Belts, as having the Brakes fail as they have done when one of the Valves packed up in the ABS

Is 'Toy Town Stuff' compared to Sensor Failure, as once the engine Stops you've no Power Steering, which suddenly gets so heavy that its like trying to Steer a Galleon, in a Force 10 Gale!

Not that I've ever wanted to try that either!

All good fun though eh!

Although I can't help wishing that they'd made that skinny Alternator Belt a little bit Stronger, as I am now even more Paranoid than I was before!

Alex

 

Alex,

The water pump belt on my X Type is a 3PK, so thinner again, and a water pump drags about the same as the alternator.

The service book states 120000kms or 10 years, whichever comes first. I noticed that belt was starting the shred ever so slightly when I got the car home is Sept last year, but since my "major service" was about to begin, it mattered not. That car was then 9 years old and 99000kms, so way inside any service schedule.

ALSO

You are NOT going to move to the other side just yet, you got too many XJS that need your TLC, and you have that TWR "thing" sniffing at your garage space I hear, so drink up, motor on, drink some more.

 

Hey Greg,

with the introduction of ABS into the XJ-S, it needed a more powerful alternator. The original Lucas one was just too weak. So the result was they fitted a Bosch one, which needed the shown flat, ribbed belt. So from ~1988 onwards, the XJ-S with V12 had 3 V belts and one flat belt.

 

Hi Daim

Just out of curiosity, which one of those two types of belt do you think was stronger and less likely to break/snap

 

I don't think one is inherently stronger than the other. The multi rib belts were introduced because they have greater surface area and can transmit more power than a V belt without slipping. One thing that is very important in belt life is the tension, and you need a special gauge for determining the belt tension on a multi rib belt. Pressing with your finger indicates nothing.

If the belt was over tightened, I could see it having a premature failure. That's part of the reason that many cars now have a spring loaded self tensioner, as most mechanics improperly tension these types of belt. You naturally used the proper gauge to tension your replacement belt on instillation didn't you?

 

Hi Jagboi

I didn't have any tools with me and certainly not one of those, so as a temporary fix I had to guess the right Tension by twisting the longest section of the belt to 90 degrees till it felt about right

But all the same I'll get a Garage to check it, just to see how accurate that was, or not as the case may be?

Thanks for your help

 

OB: Take a look at this gauge: https://www.rockauto.com/en/moreinfo...101692&jsn=487
I was able to buy it for the equivalent of about £10, so the proper tool doesn't have to be expensive.
Here is how to use it:

 

I've used that device to tension 944 and 928 timing belts, it's great!

 

Most modern brands also now have tensioner free setups, where as the belt tensions itself. My former Volvo C30 had 3 types because of issues regarding slip. The first was a standard preloaded tensioner. The second was a adjustable tensioner. The last and Ost durable was the mentioned self tensioning belt with no tensioner.

 

Pesonally I prefer the ribbed belt. Not because stronger or weaker but because they are less likely to squeal upon start up. They have a larger surface to grip and therefore just seem to do it better. But then again a standard V belt is just cheap and easy to replace.