1994 Jaguar XJ12

Up to about three weeks ago, the XJ12 started, ran and drove fine with no issues. It always started right up without the need for any extended cranking, and ran smoothly. The car was driven every week or two.

Then one day I went out to start the XJ12, and it cranked but would not start. I thought the battery may be low so I tried charging it, and after charging for several hours it still would not start although it cranked fine. All other electronic functionality was good, including all warning lights illuminating and then extinguishing.

I then hooked up the XJ12 battery to our Honda Pilot to see if perhaps the XJ12 battery was faulty, and indeed the XJ12 started up by jumping with the Honda running. Then I drove the XJ12 around the block. So at this point I thought I simply had a bad battery.

I took the old XJ12 battery to the auto parts store, and replaced it with a new battery. Upon installing the new battery in the XJ12 though yielded the same results where the XJ12 would crank but not start. I tried jumping it again with the Honda, but the XJ12 would not start this time.

I put the new battery on a trickle charger overnight, but that made no difference.

Although I haven’t put a multi-meter/tester on the ignition or injector harness yet, I can smell fuel so I think the injectors are opening. I also did a quick screwdriver test with one of the sparkplug leads and did not get a spark. So my suspicion is that I am not getting ignition coil voltage.

I’ll try to more definitively validate if the injectors are opening and if I am getting voltage to the ignition.

Again, up to this point the car started and ran fine, and nothing was changed or repaired before this problem began. The car had simply been sitting for a week or two.

Until I validate if the injectors and ignition are getting voltage, any ideas about first things to check such as a fuse, relay, immobilizer, etc on XJ40/XJ81 that may play a part in this scenario and are likely suspects?

Thanks in advance!

 

So I saw this post in another recent thread:

https://www.jaguarforums.com/forum/x...8/#post2233620


So I'm going to have to double-check this, but I think the dashboard voltmeter drops to around 9V while cranking.

I'll confirm tomorrow.

If so, what might cause that issue with a brand new battery, where the car was previously running/starting fine?

 

With the 6-cylinder cars, the CPS is usually first suspect. Check to see if you rev counter shows any movement when cranking. If not showing cranking rpm then good chance it's the Crank Position Sensor or it's connector.

Larry

 

Hi Al,

The voltage seen by the ECM while cranking can be too low even with a new battery if there is corrosion or looseness on any or several of the battery power connections between the battery, starter and alternator, or on the associated ground points, like the engine ground strap, battery negative cable, etc. I would start by cleaning the battery positive (B+) connections at the firewall jump post, the alternator and starter, and cleaning the engine ground strap and battery negative cable connection to the body.

Cheers,

Don

 

The 1994 XJ81 has the infamous Magneti Marelli ignition system that was also used on the XJS V12 cars in 1990 through to end of production of the XJS
This ignition system and whole engine management system is totally different than what is used on the 6 cylinder XJ40 cars.
There are many unique ways for this system to fail including half of the ignition system failing while the other half still operates causing raw fuel to hit the hot catalytic converter and starting a fire.
I would suggest reading through the excellent free book by Kirby Palm that details LOTS of help guide info for the XJS since most of the Marelli issues are well covered in there.

 

doesent the marelli dizzy use two rotors? Usually one fails first leading to the fire, right? usually though if that is the case (one bad bank) car still fires up (maybe a bit literally, but it does usually start on 6 cylinders)

Larry

 

This is the Marelli rotor. It's a "Siamesed" kind of a thing. Not much help there, but anyway, Al, I would still check that crank sensor. Good luck

 

BTW, these screwy rotors are also prone to burning through and shorting the spark signal out to the center post of the distributor.

This whole idea of one management system for the fueling of the whole engine and then so many components of the ignition system that can fail on a per bank basis leaving the other bank operational and the failed bank still spraying raw fuel down into the cylinders which then goes down into the hot catalytic converter is just a messed up design.

 

OK, sorry for the late reply... been busy.

All great advice and much appreciated!

Due to lack of personal time to dive into this at the moment, I had the XJ12 towed to our mechanic who is always willing to dig into mysterious issues like these, and work on cars like our Jags and Saabs that other mechanics just refuse to even look at.

That being said, he called and said so far all the voltages look good end-to-end and the grounds look clean, but he noted that the tachometer isn't rising at all during crank and in fact drops a little. Somehow I hadn't taken notice of that, which I kick myself about since this is such a common symptom and indicator

I'm not sure if my prior observation of potentially low voltage was just due to a weak battery, lengthy cranking, or I was just mistaken. Or maybe that's going on too.

So yes, the crank position sensor is definitely suspect.

But, he says the XJ12 has two crank position sensors, one on the front and one on the rear of the engine.

With that in mind he's not sure which of the two crank position sensors drives the tach, so he's ordering one and going to do some testing.

So a few questions:

1) Can you confirm that the 6L V12 has two crank position sensors?
2) Which crank position sensor drives the tachometer?
3) If there are two, does is make sense to replace both at the same time, or just the broken one?
4) Anything else I should keep in mind while troubleshooting/replacing the crank position sensors?

Thanks again,
Al

 

I basically know dick about the 12 cylinder engine but as it has only one crankshaft I can't see why it would need two but apparently it does. The "one on the front" is what you have to change IMO, but that's just a guess, maybe you should change both or at least get spares?

All this could be confirmed in the XJS forum, I would think.

A 5-minute job on the 6 BTW

Larry

 

My 4.0L Xj40 starts just fine but it has a tach that acts EXACTLY the same way during cranking (tachometer isn't rising at all during crank and in fact drops a little). So I have never put a LOT of faith in this whole idea that if the tach doesn't go up to 200rpm during cranking that it must be the crank sensor.
I would have your mechanic actually CHECK the crank sensor by seeing if it outputs a ac voltage signal when cranking.

 

Hi Al,

The V12 has a Top Dead Center (TDC) sensor on the crankshaft, and an Engine Speed Sensor on the flywheel. However, if the XJ81 is like the V12 X27/XJ-S, the tachometer signal comes from the Bank A ignition coil via a 6.8K resistor. You can download the wiring diagrams at the link below:

Jaguar XJS V12 Electrical Guide

I think it may be normal for the V12 tach to register 0 rpm until the engine starts, but I'm not certain. I worked on a V12 XJS last fall, but the most recent V12 I worked on was a '73 E-Type which was wonderfully unencumbered by computers.

Cheers,

Don

 

Howdy Al, any progress yet? Coming in a couple of days late here. The 6.0 V12 should float the tach when cranking just like the sixes (usually) do, via the front CKPS reading. Like Don says, the second rear sensor is the engine speed sensor. It is mounted on a peg accessible at the top rear of the V. I remember Bob G saying that the two sensors are the same except for their physical mounting. The engine will run, although poorly, if the rear sensor is bad. The engine will not run at all if the front sensor is bad. Maybe try swapping the two if testing doesn't help clarify.

They can be intermittent too as we know, sometimes working just fine, other times failing.

Cranking tach float is not a guarantee of a good CKPS, just like the sixes! Sorry.

 

Hi Al,
the tachometer is fed by the ignition ECM. No coil feed on these cars. Therefore: No CPS signal, no tach. Have the CPS measured across its terminals with an ohmmeter, if indefinite, it's dead. There is also a rear flywheel sensor, also mounted at the bottom of the engine. This one gets quite a bit of heat and possibly oil from leaky rear crankshaft seals, which kills its insulation. I have no clue about its effects if dead, but I think it's only used for feeding the auto trans ECM. Both sensors are accessible from BELOW.

I would indeed start with the CPS. Proper diagnostics are essential with these cars or you will first lose many dollars and your mind subsequently

 

Sorry for the delay everyone... .it's fixed!

It was the rear crank position sensor, and yes is located near the exhaust and per my mechanic was "cooked" and so brittle that it fell apart.

According to his research and troubleshooting, the front sensor identifies top dead center and the rear is actual crank position.

Replaced and good as new and according to him running perfectly smooth now, where it had the slightest hiccup here and there before so the sensor was likely intermittent for a while.

I really appreciate all the troubleshooting advice.