1992 4.0L facelift AJ6.

No spark on the plugs. Correct voltage in and out of the coil. Rotor arm ok and turning. Leads and connections reseated ok on the .

Could this be that old chestnut the crankshaft position sensor?

The problem has happened four of five times before over the last few weeks. On each occaision I have left the car for a few days before returning to try again and it has fired up ok first time.

When cold though I have noticed over the last 3 or 4 weeks that the engine stutters and won't rev over 2500 RPM. Eventually when it warms up it is ok and revs at 4 or 5000 rpm.

I last started and gave the engine a good run yesterday.

 

The crank position "sensor" doesn't sense anything. It is a small electromagnetic generator. It does fail after time and heat, as the magnet loses it magnitism. I would replace it. I had the same issue with my 97 XJR, and it fired right up, hasn't stalled or misfired again.

 

Typical, when I tried turning the engine over today it started first time.

Rather than fitting the spare crank sensor I had stashed away for just such an occaision while I had a window, all I could think of was reversing the car out of the garage and pressing on with the project work I'm eight months into and and 95% of the way through completing. I'm at that point where the end is tantillisingly in sight and all I can think of is victory!

 

Well goodness me. Tried starting her again tonight. Work must continue on the restoration after all. Of course nothing. Not firing again as before.
Ideal opportunity. I pushed her back out of the garage to make enough room to work on her from the front. Trolley jack then axle stands.
What limited access to the crank position sensor even from underneath!

It looks like an allen key fitting of about 1/4". That seemed to fit but no room to lever and turn. Just in case I dowsed it in WD40, as didn't want to take any risks then left it. Back of the axle stands and managed to shove her back into the garage even over the lip of the width wide rain ramp on the garage entrance.

Tomorrow I will need to try a long allen key or a very long screwdriver with an allen key fitting attached. Probably need to remove the air filter pipe and radiator hose to make more access room too. Will have to see.

Wish I'd taken some pictures while I was there, sorry about that.

 

Ok here's today's progress.

First of all when I tried turning the engine from cold it still wouldn't fire up ie. show a spark on the first three cylinder plugs.

I fianlly managed to find the spare ignition amplifier I'd bought a couple of year's ago. I fitted this. Still not firing.

Now to make access to replace the crankshaft sensor.

1. I removed the air filter inlet pipe.

2. I removed the large radiator/cooling hose:




Radiator hose. This is looking from the drivers side.








Radiator hose/Jubilee clip being removed close up.


3. I could then at least see the sensor from above, even if it was quite deep down:



The crank sensor mounting allen screw can be seen as the silver object slightly above and to the left of the dead centre point of this picture.


Here it is closer up:



Closer up. The crank sensor itself can also be seen at this angle.

5. The crank sensor allen screw came undone relatively easy thank goodness.

The crank sensor close up:



The sensor.




The crank sensor up closer still.

6. Now I could see exactly what thesensor looked like I was able to see that the spare one I had was nothing absolutely like thisand in fact the completely wrong type. I did not photograph it as I felt like giving up the ghost.

The car still will not start and I was still desperate to get on with the project work I am so tantillisingly close to completing. This is repair work on the rear sill/wheel arche areas however, so as the car was pushed back half way out of the garage, I was able to at least get some access and carry on.

 

Don't know about Jags but my son's Corsa had very similar symptoms before it finally died and then diagnosed by RAC - the replacement looked like yours so I guess they get used on various cars. Apparently if the sensor gives up the ghost the car's brain thinks the engine isn't turning and so lots of other stuff doesn't happen that is needed for the car to fire.


Good luck and thanks for pics in case I ever have to do mine.


LeeP

 

The crank "sensor" provides a AC signal to the ECU. On the V12, it has two of them. The front one provides a different sine wave than the rear, as the damper has a ring with three "fingers" on it, whereas the rear one pulses with every passing tooth of the flywheel. The front one gives about a .3VAC signal during start, and the rear one is about .6VAC. If either is out of the parameters of the tables built into the ECU, the car simply won't run. My 97 XJR was running happily when it began to run irratically. After checking fuel pressure, fuel contamination, etc, I put a new "sensor" on it. It immediately started as it always had. You can put a ohm meter on it, and it should read around 720ohms. But this isn't always the case. I would simply replace it. It is a service item.

 

Thanks superchargedtr6. Yes, I must admit I wished I'd measured the impedance across the sensor while I had it out. I'm trying to source another one at present. I'd read somewhere to expect about 700R across it if it was working.


Leep123, good to hear your observations on the matter. Thanks for sharing. Hope this helps you in the future. Or anyone for that matter.


I must stress, the first symptoms of anything wrong was a few weeks ago when revving the engine from cold. It would splutter at 2500rpm and refuse to rev any higher until warmed up. This is still the case on the occasions the engine does fire up.

 

Well it's the damnedest thing. The engine fired up first time again tonight. After the last couple of days of it not doing so I was really beginning to give up hope.

I still didn't measure the impedance though. Rather than mess about I just wanted to reverse the car out of the garage and get on with the bodywork restoration. The light summer nights are drawing in a bit here now in the UK, and it was pretty cloudy, so I just wanted to make the most of the light while it was there.

I noticed also when it did start that it won't rev over 900 rpm now - barely above tickover - so the problem's definitely getting worse whatever it is. I left the engine running for about 40 mins to charge the battery a little. It rev'd ok at 4-5000 after this.