Red October, you need to get out more. No, seriously, that was a great write-up.

 

A lovely and concise description there Red October....but may I clarify one little point.

These engines are fitted with a 'pre-engaged' starter which as you would know has power fed first to the solenoid which then operates a lever which 'pre-engages' the pinion before full power is fed to the motor itself.

This opposed to the older style starter which has the common 'inertia drive' system which relies on centrifugal force to 'spin' the pinion out against the detent spring to physically engage with the flywheel.

The later system of course is much kinder to the teeth on the flywheel and provides a much more positive engagement ;o)

 

Provided they match reality .. which thankfully they do in the case of the X308's

 

Haha-the weather is absolutely foul here & tipping down with rain, so I've got nothing else to do except watch telly, drink beer & write technical descriptions here

 

Yep, you're absolutely spot-on there & the fact that the Jag's pre-engaged starter has 2 positive inputs from different sources made me wonder if one of the big fuses had blown in the OP's High Power Protection Module.

From his description, it looks like the solenoid is indeed getting it's power from the ignition switch when it's in the 'start' position-the pinion is slowly rotating & the solenoid is pulling the pinion along the shaft into enagement with the flywheel.

However, when the pinion is engaged with the flywheel & the solenoid high-current switch contacts have been bridged as the solenoid pulls in fully, it looks like the starter is now not receiving the main high-current supply to allow it to develop full power & crank the engine.

As these 2 starter positive inputs come from different routes, it made me wonder if the main 500 amp fuse (2 x 250 amp in parallel) in the High Power Protection Module had blown & was not supplying the main cranking current to the starter.

It looks like he's got the lower-current solenoid supply from the ignition switch which is pulling in the pinion & slowly rotating it into engagement, but there's no main cranikng current supply available.

 

Red October,
It's very are indeed for one of the main power fuses to blow....in fact, I have never heard of it....not to say of course that it could happen but 99% of the time, it is down to one of two relays that control this circuiit....not having the diagrams in front of me but there are two relays in the first fuse box under the bonnet/hood and one in the covered section containg both engine and gearboxx ECM's
The veeehickle hand book should describe their locations.
Other than that, I would suggest that there is a bit of retracing back to the battery with a multimeter on different parts of the starter circuit, starting from the starter and going back 'upstream'.

 

I have to say I've never heard of the main fuses blowing either, but from the circuit diagrams there's only 2 sources of +12v feed to the starter motor.

The first is the permanently live main high current feed from the massive fuses in the boot.

The second is the solenoid supply which comes from the starter relay, which is itself energised by the ignition switch when in the cranking position.

As has been said on here previously, if the solenoid terminal is carefully bridged with a screwdriver tip to the main +12v starter feed, then you can tell that way if the starter is working. Provided, of course, that the main 500 Amp fused feed is still live...

But certainly, tracing back with a multimeter to check the system voltages from the starter backwards is the best method.

 

Red:
I wouldsay you are onto something referring to the two solenoid inputs BUT I am pretty sure that control circuit does not actually provide enough current to make any sort of motor torque. It is not clear from the schematic, but what I think we are looking at is a "pick" (or "pull in") winding and a holding winding on the solenoid. So, if the OP had the holding winding burned out, he would get the effect described. The pick winding pulls in the solenoid armature (through the low inpedance motor circuit), but once the main contact is made, 12 volts appears on both sides of that winding and it no longer pulls current. At least that is what the circuit looks like to me.

 

Looking at the internal schematic for just the starter motor itself, the 'pull-in' winding is the one in series with the starter armature. The actual solenoid supply to terminal ST3 on the starter motor body comes from the starter relay which is energised when the ignition switch is in the 'cranking' position.

These relays are usually rated at around 20 amps or more & this level of current will pass through the starter armature via the 'pull-in' winding which is in series with it. The 'pull-in' winding will get this level of current & have enough strength to slide the starter pinion along the shaft & into engagement with the flywheel.

At the same time as this is happening, the solenoid 'pull-in' winding current is actually of a high enough level to slowly rotate an unloaded starter motor armature.

I've done experiments with old starters & found that a current of 30 amps will spin an unloaded starter armature reasonably fast, while 20 amps will certainly make it rotate fast enough to engage with a flywheel.

This level of current can easily be supplied from a ignition-switch controlled relay to the solenoid.

The second solenoid winding which is in parallel & goes directly from ground to the starter ST3 terminal is the 'hold-in' winding which is less powerful than the 'pull-in' winding & only needed to keep the engaged starter pinion in continuous engagement with the flywheel as the engine is cranked over.

Once the speed of the starter armature & pinion increases, the 'back-emf' that it creates will oppose & reduce the voltage & hence the power of the 'pull-in' winding', so the parallel-connected 'hold-in' winding will keep the pinion in engagement with the flywheel now.

At the same time as this happens, the solenoid contacts close & the main high current supply is applied to the starter armature at terminals ST1 & ST2 so it can develop full cranking power. When the solenoid contacts close & fully energise the starter armature, you can trace from the circuit diagram that the 'pull-in' winding will now have +12v on both sides, so it has no 'potential difference' across it & no longer play any part in the starter motor operation.

At this point the pinion is kept in engagement with the flywheel by the 'hold-in' winding, as it has the full +12v across it's terminals at that point.

I really hope I've got that in the right order

 

Hi guys, did we ever get a resolution to this problem? My XJ8 has just done exactly the same thing.... tried to start briefly, then when I tried again it just whirrs...

 

Welcome to the forum.

Clarification please. The starter motor spins but the engine does not? That is, the serpentine belt is not moving?

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thanks for replying... I will do the into very soon....

Yep the starter spins up, but does not sound engaged... just as in the above thread. checked the power protection module and the false bulkhead connection. all good. My next step will be to locate and check/swap the relay for the solenoid, then remove the starter and bench test it, but from how it sounds It is ok. It did engage and crank briefly, but then stopped and just whirred.

 

After all this great technical info it's a shame we never got the solution to the original problem !

 

have to add to mine... after checking relays, I re checked to be sure that the belt had not moved, and it has so I'm guessing this is somewhat more of a problem than I first thought.

 

You say the serpentine belt is moving, therefore the engine is turning over.

It is probably just bore wash. The engine caught initially then immediately shut down. The rich fuel mixture for startup washes the oil from the cylinders and compression is too low to restart.

Push the gas pedal to the floor and turn the engine over for about 10 sec. Repeat several times, then lift pedal to about half way and see if it starts.

If that fails, a more complicated fix is to remove the plugs from one bank, squirt 1 oz. oil into the cylinders, turn the engine over for a few seconds with a old towel on top of the plug holes to catch excess oil. Then replace plugs and coils and try to start. Be ready for a lot of smoke.

 

Thanks very much for the advice... I will try it in the morning. hope I don't have to take the valve covers off... if so, do i need any sealant to replace them?

 

If you have to access the spark plugs you just remove the coil cover and coils, not the valve cover. Make sure the battery has a full charge.

I assume you have not removed the valve covers to check the secondary chain tensioners, or do you have documentation that they were changed to the 3rd gen. metal body type? This is very important. In fact, many would warn you not to start the engine again unless they have been changed.

 

I don't have any evidence that they have been changed, and was contemplating checking them myself. is it possible that this has been caused by a tensioner failing?
I'm not reliant on the car for everyday transport, so I can afford to have it off the road whlist I carry out work on it.... as it is the diff is noisy and will most likely need output shaft bearings, or worse... but that is less important in the short term. I guess I'll need to remove the valve covers to check the tensioners. The car was running smoothly before it failed to start today, so all should be well. no rattle on start up or anything.

 

If I take the valve covers off do they require replacement seal kits, or is it possible to reuse the ones fitted?

 

I doubt your problem is tensioner related. If a chain jumps one tooth the engine runs poorly. If two or more, valves and pistons meet and you will probably know something is wrong.

You can reuse the gasket. Clean the gasket and head carefully and apply a little dab of RTV sealer at the joint of the front timing case and head. If you do find it necessary to replace the tensioners it's not a bad idea to renew the gaskets.

It's very important that you do not overtighten the screws for the covers. They have brass inserts and you could shear the screws or crack the covers. I can't seem to find the numbers right now, just cleaned my desk top. Maybe tomorrow I'll be smarter.

I hope you are able to do the necessary repairs and upkeep yourself. These cars can get very expensive if you have to take them to a shop.