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Flex plate - what causes them to fail?

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So, following on from my breakdown (the car, then nearly me!) last week I presume the flex plate has failed.

What happened was: I was driving along the M4 on my way to an important appointment, doing about 60mph in 'Drive', 2500RPM on the possibly inaccurate tachometer, TC locked (it locks at ~45mph with the 1.4 LT230). Suddenly I became aware that the engine was revving wildly so I removed my foot from the throttle, wondered if it had jumped out or range, made sure it was in drive and then coasted to a stop on the hard shoulder. Checked high, low, diff-lock, forward, reverse and 1,2,3 but all to no avail. When shifting from neutral to drive or reverse there was the slightest suggestion of a jolt but revving the engine led only to disappointment. I turned off the engine and called for the waste of space (and certainly time!) that is the AA.

When the AA had arrived, after 1hr45 on the hard shoulder, had recovered the car off the motorway and planted me in the services 20 miles in the wrong direction to await an AA mechanic to come and tell me what I already suspected, and while I was bored, I thought 'what if I try the starter?'. I did and it made a whir but did not turn the engine over. When he got there after a further hour, and after I had explained what a flex plate was, the AA man confirmed that the starter would not turn the engine over, but that the ring gear did turn - quel surprise! He did recover me home himself though rather than making me wait for another flatbed.

post-209-0-29670800-1320350532_thumb.jpg

So there it is. Engine will not turn gearbox, starter will not turn engine. Either the flex plate is knackered or the back has fallen off the crank. Unless anyone can think of something else that might give the same set of symptoms.

The car is a 1987 Ninety with a 300Tdi and auto from a Discovery. The auto and torque converter are both quite low mileage reconditioned units from a well known and respected Landrover transmission specialist. I have only had the car for about 6 months and done a few thousand miles in that time, I doubt that the previous keeper has done many miles in the preceding six months as it was a second car/off-roader. What I do know is that the engine flex plate was replaced with a new genuine one about 12 months ago having failed. The previous day there had been an odd noise, the last owner suggested it sounded like when the flex plate had gone...

So, why has the new flex plate failed? I am assuming that it was fitted correctly. What circumstances are likely to cause premature failure of a flex plate? How can one fit it wrongly and how should I correctly fit a replacement - any pictures, instructions, top tips???

The current plan is to move the engine forward enough to allow access and change it that way.

Thanks

Chris

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Chris,

Having to chatted to James at the weekend, he said something that I was not aware of. The bush in the end of the crank that supports the nose of the torque converter comes in a couple of sizes (large and small, but I don't know what the dimensions are). I suspect that the bush in the end of the crank is of the smaller size where as the nose of the torque converter is of the larger size.

This would cause the flexplate to dish as the bolts that hold it to the torque converter are tightened causing it to be stressed in away it was not designed to deal with leading to the failures.

Unfortunately without taking the thing apart and actually measuring the two bits I can't think of another way of confirming it. It is as far as I can remember the only dimension I would not have checked as I wasn't aware of the differences when I last had it apart.

It would however explain the problems which only started after the new box and torque converter were fitted when the engine and box were still in the Discovery.

Pete.

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If someone has been in there pissing about, make sure that firstly the two dowells that should be in between the engine backplate and the bellhousing are present. My 300tdi rangie broke a flex plate, and when i stripped it i found that these had been left out by a previous owner. This casued the failure on mine.

In addition bear in mind that you are supposed to shim the flex plate to the correct depth (see the workshop manual for details). Obviously if the simming is incorrect it will cause problems.

Jon

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If someone has been in there pissing about, make sure that firstly the two dowells that should be in between the engine backplate and the bellhousing are present. My 300tdi rangie broke a flex plate, and when i stripped it i found that these had been left out by a previous owner. This casued the failure on mine.

In addition bear in mind that you are supposed to shim the flex plate to the correct depth (see the workshop manual for details). Obviously if the simming is incorrect it will cause problems.

Jon

(I am the previous owner)The dowels were there last time I took it apart and the alignment was double checked as I was aware of the issue with miss-alignment when I fitted the new box and torque converter/ flexplate.

Pete.

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I take it the difference in the bush size could be due to if the Engine is Defender or Discovery derived, or is it specifically between having a manual box or auto fitted or just pot luck?

as in which is larger/smaller bush ?

I've not run mine yet as still building, so dont really want this to happen bit late I know now as its bolted up. but knowing the difference might put my mind at rest :unsure:

Ralph get yer parts manuals out ....... :i-m_so_happy:

Regards

Les

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Did they remove your propshaft to tow you home?

Yes, and without prompting.

Pete: Thanks for the input, I will ask Ashcrofts about that when I call them for a flex plate.

Jon: Any details on the shimming? I do not have a WSM, anyone know if there is one online that might offer the correct numbers, measurements, drawings etc.?

Cheers

Chris

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Chris. A long time since I changed one of those....1971 I would think. BMC 250 JU delivery van...Yep is just cracked and the firm had had it from new.....

A couple of years ago I was down at Maddison 4x4. They had an IBEX on the ramp. Gearbox out fitting new flexi plates, they were fitting TWO plates instead of the one..... . Mind you that had a V8 diesel fitted....

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I take it the difference in the bush size could be due to if the Engine is Defender or Discovery derived, or is it specifically between having a manual box or auto fitted or just pot luck?

I don't know, however this engine has always been an auto and the only things changed prior to this problem were the gearbox and torque converter (and now 3 flex plates have failed the original 100,000 plus miles one and then 2 replacements both from Ashcrofts in a relatively short space of time.)

The box and converter were supplied by Ashcrofts so I am happy with them as units. I put the first (original flexplate) failure down to the mileage it had done, and the second one I attributed to the fact it was made of a different material to the original one (it seemed more brittle especially one compared to the current one that is a genuine Landrover part). However now this has failed also, I can only assume that in the process of replacing the box and converter I have missed out checking some critical measurement that has led to the failures.

Pete.

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Staight form Ashcrofts Site:

Check List for fitting ZF Autobox

Please note that a good 50% of our "warranty returns" are due to two basic installation errors.

1)debris and contamination in the oil cooler and oil cooler lines, even if you have flushed them out this is no guarantee they are clean, if the old box has suffered a bad failure you would be wise to replace these.

2)broken oil pump, by not fitting the torque converter correctly, read below.

Assembly

Check engine to bellhousing and Autobox to transfer case dowels are all fitted.

It is very important that you locate and install your torque converter correctly. One of the most common causes of failure is the torque converter not being located correctly onto the pump before fitting causing the pump to be broken on start up.

To avoid this we would suggest that you stand the autobox upright and lower the torque converter into the bellhousing.

When the torque converter has fully located onto the pump it is essential to then measure the distance from the bellhousing front face to the torque converter feet ( also known as the flange that the flexplate is bolted to).

Once the torque converter is inserted and checked make sure it does not fall forward when offering the autobox up to the engine. Do not be tempted to pull the gearbox into position with the bellhousing bolts, it should fully locate on the engine with no gap between the bellhousing and engine block.

With the autobox fitted to the engine, check that the converter will rotate freely and has a small amount (about 2mm) of end float. Bolt the converter to the flex plate using loctite or similar on the thread of the bolts.

Ensure the breather vents well away from the exhaust.

Oil Fill

Always use a good quality oil, preferably a Dextron III or similar. Put the transfer case in neutral and fill. Start the engine, the oil level will drop immediately, top back up straight away. Move the shifter lever up and down through the gears slowly whilst checking the oil level and topping up as required. Once the level has stabilised, put the transfer case in gear and the unit should now be picking up drive, this is indicated by the engine revs dropping when put in Drive.

Set up

If the vehicle is a Disco I or Range Rover Classic under no circumstances drive with the kickdown cable disconnected, as this will cause premature autobox failure.

Ensure that the shift assembly is set correctly and full travel of the shifter and engagement of all gears is positive, partial shift will cause premature autobox failure.

If the vehicle is a Disco I or Range Rover Classic you may want to adjust the kickdown cable if you want to fine tune the shift pattern, the looser the cable the softer and earlier the shifts, the tighter the cable, the harder and later the shifts.

Torque Converter Depth Table

The box code is the last 3 digits of the second number down on the name plate on the left hand side of the autobox.

Box Code.....................Depth in mm

061.................................. 22

064....................................22

065....................................27

332....................................51

593....................................51

699....................................51

727....................................27

728....................................83

740....................................51

741....................................51

742....................................51

744....................................18

748....................................18

747....................................27

757....................................27

759....................................27

763....................................83

764....................................27

765....................................27

766....................................96

767....................................27

768....................................83

769....................................83

Please call if you have any other queries.

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Thanks for the link Les. That has led me to the 300Tdi rebuild manual which gives clear instructions for measuring and shimming the flex plate. I assume that nothing will have changed from original though...

Chris

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The list and fitting guide, Les has posted was also supplied with the new box (it may actually be in the paperwork I gave you with the car) and I double checked the torque converter depth when I fitted the box originally and also when I last changed the flex plate.

Pete.

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I take it the difference in the bush size could be due to if the Engine is Defender or Discovery derived, or is it specifically between having a manual box or auto fitted or just pot luck?

as in which is larger/smaller bush ?

I've not run mine yet as still building, so dont really want this to happen bit late I know now as its bolted up. but knowing the difference might put my mind at rest :unsure:

Ralph get yer parts manuals out ....... :i-m_so_happy:

Regards

Les

the only crank spigot bushes I can find are these

Defender 300tdi LFB500050 previously numbered as 8566L

Discovery 300tdi 8566L now the LFB500050 as above

Flywheel bush for all engine models up to and including 300 TDi (not auto transmission or V8 models).

both for vehicles with manual gearbox, microcat doesn't show a bush that relates to a automatic gearbox.

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Forget the torque converter depth issue, if it's not right the oil pump gets minced on fitting and it wouldn't ever have driven.

Realy we need to see exactly where the failure has occured to give a meaningfull answer.

What I would say is, if the failure is around the centre boss, check the plate the bolts pass through is fitted the right way, there is a chamfer on one side and this must face the flex plate.

Fit it the wrong way and the flex plate will crack.

Anyway get it in bits and let's see.

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Forget the torque converter depth issue, if it's not right the oil pump gets minced on fitting and it wouldn't ever have driven.

Realy we need to see exactly where the failure has occured to give a meaningfull answer.

What I would say is, if the failure is around the centre boss, check the plate the bolts pass through is fitted the right way, there is a chamfer on one side and this must face the flex plate.

Fit it the wrong way and the flex plate will crack.

Anyway get it in bits and let's see.

both times it failed on me it was the centre that broke out. I suspect it has failed in the same way again as the ring gear is still engaged with the starter.

Pete.

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untitled.jpg

What part previously failed?

Re read the first post ,the flex plate transmits no drive, in this set up it merely holds the ring gear. breakage of the flex plate(1) wont cause loss of drive, the engine wont turn over on the starter though.

If the drive plate (6) fails there will be no drive, the engine will turn on the starter however.

The only thing I can see that would cause both loss of drive and failure to turn on the starter is the bolts(12) have all broken, as unlikley as that seems.

I don't think they can just come out as the boss(5) covers them, possible to come loose and then break I suppose.

Incidentally, items 10 and 7 have a chamfered edge that should face the flex plate and drive plate respectively.

Incorrect fitment of either will cause problems.

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untitled.jpg

What part previously failed?

Re read the first post ,the flex plate transmits no drive, in this set up it merely holds the ring gear. breakage of the flex plate(1) wont cause loss of drive, the engine wont turn over on the starter though.

If the drive plate (6) fails there will be no drive, the engine will turn on the starter however.

The only thing I can see that would cause both loss of drive and failure to turn on the starter is the bolts(12) have all broken, as unlikley as that seems.

I don't think they can just come out as the boss(5) covers them, possible to come loose and then break I suppose.

Incidentally, items 10 and 7 have a chamfered edge that should face the flex plate and drive plate respectively.

Incorrect fitment of either will cause problems.

You're looking at a 200tdi diagram by the looks there. The 300tdi is different.

On a 300 the flex plate DOES transmit drive, and if it is broken then the engine will not turn over on the starter.

All the symptoms are classic symptoms of a broken flex plate.

Jon

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Ah, wasn't aware 300 was different.. RRCv8 and 200 are as above.

Yes if the converter bolts directly to the flex plate, failure will cause the symptoms described.

My advice still stands though, that any plate/spacer next to the flex plate should have a chamfer that faces the plate.

Also, check the bellhousing to back plate locating dowels are fitted.

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Check for crankshaft endfloat as this can also be a contributiing factor. I had to replace the thrust washers on mine as with 200000 miles on it the endfloat was excessive!

Jon

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Thanks Andy, but since I already have the flywheel housing that price exceeds the cost of a new flex plate.

Bought an engine crane this morning so now I just need some motivation to get the engine out...

Chris

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Hi This is a, :rtfm: WEALTH WARNING, if you are pulling the engine forward be sure to clamp the TC in place before it can move out of engagement with the pump, if it slips forward by 3 or 4 mm it is very easy to break the pump gears on re-assembly and if you pull it out completely you are trouble as you will most likely have to remove the gearbox to re-seat it. YOU HAVE BEEN WARNED!!. good luck Ian Ashcroft

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