TC lock help

hi guys, I have a 1999 discovery td5. The fault I have with the vehicle is, the TC won't lock up when you get up to speed. Sometimes it will take the notion but most of the time it doesn't. Might sound silly but it seems to stand a better chance of working in wet weather. i have read all sorts on internet so I changed the gear box oil twice, and replaced the gearbox filter on the secondchange of oil. No burnt smell from the oil and still a red colour. Will just add, all gear changes are nice and smooth. I have done a stall test and it's around 2800rpm give or take a little just using rev counter in car. From what I have read, TC lock is a ECU sending a message to a solenoid when temp,speed and light throttle is correct. I might have the next bit of info wrong but am I right in thinking that TC lock is manually selected when you put it in to low ratio box? If I am correct, everything seems to be working fine in low ratio. Which makes me think the solenoid isn't faulty or sticking. With it working now and again in wet weather I have the notion it could be electrics at fault. The other reason that makes me think this is, it might lock up at anything between 50mph and 70mph. I have had the vehicle checked for fault codes by Land Rover dealer and the only thing that showed up was Mass air flow and that's now been replaced. If anyone could give me pointers as where to look it would be greatly appreciated. Don't want to go to the expense of putting a second hand or reconditioned box and torque converter in to find it was something silly.
thanks for any help you can throw my way

Do you have much bigger than standard tyres?

235/70 16 on front and for some reason 225/65 16 on the rear

The EAT ECU is mapped from factory for standard dimension tyres and it needs vehicle speed information form the SLABS for shift control, that 5% difference between axles cam mix up the SLABS so i'd put 4 similar tyres before going further to rule this out

Will get a couple of new tyres on the back. Price of tyres sounds much better than price of gearbox

Do the wheel speed sensors control the gear change and throttle lock?a road I use often has two thin ruts going from one side of the road to the other. Maybe it's because I put a stiffer lifted suspension kit on the disco but whe I go over the ruts it really does cause a good thud through the vehicle but it also make it go into throttle lock if I am running around the 60mph mark. Just thinking could my problem be a loose wire. Ps. Tyres made no difference

What do you mean by "TC won't lock up"? Do you mean the TC light comes on randomly whilst driving down a straight road?

The OP meant TC(Torque Converte) lock up IMO

Yes, torque converter. How can you check if wheel speed sensors are working correctly? This is driving me bloody mad. The gearbox works really well apart from going into torque lock and I'm using a load of extra fuel and the engine is revving higher than it needs to

If you have 8J rims you should have 255/65/16 all round. 225 are not fitted as standard and 235/70/16 are only fitted to 7J rims.

Sorry for being a dick but what are 7j and 8j rims?

sierrafery said:

The OP meant TC(Torque Converte) lock up IMO

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paul 1811 said:

Sorry for being a dick but what are 7j and 8j rims?

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The xj is just the width of the rim. So 7j would be a 7 inch wide rim and 8j would be an 8 inch wide rim

paul 1811 said:

Yes, torque converter. How can you check if wheel speed sensors are working correctly? This is driving me bloody mad. The gearbox works really well apart from going into torque lock and I'm using a load of extra fuel and the engine is revving higher than it needs to

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If you have a different overall diameter, then the speeds will read differently. But the overall speed seems to be averaged. However, the SLABS ECU will think that the wheel with a different diameter is spinning faster so will try to apply the traction control to bring the "traction" back as it thinks its lost traction due to it spinning faster

Try disconnecting an ABS sensor to disable the TC, ABS and HDC. It will calculate speed off the other 3 sensors instead.

Preferebly disconnect the sensors to the wheels that are of different overall diameter.

paul 1811 said:

Sorry for being a dick but what are 7j and 8j rims?

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The 7 and 8 refers to the seating width of the rim in inches to accommodate the tyre beads, the J or JJ refers to the seal seating for the bead of tubeless tyres, it is designed to stop a deflating tyre "rolling" off the bead of the rim and causing a "rapid" deflation, that can, of course, cause a vehicle to roll over.
You may use tube type tyres, (with tubes fitted), on J or JJ rims but you cannot run tubeless tyres as a tubeless tyres on a rim that does not have the J or JJ in its designation.