Hello, I’ve just bought a 2.5 dse and have been searching the web for information and came across this site, I am looking for information regarding auto gearbox and cooling upgrades for a p38 that has been chipped.

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welcome, to the pub.

Upgraded radiator (https://www.direnza.co.uk/product/cooling/radiators/range-rover-p38-2-5-td-94-99-aluminium-performance-radiator/) and ensure the water pump is good and has a metal not plastic impeller.

The diesel gearbox is the ZF 4HP22 and is also used in the 4.0 litre petrol but the 4.6 petrol uses the stronger 4HP24. A member on here, dave3d, has put a 4HP24 into his diesel so can probably offer some advice.

Thanks
It was reading about the 4hp24 gearbox being used behind the 2.5 that caught my attention. I have had problems in the past when towing with other p38 2.5 engined RR, can the gearbox mods be done or do you need to go to a aftermarket supplier eg Ashcroft. Does the Direnza rad improve cooling significantly? does it still have flow down one side and backup the other? I seen an article about fitting rad from one of the v8 RR to get top to btm flow. The 2.5 dse we’ve just bought is very clean and although it needs oil cooler pipes for gearbox and engine changing seems mechanically perfect, that’s probably jinxed it now.

To fit the 4HP24 you'd need to mix and match the torque converter and bellhousing, Ashcrofts would be able to advise you which bits you'd need to use. The rest of the gearbox is the same, just with stronger internals, so you shouldn't need to modify anything. I've got a 4.0 litre V8 with the same 4HP22 as you have and regularly tow 3.5 tonne trailers and never had a problem (despite my gearbox being the original with 450,000 miles on it). What problems did you have in the past?

I doubt you could easily fit a V8 radiator as the diesel one incorporates the gearbox oil cooler while the V8 has a separate one.

sadly a V8 RR rad won't fitt in a diesel as the closer pipes run through the middle
pluss I belive the diesel rad is smaller because of the intercooler I might be wrong on that one

That's probably it as on the V8 the engine oil cooler and transmission cooler are sandwiched between front of the radiator and behind the AC condenser, whereas that space is taken up by the intercooler on the diesel. Hence the oil cooler being in the base of the radiator and the transmission cooler being where the nearside foglight should be.

yep that's the one thats why you don't get front fogs on a diesel and if you do it rare to see

Got front fogs on my 2001 dhse. Fan on the cooler.

A few people have added them as, surprisingly enough, the wiring is there but as far as I know they never offered them as an option on the diesel.

you can fit the HP24 box it's just the torque converter is to big on the HP24 so you have to use the one from the HP22 including the oil pump and bell housing.

My experience with the automatic was travelling about 50 mph and barely breathing on the accelerator it would change down and then back up again nearly instantly, that gearbox might have had problems I don’t know. Also the engine just seemed to rev without a proportional increase in speed, I believe changing to the larger torque converter helps with this but I might be wrong.
Are any electrical mods needed to fit the 4hp24 gearbox? I understand it needs the 4hp 22 bell housing and the props may need altering. I am not a fan of the plastic on the radiators after having one top tank split and another one disintegrate so I think I will go for an aluminium rad.

it's a common upgrade for a disco as the torque converter is smaller than the standard HP22 converter . you will need a new flex plate , boss to bolt to crank and a mod to the bell housing to do up bolts on flex plate . all available from Ashcroft. you can upgrade a 22 box or down grade a 24 box eg fit original bell housing and a 22 pump. the 24 has a bigger torque converter than the 22 so it has a bigger pump and the 22 converter doesn't fit.
PS the 24 torque converter will not fit in the diesel bell housing.

Swapping the HP22 gearbox for a HP24 box on the diesel is pretty straight forward. I fitted one to mine over 10 years ago and it has been fine since.

The HP22 was only used in the 4.0 petrol and the 2.5 diesel, while the HP24 was just used in the 4.6 petrol.
There are actually two versions of the HP24 gearbox - an early one and a later type, which is known as an 065 box. The number will be found on the gearbox plate. It should say something like 030 - 065.

The 065 box is a direct swop for the HP22 . The earlier HP24 is 15mm longer. It can be adapted to fit but it saves a lot of hassle if you find a later 065 box. You need to use the diesel bellhousing, the bolt holes are the same, and also use the diesel torque converter. I had an uprated one fitted that was supplied by Ashcrofts but I think they no longer do them.

No electrical mods are needed. Continue to use the same diesel gearbox ecu.

My car used to hunt up and down the gears before I chipped it. Towing up a hill was awful. It would drop from 4th straight down into 2nd, rev like hell then change back into top. It would then repeat the cycle, ad infinitum. Chipping the engine changed all that with later gear changes.
The uprated TC helps with later gearchanges but is or was expensive. I would continue to use the OEM diesel TC if I were you.

Cooling was a problem on mine at the start but It was only when towing in hot weather, like towing a caravan in France in the summer. Solo is no problem at all. Following Dave Ashcrofts advice I fitted a thermocouple to the gearbox sump and fitted a gearbox temp gauge in the dash.
The gearbox will overheat never mind the engine.
With a chipped engine it could be 40 to 50% more power and the torque increase even more than that. It means 50% more heat and cooling has got to be spot on.
The plastic on old radiators goes brittle with time and they can burst violently. Mine did several years ago.
I fitted an Allisport radiator which has better cooling. It looks the same as the Direnza.
More sure your viscous fan works ok. I also did a mod to the electric fans at the front to turn them on manually when needed.

Another thing I would point out is that some aftermarket radiators that are advertised have a much reduced heat transfer area. They are very shallow front to back. You get what you pay for I guess. OEM rads will be fine and the alloy ones I have seen all look OK. All diesel rads are now sold with a manual gearbox cooling section. Just don't pipe it in if fitted to an auto.

You can't fit fog lights to an auto diesel because the gearbox cooler gets in the way of the nearside one. Later cars have a fan on the cooler and have even less clearance. Manual diesels could probably be fitted with fog lamps however. You would have to fit an after market auto gearbox cooler up front and remove the original cooler under the wing to fit fog lights. The wiring is there ready for all models but would need activating in the BECM.