P38 Auto Issue With Reverse

Hi.

I have recently purchased a 2001 4.0 HSE P38 AUTO being sold as a spares or repairs vehicle. The Range Rover has two issues. The first being a failed EAS pump (easy fix), the other being the lack of reverse power! The gearbox works perfectly in drive, but is very week in reverse. The gearbox oil and filter has been changed and apart from improving the forward drive gear change, it hasn’t made any difference to reverse. When reverse has been selected, the Range Rover will creep backwards at idle. When you apply revs, no change in reverse speed, similar to huge amount of manual clutch slip. Selecting low range will allow the Range Rover to creep up a slight incline due to the low ratio, but again, no change in speed when applying revs. I have no warning lights or messages on the dash.

Any advise will be very appreciated........

Forgot to mention, the gearbox code is ZF 4HP-22.

Dylan Banachowicz said:

Forgot to mention, the gearbox code is ZF 4HP-22.

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The EAS compressor has probably failed due to leaking airsprings.
For the gearbox, someone who knows will be along in due course, but did you fill the box with the engine idling and after going through all the gears? You could try some Lucas Transmission Fix, it sorted mine.

We know the issue with the EAS, the pump seals have failed. The previous owner removed the original working pump and valve block to fix his failed one, it worked perfectly before removal. I didn’t change the fluid and filter, the original owner did so will check levels to make sure. It drives forward faultlessly, only issue is with reverse. Used boxes are cheap, may go for a ZF 4HP-24 from a 4.6 if needed.

Dylan Banachowicz said:

We know the issue with the EAS, the pump seals have failed. The previous owner removed the original working pump and valve block to fix his failed one, it worked perfectly before removal. I didn’t change the fluid and filter, the original owner did so will check levels to make sure. It drives forward faultlessly, only issue is with reverse. Used boxes are cheap, may go for a ZF 4HP-24 from a 4.6 if needed.

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Think you need more than just the gearbox if changing from a 22 to a 24

Dylan Banachowicz said:

We know the issue with the EAS, the pump seals have failed. The previous owner removed the original working pump and valve block to fix his failed one, it worked perfectly before removal. I didn’t change the fluid and filter, the original owner did so will check levels to make sure. It drives forward faultlessly, only issue is with reverse. Used boxes are cheap, may go for a ZF 4HP-24 from a 4.6 if needed.

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Reverse clutch will have failed more than likely due to a sticky valve block not allowing full pressure to be applied to it..

tomcat59alan said:

Think you need more than just the gearbox if changing from a 22 to a 24

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It's amazing how many think you take one out and bolt the other in.

From what I’ve been told, the gearbox itself is externally identical, internal parts are stronger but will need to swap the bell housing and torque converter with the ZF 4HP-22 as the 24 torque converter is deeper therefore the bell housing is longer. Maybe I have been misinformed?

wammers said:

Reverse clutch will have failed more than likely due to a sticky valve block not allowing full pressure to be applied to it..

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Thank you for the info. Will probably just swap it with a known good unit.

Dylan Banachowicz said:

From what I’ve been told, the gearbox itself is externally identical, internal parts are stronger but will need to swap the bell housing and torque converter with the ZF 4HP-22 as the 24 torque converter is deeper therefore the bell housing is longer. Maybe I have been misinformed?

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That sounds about right.

tomcat59alan said:

That sounds about right.

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The 22s torque converter fingers may not match the 24s oil pump. So the pump would need changing also.

Dylan Banachowicz said:

From what I’ve been told, the gearbox itself is externally identical, internal parts are stronger but will need to swap the bell housing and torque converter with the ZF 4HP-22 as the 24 torque converter is deeper therefore the bell housing is longer. Maybe I have been misinformed?

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Depending on the age of the HP24, it should be a straight swap using your original HP22 torque converter and bell housing. However on the V8 you may also be able to fit the 4.6 torque converter and bell housing but may then need to change the flex plate. Some HP24's do need the pump changed to match the different torque converter when this is done on the diesel.

Ashcroft can fix the guts of the HP24 into the Hp22's casing..

Would rather ebuild the unit than replace with an unknown 2nd hand one..

Henry_b said:

Ashcroft can fix the guts of the HP24 into the Hp22's casing..

Would rather ebuild the unit than replace with an unknown 2nd hand one..

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Lots of the HP24's are low mileage, and for £200 compared to the best part of £3K for an Ashcroft rebuild a secondhand one is worth a shot IMO.

Well that was easier than I anticipated. A GEMS 4.6l HP24 auto box fitted to a THOR 4.0l engine.

I picked up a know good working order GEMS HP24 gearbox for £150 to replace my faulty THOR HP22 box. There seems to be a lot discussions regarding lack of compatibility so decided to give it ago, what’s the worst that can happen!!!!!

To start with the overall gearbox length is identical as is the transfer box used on all P38’s, so no issues with length, mounts etc. The flywheel mounts on the crankshafts are also identical. The difference starts with the bell-housing length. The GEMS HP24 is longer to accept the larger torque converter. The input shaft on the GEMS HP24 gearbox is also substantial longer so the THOR HP22 torque converter WILL NOT FIT the GEMS HP24 box. To compensate for the longer bell-housing, a shorter rear section is fitted to the rear of the gearbox to maintain an identical overall length. The rear section also houses the crossmember mounting points, therefore all engine and gearbox mounts are identical for all engine / gearbox configurations.

The issue starts with the larger torque converter. The GEMS HP24 4.6l torque converter is larger in diameter and depth. This causes an issue when the gearbox is offered up to the engine. Basically there should be a 2mm clearance gap between the torque converter and flex plate with the torque converter pressed fully in to the gearbox and the gearbox bolted to the engine. The torque converter will be pulled up to the flex plate when the flex plate is bolted in position. This distance is important to prevent damage to the gearbox fluid pump, too small a distance will put mechanical pressure on the pump, too large a distance could cause damage to the locating slots. To overcome this issue, a thinner flex plate / flywheel is fitted to the GEMS HP24 4.6l engine. An easy fix would be to use the flex plate supplied with the GEMS HP24 gearbox as engine and gearbox dimension are identical (GEMS / THOR, 4.0 / 4.6, HP22 / HP24). Unfortunately the GEMS (Lucas) and THOR (Bosch) engine use a different crank-position-sensor pickup on the rear of the flywheel / flex plate, so that won’t work. After a bit of comparing between the flywheels / flex plates, I discovered a spacer between each of the flex plates & crank mounting bosses, problem solved. Swap the spacers between GEMS & THOR flex plates!!! With this done, gearbox moulted perfectly with the correct clearances between torque converter and flex plate.

One thing I didn’t notice, was the bell-housing / engine mounting bolts. The THOR engine uses an aluminium sump, the GEMS engine uses a pressed steel sump. The THOR engine has M12 mounting bolts all round the bell-housing, into the engine block and sump. The GEMS engine only has mounting bolts around the engine block. Due to the use of a pressed steel sump, no mounting bolts are available around the sump area, instead a cover is mounted to the gearbox using M8 bolts. Prior to fitting the gearbox, it would be a good idea to drill and tap the bell-housing to accept the correct bolts around the sump area of the THOR engine.

So I now have a THOR 4.0 engine fitted to a stronger GEMS 4.6 HP24 gearbox.

Hope the photos help explain........

Dylan Banachowicz said:

View attachment 184673 View attachment 184681 View attachment 184682 View attachment 184684 View attachment 184687 View attachment 184688 View attachment 184689 Well that was easier than I anticipated. A GEMS 4.6l HP24 auto box fitted to a THOR 4.0l engine.

I picked up a know good working order GEMS HP24 gearbox for £150 to replace my faulty THOR HP22 box. There seems to be a lot discussions regarding lack of compatibility so decided to give it ago, what’s the worst that can happen!!!!!

To start with the overall gearbox length is identical as is the transfer box used on all P38’s, so no issues with length, mounts etc. The flywheel mounts on the crankshafts are also identical. The difference starts with the bell-housing length. The GEMS HP24 is longer to accept the larger torque converter. The input shaft on the GEMS HP24 gearbox is also substantial longer so the THOR HP22 torque converter WILL NOT FIT the GEMS HP24 box. To compensate for the longer bell-housing, a shorter rear section is fitted to the rear of the gearbox to maintain an identical overall length. The rear section also houses the crossmember mounting points, therefore all engine and gearbox mounts are identical for all engine / gearbox configurations.

The issue starts with the larger torque converter. The GEMS HP24 4.6l torque converter is larger in diameter and depth. This causes an issue when the gearbox is offered up to the engine. Basically there should be a 2mm clearance gap between the torque converter and flex plate with the torque converter pressed fully in to the gearbox and the gearbox bolted to the engine. The torque converter will be pulled up to the flex plate when the flex plate is bolted in position. This distance is important to prevent damage to the gearbox fluid pump, too small a distance will put mechanical pressure on the pump, too large a distance could cause damage to the locating slots. To overcome this issue, a thinner flex plate / flywheel is fitted to the GEMS HP24 4.6l engine. An easy fix would be to use the flex plate supplied with the GEMS HP24 gearbox as engine and gearbox dimension are identical (GEMS / THOR, 4.0 / 4.6, HP22 / HP24). Unfortunately the GEMS (Lucas) and THOR (Bosch) engine use a different crank-position-sensor pickup on the rear of the flywheel / flex plate, so that won’t work. After a bit of comparing between the flywheels / flex plates, I discovered a spacer between each of the flex plates & crank mounting bosses, problem solved. Swap the spacers between GEMS & THOR flex plates!!! With this done, gearbox moulted perfectly with the correct clearances between torque converter and flex plate.

One thing I didn’t notice, was the bell-housing / engine mounting bolts. The THOR engine uses an aluminium sump, the GEMS engine uses a pressed steel sump. The THOR engine has M12 mounting bolts all round the bell-housing, into the engine block and sump. The GEMS engine only has mounting bolts around the engine block. Due to the use of a pressed steel sump, no mounting bolts are available around the sump area, instead a cover is mounted to the gearbox using M8 bolts. Prior to fitting the gearbox, it would be a good idea to drill and tap the bell-housing to accept the correct bolts around the sump area of the THOR engine.

So I now have a THOR 4.0 engine fitted to a stronger GEMS 4.6 HP24 gearbox.

Hope the photos help explain........ View attachment 184681View attachment 184682View attachment 184684View attachment 184687View attachment 184688

Click to expand...

The HP22 to HP 24 swap has been done many times on the diesel. Just use the smaller torque converter.

Dylan Banachowicz said:

View attachment 184673 View attachment 184681 View attachment 184682 View attachment 184684 View attachment 184687 View attachment 184688 View attachment 184689 Well that was easier than I anticipated. A GEMS 4.6l HP24 auto box fitted to a THOR 4.0l engine.
/*SNIP!*/
Hope the photos help explain........ View attachment 184681View attachment 184682View attachment 184684View attachment 184687View attachment 184688

Click to expand...

Thanks for sharing. Great write up and hopefully others swapping a GEMS into a Thor will find it useful!

Datatek said:

The HP22 to HP 24 swap has been done many times on the diesel. Just use the smaller torque converter.

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That was my first idea, but the input shaft on the THOR HP22 & GEMS HP24 gearboxes are hugely different in length, some 24mm. Can’t comment on the difference between the THOR HP22 & HP24 gearboxes, I would assume the input shafts are the same as they both use the same size torque converter. Using the THOR HP22 torque converter in the GEMS HP24 gearbox would result in the turbine splines locating correctly, but not the fluid pump, the torque converter may not even reach the gearbox oil seal.