All The Reasons Why/How Your Transmission Might Be Slipping (And How To Fix It)

►The ZF 6HP, found in tons of BMW, Land Rover, Audi, and Jaguar vehicles, has a reputation for being largely reliable. However, any issue they have is likely caused by a pressure loss within the mechatronics, which will trigger some annoying symptoms. Everything in the video here will diagnose the possibilities related to the hydraulic side before getting into the more costly and labor-intensive mechanical side. 

Symptoms of a Failing ZF 6HP Hydraulics
Delayed Shifts
Hesitation
Slipping under acceleration

Automatic transmissions use a combination of a hydraulic system and several large mechanical components ( physical clutch packs, cogs, and gears) to turn engine power into forward motion. Tying the two together is the mechatronics unit. It controls both systems, determining hydraulic pressure and to which mechanical parts the pressure is sent at any point. Within that, the hydraulic pressure activates the shift points, shift aggressiveness, torque converter lock-up, and so on. Any loss of pressure across the mechatronics unit will cause some noticeable faults, like what happened with this Land Rover LR3.

Causes of ZF 6HP Faults
Failed sealing plate
Clogged/damaged shift solenoids
Worn valve body bores
External fluid leaks

Owner Mike Roselli was experiencing some funky behavior in the form of hesitation and slippage under load, tipping him off to a transmission issue - the question is, which side would the issue be on (mechanical or hydraulic)? ZF's 6HP automatic has a few wear points, including the valve body sealing plate and the valve body pistons. Worn valve body piston bores are a common cause as the metal-on-metal contact will cause wear with significant mileage. The same goes for the sealing plate, as its rubber/silicone seals can dry and shrink. Replacing the pistons or sealing plate means draining the transmission and removing the valve body for disassembly. It's a fairly straightforward procedure but one that does require caution, patience, and a bit of dexterity. 

In the end, addressing all of the seals and possible wear points in the hydraulic side of the transmission proved to address all of the symptoms the LR3 was exhibiting, avoiding costly (and more labor-intensive) mechanical repairs. 

Watch along as FCP Euro's BMW Expert, Gareth Foley dives into the 6HP with a bunch of parts to give it a long-overdue service and solve the Land Rover LR3's slipping transmission. 

Expert Tip For those with a 6HP looking to start this repair on their own, start first by identifying the specific transmission you have by locating the number plate on the driver's rear of the transmission. From there, you can order parts and have everything ready to complete the repair. 
Also, pay special attention to which separator plate to order, as there are several options available.

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