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ZF6HP26 bumps and clunks

Friday, July 31, 2015 - 07:00
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After rebuilding a ZF6HP26 transmission, it is frustrating to say the least when you discover harsh, bumpy or clunky type shifts on a road test. If your scan tool is capable of resetting the adaptations you try that. When that doesn’t work, you bite the bullet and bring it to the dealer for software updates and adaptation reset. The shifts feel a little better especially the 2-1 but the shifts are still not clean enough to deliver the car.

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Figure 1
Figure 2

By this time your thinking maybe if I drive it long enough with my fingers crossed it will drive itself out. But then you begin to wonder, I know I didn’t use OE fluid and maybe that is why I am having the problem. After all, with Shell M1375.4/Life Guard 6 fluid selling around 25 bucks a liter with a trans that takes 9.5 liters on a dry fill, it’s a tough call. This is especially so when you say to yourself, my Amtecol Duralife ATF DVI synthetic fluid at near 5 bucks a quart has always worked before*. Why would it be a problem now? So do you take the plunge (no pun intended) and buy OE fluid to see if it fixes the problem? This problem becomes more frustrating when you discover that the problem was self inflicted. As a rebuilder, sometimes tightening up a clutch pack makes for a nice shifting transmission. Not so with ZF6HP transmissions (figures 1 and 2). I have been told that the adaptive process is based on friction surface area per friction plate which requires looser clearances for proper shift overlap to take place. When you disturb that clearance to the tighter end, you interfere with the computer’s calculating process to make that shift. In other words, you tighten the clearance and the computer will not be able to properly adjust for the overlap resulting in bumpy shifts.

Adjust clutch clearances as follows:

Clutch A: .062” - .080”

Clutch B: .055” - .073”

Clutch C: .054” - .072”

Clutch D: .082” - .106”

Clutch E: .072” - .093”

*Caution: There is a law in California where it recognizes OE fluids as the only legitimate replacement fluid to use. The cost of Shell M1375/ Life Guard 6 from Land Rover or Jaguar in California can be as high as $60.00 per liter.  Regardless of the cost, it is the only fluid California law allows. 

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