Back on the Bench - June - 2019

Tracking Down the ZF6HP21 E Clutch Failure

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Normally, when a road trip is planned, it’s planned out using some form of map or GPS. Imagine going on that trip without the use of either. There’s a pretty good chance of getting lost on the way to the destination.

This also applies to transmissions. Hydraulics are the road maps for the circuits, and without that road map, it can lead to some difficulty tracking down the cause of the problem. Today we have a ZF 6HP 21 on my bench, with the common E clutch performance code. Without using hydraulic diagrams, let’s follow the path of the fluid, starting from the clutch to main line pressure. And the solenoid pressure that operates the valves.

During disassembly there are a few things I like to do that maybe considered backwards. Once I get the transmission on the bench I like to verify the torque of the valve body and pump bolts. After the pan and converter has been removed, use your torque wrench and verify the torque on the pump and valve body bolts.Why, you might ask?These bolts may contribute to a cross leak between components. This can be overlooked if an impact is used to remove bolts.

After removing the valve body, it’s a good idea to air check for any kind of circuit leaks before removing the drive train. Make sure to check the unit endplay while disassembling the drive train, also check clutch clearances and air check each component. Often times, the cause of the problem can be found before getting too deep into the transmission.

Starting in the E clutch drum, check the seal and the bushing surface (figure 1). Next, look at the rear of the stator support. It’s common to see damage on both the surface where the sealing ring rides and the bushing (figure 2). When this bushing wears enough, the input shaft can damage the surface inside the support where the seal on the input shaft contacts the inside of the support.

Typically when air checking the drum for leaks, we only use 35 PSI. In this case it’s recommended to use full shop air pressure and some fluid. Low pressure might not be enough at times to reveal certain leaks.

When looking inside the feed hole, the tube that separates the E clutch from the torque converter circuits is visible. If there is any air coming out of the hole above it, or around the base of the tube (figure 3), the drum will need to be replaced; the tube is not sold separately at this point in time. Another common problem that has been seen in other ZFs and 6R80s, is the drum cracking around the area at the base of the shaft (figure 4). Take some Transjel® and apply a thin layer around this area. Apply full shop pressure and look for bubbles to form. This can often be a small leak, that becomes much worse with heat.

Be sure to check the piston in the drum for cracks and seals for damage. Check the surface were the seals ride for excessive wear. Even though this isn’t a common issue, it’s worth the time to inspect these components.

Heading into the stator support area, through the feed hole (figure 2) into the back half of the pump (figure 5). Then through the pump (figure 6) and into the case. In figure 6, you can see E clutch oil comes very close to the suction side of the pump. There are no gaskets in the pump, or between the pump and case, and any kind of surface damage, warpage, or loose bolts can allow cross leaks. Always flat sand the pump body halves to prevent E clutch oil from being pulled into the suction side of the pump. From the pump, the fluid path leads through the case and into the valve body.

In the next article, the real fun begins, mapping the path of the fluid through the valve body.