Back on the Bench - September - 2019

Tracking Down The ZF6HP21 E Clutch Failure

Back On The Bench featured image

The ZF6HP21 has been around for some time. If you haven’t seen one yet you’re sure to see it soon. This transmission uses other names like the 6R60, as well as other variations. Whatever name it has, they all work pretty much the same and have a common failure point; the ‘E’ clutch, which controls 5th gear. In the June 2019 issue of GEARS we covered the internal failure points, so in this issue we’ll address problem areas of the valve body.

For this, we’ll map the valve body, which is to follow the flow paths for ATF that involves the circuits for clutch apply. We do this so we can examine all of the areas that might leak and cause a failure. Flat sanding the castings will also point out low spots that might be the cause for the failure. The point being, we’ll check every pathway so we don’t have a repeat failure.

Figure one shows the EDS4 solenoid, its feed and control circuits and then clutch valve ‘E’, which controls the clutch.

Figure two shows the pressure reducing valve, which feeds all of the solenoids. We’re starting with this valve but it’s important to know that you had a problem here then you’d have other failures too. Since we’re just covering the ‘E’ clutch we’ll keep moving.

The pressure from the solenoid works its way to three valves, and the solenoid accumulator. Pressure is applied in between clutch ‘E’ valve and the plug and sleeve (figure 1), and then between the select valves and at the end of holding valve ‘E’ (figure 3). The bores and valves should be checked for damage and broken springs. You should also take some time vacuum-check these valves since their wear isn’t always noticeable.

Following the pressure from mainline pressure to the ‘E’ port in the case. Select valve ‘3’ opens a path from main line pressure to holding valve ‘E’ (figure 4) and clutch valve ‘E’ (figure 5). In some ZF applications, the select valve is in the rested position to allow fluid through, and the solenoid does not stroke the valve. Following the path of solenoid feed leads us to find something we might not have found looking at a hydraulic diagram.

Clutch valve ‘E’ opens the path from the select valve to the ‘E’ clutch apply circuit while the holding valve ‘E’ blocks off fluid pressure from exhaust.

ZF valves have been known to lose some of the anodized coating (figure 6) so it’s a good idea to disassemble the entire valve body, even if you don’t yet have other problems.

Next, we want to bench test the EDS4 solenoid (figure 7). Use a small piece of rubber hose to block off the solenoid feed area and then apply about 30 psi of air through the end of the solenoid that goes to the valves. Air should pass through freely. Now energize the solenoid and check that it seals off exhaust area. If the solenoid passes, and everything else checks out, we might have a command issue with the mechatronics unit, however, you’re much more likely to find the leak in the areas we’ve detailed here, or in the internal components we covered in the previous issue.

All this may seem a bit tedious and time consuming but we’ve seen shops struggle with this problem; having the unit fail after rebuild only to find that some of these areas were over looked. It’s not necessary to go through this on every unit that hits your bench for an ‘E’ clutch failure it’s better to take the time.

Mapping a valve body takes time but it gets easier as you practice it more often. And as you get better at it you’ll be amazed at the imperfections you run across that you’d miss otherwise and you’ll keep these units from showing up ‘Back on the Bench’.