Shift Pointers

• Subject: Melted relief valve blocks fluid flow through pump cover
• Unit: ZF 6HP26
• Vehicle Application: BMW X5
• Essential Reading: Rebuilder, Diagnostician
• Author: Pete Luban, ATSG

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A BMW X5 SUV came into the shop on the hook with a no-move condition and a major leak from a cooler line.

The ZF 6HP26 transmission was removed, disassembled and inspected, and no clutch damage was found. However, some strange pieces of rubber were found and the converter was blue.

The transmission was rebuilt and a brand-new converter from ZF was installed, along with a new set of cooler lines. When the SUV was started up, the converter started to make a horrendous rattling noise.

The transmission was removed and inspected again and no damage was found, so another brand-new ZF converter was installed with the transmission. Once more, when the SUV was started up the same rattling noise was coming from the converter.

At this point some spare transmission parts were located, and in the meantime the original valve body was disassembled. Upon inspection of the valve body, one of the plastic pressure-relief valves was found to be severely melted and welded to its spring (see Figure 1).

The damaged relief valve was replaced and the valve body was put back into the transmission, which was reinstalled in the vehicle. The noise was gone, the SUV road-tested beautifully for a few days, and the vehicle was delivered to the customer.

A short time later, the transmission started to operate poorly, and the vehicle surged when coming to a stop. The shop removed the transmission again and disassembled and inspected it but found nothing to which the complaints could be attributed.

Again the spare-parts bin was raided and a new pump was disassembled and inspected, then compared with the original pump. When both pump covers were laid side by side, the cause of the problem reared its ugly head. Figure 2 shows the new pump cover; the indicated area shows that the lube hole is clear. In Figure 3, showing the original pump cover, this same lube passage is blocked by some of the melted plastic relief valve from the original valve body. Once the transmission, with the new pump, was reinstalled in the SUV, all was well.

Sometimes when a part is found to be bad, that may be only the tip of the iceberg; careful inspection and, sometimes, comparing a known good part with the original one may solve the mystery.

One more point to this story: It could be assumed (I hate that word) that the leak in the cooler line and a low-fluid condition probably caused the relief valve to melt and the converter to turn blue from excessive heat, but look at the radiator in Figure 4 that was removed from the SUV. I think it would be safe to say that that also could have been a contributing factor to the damage seen.

Many thanks to Sean O’Connor from AC/A&M Gearbox Center in Dublin, Ireland, for sharing his experience and also for the photos.