Is ATF incompatible with the ZF steering rack?

[Graeme]

Is it known that ATF is incompatible with ZF racks?

I used Mobil ATF in my rack due to not able to travel to get the correct oil with a badly leaking input shaft seal, for which I had a replacement on hand. The rack moved smoothly during multiple attempts to use hydraulic pressure to push out the seal block but became extremely stiff immediately once driven. The rack seems to be binding as without the engine running and the vehicle on stands, turning the wheel can cause a groan.

I suspect that the main piston seal might not be compatible with ATF.

[shanegtr]

Is it known that ATF is incompatible with ZF racks?

I used Mobil ATF in my rack due to not able to travel to get the correct oil with a badly leaking input shaft seal, for which I had a replacement on hand. The rack moved smoothly during multiple attempts to use hydraulic pressure to push out the seal block but became extremely stiff immediately once driven. The rack seems to be binding as without the engine running and the vehicle on stands, turning the wheel can cause a groan.

I suspect that the main piston seal might not be compatible with ATF.
I ran Dexron III ATF in my D3 for nearly 10 years without issue

[p38arover]

I’m using a Penrite equivalent to the specified fluid in my L322 zF rack.

[Graeme]

Thanks for the comments.

Draining the ATF then filling with Penrite PAS001 which is supposedly equivalent to the LR spec didn't make any difference. However the absence of any turbulence in the half-full reservoir at idle and only minimal at high engine revs prompted a test of the steering weight at high revs which was about normal for slow speed driving. This confirmed that the issue appears to be low output from the pump rather than an issue with the rack. Being variable displacement, something has gone awry with the pump's pressure/flow control mechanism.

[Graeme]

A mostly blocked filter in the reservoir would restrict supply to the pump. I had considered that my draining by removing the servotronic valve indicated that the flow wasn't restricted, but perhaps the draining is via the return port in the reservoir.

[Graeme]

The partly blocked filter in the reservoir was the culprit.