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  • Poor line pressure control
  • Poor Reverse engagement


Oscillation of the aluminum valve wears out the pressure regulator bore.


Eliminate excessive leakage by reaming the valve body bore and installing the oversized replacement pressure regulator valve.

ZF4HP14, ZF4HP18

Oversized Pressure Regulator Valve
Part No. 85991-01

View Instructions

Many vehicles with either a ZF4HP14 or ZF4HP18 transmission experience poor line pressure control due to a worn pressure regulator bore. This bore wear can be verified by testing line pressure and/or performing a Wet Air Test on the valve body. The past fix was to replace the valve body, which is expensive and hard to find. These problems are often caused by wear from the oscillation of the aluminum valve in the valve body bore. Line/balance, modulated TV, and converter/cooler pressures then leak past the valve and exhaust to the sump or cross leak into other circuits. Excessive leakage can be eliminated by installing the oversized Sonnax replacement pressure regulator valve 85991-01.

  • The five spool diameters on this valve are slightly larger than the OE valve, allowing the valve body bore to be modified to restore the hydraulic integrity of the assembly.
  • Valve is manufactured from hard-coat anodized aluminum to resist wear.
  • The valve body bore can be made large enough to accept this replacement valve by reaming it with the specially designed Sonnax reamer tool kit 85991-TL.

September 30, 2005

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