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  • Low line pressure
  • Soft shifts
  • Poor modulator control
  • Delayed Reverse


A worn boost valve or boost valve sleeve.


Restore proper line pressure control with closely toleranced boost assemblies, made from wear-resistant materials.

4R44E, 4R55E, 5R44E, 5R55E

Boost Valve Kit
Part No. 37947-03K

Increased Ratio

View Instructions

Some typical problems in 4R44E, 4R55E, 5R44E and 5R55E units are soft shifts, delayed reverse, poor modulator control and/or low line pressure in reverse. A worn boost valve is often the cause, allowing reverse and/or TV pressure to cross leak or exhaust. Sonnax offers two replacement boost valve kits for these units: the OE ratio kit 37947-01K and the increased ratio kit 37947-03K. The 37947-01K has OE spool ratios, to return line rise to stock condition. The 37947-03K assembly has increased ratios for slightly higher line pressures.

  • Sleeves are manufactured from highly wear-resistant aluminum alloy.
  • Valves are made from hard-anodized aluminum for better wear resistance.
  • Tight tolerances restore the hydraulic integrity of the assembly.
  • Kits improve engagement related to loss of boost oil and increased line rise in forward and reverse.

Pressure Comparison Between OE and Increased Ratio Boost Valves

EPC PSI 37947-01K
(OE Ratio)
(Increased Ratio)
Line Reverse Line Reverse
30 107 135 114 143
75 178 224 194 243
120 250 313 275 343

OE sleeves may come in either a 3- or 2-circuit design. Boost valve kits 37947-01K and 37947-03K may be used to replace either version

  • Valve
  • Sleeve

Sonnax valve body layouts provide a detailed overview of individual units making it quick and easy to determine what’s available for the specific valve body you’re working on. See all Sonnax valve body layouts.

Ford 4R/5R44E, 4R/5R55E Valve Body Layout

Ford 4r 5r44e thumb

May 21, 2013

Ford 4R44E, 4R55E, 5R44E, 5R55E Vacuum Test Guide

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  • 37947 03k