37947-03K / 4R55E / 5R44E and 5R55E, Oversize Pressure Boost Valve

$ 48.00


Increased Ratio 37947-03K


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


A worn boost valve or boost valve sleeve.


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

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

Additional information

Weight 1 lbs
Dimensions 4 × 4 × 2 in


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