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


A worn boost valve or boost valve sleeve.


Restore proper line pressure control and prevent future wear Sonnax drop-in boost valve kits.

4R44E, 4R55E, 5R44E, 5R55E

Boost Valve Kit
Part No. 37947-03K

Increased Ratio

You need this if... View Instructions Find a Distributor

Some typical problems in Ford 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 boost valve 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
  • Harcoat anodized aluminum valve combats premature wear
  • 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
  • Drop-in Zip Valve parts install quickly and easily

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
Learn More About Zip Valve™ Drop-In Repairs

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 4R44E, 5R44E, 4R55E, 5R55E Valve Body Layout

Ford 4R44E, 5R44E, 4R55E, 5R55E Valve Body Layout
  • 4R44E, 4R55E, 5R44E, 5R55E Boost Valve Kit 37947-03K
  • 4R44E, 4R55E, 5R44E, 5R55E Boost Valve Kit 37947-03K

You need this if...

Vacuum testing at the port(s) indicated fails to hold the recommended minimum in-Hg, or if wear is visually detected.

4R44E, 4R55E, 5R44E, 5R55E Boost Valve Kit Vacuum Test Locations