• 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. 37947-01K

OE 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 boost valve 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
  • Hardcoat 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
  • 4R44E, 4R55E, 5R44E, 5R55E Boost Valve Kit 37947-01K
  • 4R44E, 4R55E, 5R44E, 5R55E Boost Valve Kit 37947-01K

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