• Uncontrollable line rise
  • High line pressure
  • Broken parts
  • Broken clutch piston


Worn boost valve sleeve allows Reverse oil leakage and worn end plug causes imbalance of pressure regulator valve


Ensure proper main regulator valve control with Sonnax boost valve kit with O-ringed balance oil plug. Replaces '91 current OE boost valves.

4L80-E, 4L85-E

Boost Valve Kit
Part No. 34200-03K

Fits '91-later units with factory-style sleeve, no O-rings. Can be retrofitted to '89–91 units. See instructions for details.

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

Common problems in vehicles with GM 4L80-E and 4L85-E transmissions are uncontrollable line rise (upward of 500-600 psi), high line pressure in Reverse, buzzing noise, broken direct clutch drums and/or broken cases. These problems are usually caused by Reverse boost oil that enters the torque signal circuit due to a wear-created cross leak at the sleeve. High line pressure and buzz is created by oil leakage past the aluminum balance plug. Sonnax boost vavle kit 34200-03K eliminates these problems.

  • Hardened steel boost valve with over 20% wider spools to provide better durability and increased sealing contact area
  • Boost valve spools have annular grooves to reduce side-loading that causes wear
  • Valve and sleeve are manufactured to close tolerances to restore proper clearance and hydraulic integrity
  • Kit includes a replacement aluminum balance oil plug with an O-ring to ensure proper sealing and adequate back pressure on the pressure regulator valve

If you detect leakage between the sleeve and pump bore, use 34200-01K, which includes an O-ringed sleeve to compensate for pump bore wear and prevent pump scrappage.

  • Valve
  • Sleeve
  • Balance Plug With O-Ring

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.

GM 4L80-E, 4L85-E, 400 Valve Body Layout

GM 4L80-E, 4L85-E, 400 Valve Body Layout
  • 4L80-E, 4L85-E Boost Valve Kit 34200-03K
  • 4L80-E, 4L85-E Boost Valve Kit 34200-03K

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.

4L80-E, 4L85-E Boost Valve Kit Vacuum Test Locations