Sonnax Transmission Company, the leader in transmission innovation, announces a groundbreaking servo system to improve GM TH400, 4L80-E reliability and make staging more consistent and predictable. The externally adjustable low/reverse servo were named runner-up in the performance racing new product category at the 2025 SEMA show, standing out among more than 1,300 entries across 15 categories.
In drag racing, proper L/R band adjustment is critical to prevent transbrake malfunction. If the servo is out of adjustment, the piston can bottom out in the case bore, causing the band to slip and the vehicle to creep forward during staging. Until now, racers faced a major challenge: OE servos with steel covers are difficult to adjust in-vehicle, making quick checks impractical.
Sonnax externally adjustable servos overcome this challenge by making band checks and adjustments easy. Thanks to a unique design, there is no cover or access plug to remove — the adjuster itself extends through the cover. A quick check is done simply by pushing on the end of the adjuster and observing the movement of the piston relative to the servo cover. This unique feature combined with top-quality materials make these servos an ideal performance upgrade for any vehicle. Choose from kits for the TH400 (part number 34914-20K) or 4L80-E, 4L85-E (part number 34914-28K) transmission.
Key Features
- Quickly check and adjust the low/reverse band to prevent transbrake issues
- Externally adjustable design for easy in-vehicle service
- Works with or without OE 1-2 accumulator piston
- 6061 Anodized billet aluminum cover for strength and durability
Sonnax externally adjustable L/R servos are available now through authorized distributors. Learn more about these and other Sonnax performance parts at www.sonnax.com.
This story was originally published on February 19, 2026. 
