Q&A / Tech

Mailbag: Why Won’t My Trans-Brake Engage? (and Other Transmission-Related Questions)

Q: I have a 1969 Chevelle with a 402 big block, a Turbo 400 transmission, and a 12-bolt rear axle with 4.56 gears. I used to run 7.20s in the eighth-mile and wanted to go faster, so I had an 8-inch converter built and a trans-brake installed. Now the converter won’t stall. It was designed to stall at 4,500 rpm, but only stalls at 1,600-1,800 rpm. Should the converter stall by itself, or should the trans-brake be engaged? How do I fix the problem?

I also have a problem the trans-brake not engaging when the motor is running. When the engine is off, you can push the button and watch the solenoid engage. I’m pulling exactly 12 volts at the solenoid so it shouldn’t be an electrical problem. I had the transmission completely gone through (thicker clutches and so on) before installing the converter and brake.

A: If a race converter is built properly, it should stall close to the expected rpm with or without the trans-brake engaged. The only exception to this is if the power-to-weight ratio overpowers the car’s brakes before you reach the desired stall speed. The trans-brake should take care of that problem. If the brake activates with the engine off but not when it is running, the problem could be either a reduction of volts or amps to the solenoid when the engine is running, or the solenoid is too weak to activate when there is fluid pressure in the transmission.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.

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