The valve body is a major component of any transmission. It is a control center that looks like a maze filled with valves, passages, and solenoids that divert transmission fluid where it’s needed. The transmission valve body isn’t known to fail all that often, which is a good thing. But when it does fail, you are left with one of two options, both of which will be expensive – either rebuild or replace it. Get familiar with the symptoms of a failing transmission valve body so you can take some preventative measures.
How It Works
All automatic transmissions have a valve body that acts as the brain of the transmission. Within, there are dozens of passages and channels that direct the flow of hydraulic fluid to different parts of the transmission. This is in order to activate the right clutch pack or band and shift to the most appropriate gear based on the speed of the vehicle. Each valve has a different purpose and they are named to reflect that job. The valve body has a 1-2 shift valve that is responsible for shifting up from first to second gear.
Symptoms of a Failing Valve Body
Delayed Garage Shifts
One of the most common symptoms of a failing transmission valve body is a delay in shifting or harsh shifting. Usually called garage shifts, it’s when there is a harsh shift from park to drive or drive to reverse. Try to limit the number of times you are in that situation, even if it means backing into the garage.
When driving, you could notice the transmission shifting into higher or lower gears at the wrong time. You could be struggling to get up a large hill, and for no reason the Allison transmission can shift into a higher gear, making the climb even harder. Another example is driving on a flat highway at 65 mph and the transmission shifting into another gear for no reason at all.
When a transmission is working properly, there should be a drop in the RPMs on an upshift to match the higher gear in between every shift. A shift flare is when the opposite happens; the RPMs will increase, and the engine will rev up in between gears.
Gear slippage doesn’t always mean a failing valve body. The culprit could sometimes be low transmission fluid or worn transmission bands. But, it could also mean the valve body is going bad. A slipping transmission means your truck is trying to upshift but then falls into a lower gear. It might even refuse to shift into higher gear, which makes the engine higher than necessary.