The valve body is one of the pieces of equipment that allows an automatic transmission to function correctly. By channeling hydraulic fluid through a series of paths, the valve body is able to trigger the correct clutch pack for the selected gearing.
Inside of the valve body, there are a number of valves, each of which is referred to by the shift that it’s responsible for. For example, the third-fourth valve activates the upshift from third to fourth gear, and the fourth-third determines when to downshift from the fourth to the third gear.
Because the valve body is responsible for determining when to be in drive or reverse as well, it is connected to the shifter in the vehicle. The valve that’s connected to the shifter is referred to as the “manual valve” since it’s the one valve that takes input from the driver.
The valve body location
Types of Valves
As its name implies, a valve body is really just a housing apparatus and selector for a variety of valves that are able to control the gearing of your vehicle. Because every valve body and transmission is different, these are just the primary types of valves that you’re likely to find inside of the valve body.
Valve Location Will Vary
The vast majority of valves inside of a valve body will be shift valves. These are simple balance-type spool valves that operate the bands, servos, and gearsets of your engine.
Essentially, shift valves are the reason that an automatic engine is able to have different gears at all. It’s important for these valves to be sensitive because they need to be able to respond to the engine load, the engine speed, and gearshift position.
The shuttle valve exists to allow fluid to flow through it. Inside the valve is a block of some kind (it’s usually a ball). What makes a shuttle valve different than say an empty pathway is that it can allow fluid from two different sources to enter and push fluid to a third point. Considering this whole system relies on hydraulic pressure the shuttle valve is fairly important.
This is the valve that is controlled by the driver and allows the engine to be placed into park, reverse, drive, or neutral.
Pressure Reg Valve
There are usually two of these, a primary and secondary regulatory valve. The first determines the amount of oil coming from the transmission pump. The second regulates the oil going to the torque converter.
This is essentially an override valve. If the kickdown valve detects that acceleration is occurring quickly, the transmission will shift into a lower gear. The kickdown valve is able to ignore shift control pressure.
This valve responds to pressure from the gas pedal. Though this isn’t a precise way for the driver to control the valve body, it is still a valve that the driver has some control over.
The governor valve has been replaced with an electronic solenoid in many modern engines, but the process is largely the same. The governor (or electronic solenoid) senses the speed of the output shaft and uses that indicator to help control gear shifts. The pressure coming from the governor increases with vehicle speed.
Vacuum Modulator Valve
This is a diaphragm device that works to read the engine load on the shift valve. As the load rises and falls, the diaphragm expands and contracts, which changes the throttle control pressure.
Symptoms of a Bad Valve Body
Given how complex valve bodies are, it’s not surprising that from time to time they fail. This is to be expected. If your car starts to exhibit these behaviors, it’s possible that you’ll need a new valve body.
Some have said that these noises sound almost like a banging, and tend to start when you’re using the brake pedal to slow down.
You don’t have to touch your shifter frequently in an automatic, so it should be very noticeable if one of the few times you go to change gears your shifter suddenly “slips.” This can definitely be alarming, but is one of the issues that’s easiest to fix. This is usually caused by either a low fluid level or loose valve body bolts.
It isn’t just upshifts that are controlled by the valve body. Downshifts are also controlled, and when your hydraulic pressure starts to get squirrelly, your vehicle will have a harder time downshifting. This is another time when it's possible your problem is caused by loose valve body bolts.
If you’ve ever had a long pause between going in reverse and being able to slide into drive, then you understand the pain of a delayed shift. The honks from the cars impatiently waiting behind you, the annoyed stares, and the shame are just the beginning of it. This issue can be caused by valves sticking.
Irritatingly enough, erratic shifting can be caused by both low fluid levels or high fluid levels, proving that with pneumatic systems it’s really important to get it “just right.” This can also be caused by a misadjusted kickdown rod.
Fixing a faulty valve body can be expensive, but it’s hardly the most expensive fix in a car, and it’s best to take care of it as soon as you notice a problem. If you haven't noticed these symptoms but feel like your shifts are a little mushy and can feel what seems like a slipping sensation from time to time, then it may be a good time to consider a shift kit.
If your valve body isn’t in need of replacement but it starting to show its age, a shift kit can help you enjoy your car again.
Though valve bodies are complex and taking them completely apart is best left to the pros, there are shift kits available that can provide an easy way to eliminate some of the problems that older valve bodies may suffer from.
Shift kits give you the ability to alter your valve body without getting an advanced engineering degree. It’s important to make sure that you have the correct model for your model of valve body so that everything lines up correctly.
Shift kits can also allow you to customize your shifting based on your driving style. Some shift kits have multiple stages so that you can select whether you’re using your vehicle more for daily commuting or racing and account for your shifts accordingly.
Because working on your valve body means draining the transmission fluid, it’s also important to think about this in advance and have a surplus of new fluid to restock the valve body with. Without fluid, the valve body can’t work, and if you can’t shift, it’s going to be very hard to drive to the store to get more.
Valve bodies are the central control of automatic transmissions, and as long as they’re working correctly they’re miraculous. By using either a governor and modulator or electronic solenoids, valve bodies are able to correctly determine the right clutch to engage and shift accordingly. Unfortunately, when they’re not working, valve bodies can mess things in your car up very quickly and they can be challenging to fix.
If you’re not satisfied with the way your automatic transmission is shifting, but it seems to still be functioning correctly, a shift kit can create cleaner shifts by changing the amount of pressure necessary to up and downshift.
Source: Construction Mechanic Basics Volume 2 | Etereman Image Credit: Construction Manuals, GMC Sierra | High Tech Performance