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Three Causes of Transmission Slipping

Auto Trans cutaway photo

Automatic transmissions are sometimes touchy things. One of the most difficult things, and most common, to diagnose with an auto trans is slipping. Sometimes it happens only at certain speeds, sometimes only in some gears, and sometimes only when under specific torque. Most of it time, it’s caused by one of three different things, all of which have to do with the transmission’s fluid.

Transmission slip is usually caused by low fluid levels, low fluid pressure, or faulty fluid pressure regulation. All three of these likely point towards other, more serious problems within the transmission. Fluid pressure problems not related to low fluid levels are almost always going to lead to a transmission rebuild or replacement.

Low Fluid Levels

The easy one comes first, of course. This is easy to check and relatively easy to solve (for the short term). First, you should know why fluid levels are important, so click here.

If there isn’t enough fluid in the torque converter to create a smooth spin, the transmission will slip because the converter won’t be able to smooth things. Transmission fluid is also the lubricant throughout the transmission, another of its vital tasks.

Checking transmission fluid levels is fairly easy, which is why we’re doing that first. In most vehicles (check your owner’s manual), fluid levels are checked with a dip stick located in the engine compartment, usually with a yellow finger loop to designate it as transmission. Fluid levels are usually checked with the engine warm and running and the transmission in Park. This has fluids at a specific level, since the transmission will have the torque converter full, but not be circulating fluids in most of the gearing. So the transmission pan will not be completely full. Make sure your park brake is set and the transmission is in Park!

With the engine running and the vehicle parked, pull the dipstick, wipe it clean, and replace it. Then pull it again and check the levels. They should be at or very near the FULL mark. There is usually an “ADD” mark a bit below that, so if fluid is above that ADD line, you probably do not need to add any. If not, double-check your reading (wipe, replace, pull) and then slowly begin adding fluid until it reaches the FULL mark.

Using a funnel, pour directly down the dipstick tube for the transmission. Add fluid at one pint intervals and only use the fluid specified for your vehicle, pouring from a NEW container. There are various types (weights) of automatic transmission fluid (ATF). Be very sure that the type you’re using is exactly what’s specified for your transmission. You’ll usually find the recommended fluid type etched into the dipstick. It will also be listed in your owner’s manual.

If your transmission fluid is low, there is a reason. It’s most likely not good. Leaks are common, usually from bad seals that let it dribble out from the pan or near the pump. It’s important that you find out why your transmission is losing fluid.

Low Fluid Pressure or Bad Pressure Regulation

These other two problems are more serious and are related. Low pressure or bad pressure regulation lead to basically the same overall problem. Low fluid pressure will usually trip an engine diagnostic code, but that’s not always guaranteed. If your fluid levels are good, but the transmission continues to slip, you likely have a pressure issue. To fix it, start with the easy stuff and move on.

First, drain the fluid and remove the transmission pan. On most automatic transmissions, there are maintenance intervals for fluid flushes and filter changes. Find out if yours requires one or is getting close - at least find out when it was done last. Sometimes, a slipping transmission, if caught early, can be fixed by simply changing out the filter, cleaning the pan, and replacing the fluid. If not, you may have a worn fluid pump or a bad pressure regulator. In both cases, you’ll be looking at a transmission rebuild or replacement. That’s not going to be cheap and it’s very unlikely to be done DIY.