by Sam Dillinger (comments: 0)

Why Is My Transmission Slipping

my transmission is slipping

This is a question I get asked an awful lot- “why is my transmission slipping?” When I get this question, it’s always with good intent, but it’s almost never with any qualifying information. The reality is that this question has just about as many answers as there is types of transmissions. One transmission may slip for a reason that wouldn’t cause that in another transmission. One transmission will have components in it that could be the culprit for that unit, but those parts don’t exist in your neighbors transmission. So let’s take a look at some of the qualifying information needed to answer a question like this, and some of the reasons for why your transmission could be slipping.

The very first place any good diagnosis has to start with is “under what specific conditions does the problem occur?” Since we’re talking about a transmission slip for this article, we’re looking for the specifics of when the slip occurs. Does it slip on takeoff? Only at 45 MPH (Miles Per Hour)? Only when the vehicle is cold? And the list goes on from there. There are numerous conditions when the transmission can slip, so the first thing we need to know is when does it happen for you. Once that question has been answered, the very next step is to check the transmission fluid level and quality. This is always, ALWAYS, the first mechanical thing to check. Why? Because low fluid can cause a slip under just about any driving condition you can think of. Some units will only slip when going around a corner when the fluid is low, some only in reverse, some only on takeoff, some only when the unit gets hot, and so on. As for the condition, what does the fluid look and smell like? There is no such thing as “dirty” transmission fluid. This is a tactic used to try and sell you a transmission service. The reality is that if the fluid is dark brown or black, something has failed. Even if that something is the viscous ability of the fluid itself, something has failed. Also, there is a certain smell (it actually smells like burnt metal, and is unmistakably not normal) that will tell you if there is a major failure inside the unit. So we always start with checking the fluid.

Let’s assume the fluid is in relative good condition. What do we do next? We go for a ride! Not really, but soon. Next we need to scan the vehicle and see if there are any Pcodes in the system. This is an important step that gets overlooked all the time. If the check engine light is on, this step is a no-brainer. If it’s not on, it still needs to be done. There could be Pcodes stored that haven't turned on the light yet, but can provide valuable diagnostic information. NOTE: it’s important to scan the TCM, Transmission Control Module, when looking for transmission Pcodes. Some vehicles don’t have a separate TCM, so scanning the PCM, Powertrain Control Module, will be sufficient. If your vehicle does have separate controllers, it’s crucial to check the TCM for Pcodes because they won’t show up in the PCM at all. See my article about Pcode P0700 for some further useful information on this. There is some other useful information to be found in the “data” section of the scanner, but unless you're very familiar with automotive repair, not much (if any) of it will make any sense to you. In the interest of keeping this to what everyone can understand, we’ll skip that part. Now that you’ve scanned, the TCM, let’s assume there are no Pcodes set.

Now we get to go for a ride! The purpose of taking a test drive seems like it would be pretty straight forward, but let’s take a look at it, to cover all the bases. When you go for a test drive for diagnostic purposes, it’s really important to do so as closely to the same conditions as when the slip occurs the most. For example, if the slip mostly only occurs in the morning, it’s a pretty good bet that we’re talking about a transmission that is “cold”. I put cold in quotation marks because that temperature can be relevant to where you live or what time of year it is. First, pay attention to how the transmission engages when the gear selector is put in reverse and when it’s put in drive. Is there any delay, harsh engagement, shudder, any kind of abnormal actions other than just engaging the gear selected? This is important because if there’s an issue right at the beginning, it can help lead to the correct diagnosis. If we've engaged the gear correctly, let’s go for a ride.

So when you take off, you’re already in first gear. That means the first upshift is from first gear to second. This is important because many people get tripped up when counting the gears. The first shift will trick them into thinking they've just shifted into first gear. It’s the first shift but it’s into second gear. Counting the gears is crucial because a lot of times when the transmission exhibits what is commonly referred to as “slipping” it happens when you're changing from one gear to the next, and the overwhelming majority of the time it happens when upshifting as opposed to downshifting. Especially with modern transmissions that sometimes have 8 or even 9 speeds, knowing what gear change the slip occurs in is critical. Just saying it happens at 40 MPH won’t always tell you the correct gear range where the problem occurs. For the sake of this article let’s assume you have a 4 speed transmission and you notice the slip occurs after third gear but before fourth gear. What gives? Why would the transmission slip in between gears? Usually if the transmission is going to slip in a certain gear, it’ll occur right at the outset of the gear, and then continue throughout the gear, assuming it’s able to achieve full engagement.

The answer is the torque converter. Modern torque converters have an electronically modulated clutch that turns on to about half of it’s available capacity between third and fourth gears. This is known as partial lockup. I’ve experienced more slipping into partial lockup than just about anywhere else in the transmission. There are a few reasons the torque converter clutch will slip on partial engagement. The wrong type of fluid will cause a shudder in the torque converter clutch in quite a lot of transmissions. The seal for the piston that applies the clutch could be weak, or the clutch could be worn from heat. This clutch is used more than any other in the transmission, so by default it will wear quicker than any other. When the torque converter clutch slips, the torque converter needs to be replaced. To be fair, it can be rebuilt, but it’s an exceptionally difficult process that requires very expensive tools, and an intimate knowledge of the components. It’s better to just replace it. NOTE: my personal advice is to have the transmission overhauled when you need to replace the converter. It may not be technically required, but it will save you a ton of potential headaches down the road. I personally don’t ever just replace the converter- the chance of a repeat repair is astronomically high when doing this.

Now let’s look at some other scenarios. The transmission slips on takeoff. There are a minimum of two components needed for your transmission to achieve forward (or rearward, if you're going in reverse) movement. You need a driving member and a driven member. If it helps, you can think of it as one part that pushes and one part that is being pushed. That’s an imperfect analogy, but it’s close. On takeoff, you have two specific components that will be used every single time you take off, no matter if it’s the first time you engage drive for the day, or if you’ve been driving for two hours. In a lot of cases, one of these components will only be used in first gear, and not after, so this will explain why it only slips on takeoff/during first gear travel. There is a chart that applies to each specific transmission that will tell you what components are being used for each gear of your specific transmission. This chart can be an invaluable tool for diagnosis. Usually a google search will help you find the chart in question.

The transmission only slips in reverse. Again, there will be normally two components used to propel the transmission. In reverse, there is one component, appropriately named the reverse clutch usually, that is only for this gear. If your trans is only slipping in reverse, this is a really good place to start looking for the problem.

As for what is actually slipping when the “slipping” condition occurs, this is the clutches not holding completely. When a clutch pack is applied, all the clutches in the pack are squeezed together between metal plates, and aren’t supposed to move anymore, until the pack is released. If one or more of them moves when the pack applies, this is what is giving the actual slipping.

This covers the most common causes and explanations for why your transmission is slipping. One thing I’ve learned in all my years of fixing transmissions is that there is an exception to every rule, and even things that you think could never possibly apply to your condition, can be the exact cause of your problem. This is a good way to start narrowing down your problem, and for my members, if you’ve got further questions, need some diagnostic assistance, or want to tell me how you enjoy my writing (this doesn’t really happen, but you can if you want to!) come see me at www.mycheckenginelight.net

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Meet Sam Dillinger

My name is Sam Dillinger. I've been a professional, dealership technician for 18 and a half years. My first introduction to mechanical repair was when my own vehicle broke down in the fall of 1995. I was 18 and couldn't afford to pay to have it fixed. So I borrowed tools and asked a ton of questions and, eventually, was able to replace the clutch on my truck by myself. During the course of that project, I found that I really enjoyed having a wrench in my hand and figuring out the puzzle of disassembly and reassembly of a motor vehicle.

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