by Sam Dillinger (comments: 0)

Why is My Transmission Overheating

transmission cutaway

This is a really important question because if your transmission overheats too much, your transmission is going to suffer a catastrophic failure shortly thereafter. When you're talking about transmission repair, catastrophic failure = painfully expensive! Like everything else with transmission repair, there are a couple of different reasons that your transmission can overheat (if you're one of my regular readers, you can start to see why transmission repair is so difficult, everything has multiple possibilities for causes!)

The first reason your transmission is overheating is the most common- there’s something slipping inside the unit. When an internal component is slipping badly enough it will overheat the fluid relatively quickly. There is a sensor somewhere in your transmission that sits directly in the transmission fluid and measures the temperature continuously. I explained in much greater detail why your transmission is slipping in another article, but to cut a long story short, when a clutch pack applies it squeezes the clutch discs together between metal plates. When the clutch pack doesn't hold, that’s the clutch slipping. That creates a tremendous amount of friction because the clutch pack is still squeezing. This friction will create heat, and that will set off all the bells and whistles that happen when the transmission is telling you it’s getting too hot. This same thing can happen inside the torque converter, so it’s important not to forget about that when looking for the cause of the overheating.

The next possibility we’ll look at is the actual transmission fluid temperature sensor itself. A lot of transmissions, most of them that I can think of off the top of my head in fact, have a sensor mounted somewhere on the valve body that sits directly in the transmission fluid and reads the temperature the entire time the key is on. There are some things that I should say here that apply to specific vehicles, or transmission types. One is that in some cases, the sensor only reads when the shift lever is in drive or reverse. In park or neutral, the temperature that’s being used is the engine temperature. It does this because that value can’t be blank, so it just borrows the engine temperature for a bit. In other cases, when the key is on and the engine isn’t running, the value displayed for transmission fluid temperature is just a calculated value. This can also help in diagnosis of a bad sensor, because it should never display the calculated value when the car is running and driving. The next is that a lot of times the transmission fluid temperature sensor is built into another sensor, so it can’t be inspected or replaced separately. The only part you can “inspect” separately is the integrity of the electrical circuit with the proper testing equipment. The last note on the transmission temperature sensor is that it’s an electrical component that has Pcodes associated with it, so usually when there’s a problem with the sensor itself, it’s very likely it’ll set a Pcode for diagnostic aid.

Now we’re going to explore another fairly common possibility- a restriction in the fluid flow. The transmission fluid is used as a lubricant, but the other side of that coin, is that it’s used as a cooling agent for all of the metal components that are moving against each other at high speeds. The fluid starts out in the transmission pan (a reservoir just like what you have for brake fluid) and travels through the pump, then torque converter, then out to the fluid cooler, then back into the body of the transmission to accomplish the rest of it’s tasks. If at any point in that circuit the fluid flow becomes restricted, this will cause the transmission to overheat very quickly. In fact, the transmission will overheat in just a few minutes; the worse the restriction, the quicker the overheat. Some common examples of where to look for the restriction would be in the transmission filter, one of the cooler lines (there are 2, a pressure line and a return line), the cooler itself, and on some models, there is a thermal bypass valve that is inline with one of the cooler lines. A quick note about the transmission fluid cooler: it sits in the front grille area of the vehicle (usually mounted to the radiator or the A/C condenser) and sometimes it can be clogged externally. By that I mean by leaves, or dirt, or other debris. If air can’t flow through the vanes of the cooler, it can’t exchange the heat from the fluid to the air. Just something to be aware of. These pieces all clog for different reasons, but that is an article for a different day!

Speaking of metal components moving against each other at high speeds, these pieces are separated by bearings to help reduce the amount of friction, and by this the amount of heat, between the two metal components. If one of these bearings fails, this will cause an overheat. The bearing could seize (seems strange to think this can happen when it spends it’s whole life covered in transmission fluid, but it can), one of the rollers could come apart or come out of it’s race, or it could just fail from length of service. When the bearing fails it no longer acts as a reduction in friction, and in fact becomes a greater agent of friction instead. If this happens, it’s important to catch it early because it won’t take long before it seriously damages other internal components. The more it takes out when it goes, the more expensive the repair!

The last thing we’ll discuss is a short one, and it’s related to the last item. When one of the metal components fails, a planetary gear, sun gear, or annulus gear is mainly what we’re focusing on, but in some cases it could even be a housing for one of these gears, you’ll get an overheating condition for the same reasons as a failed bearing. The important thing here is that there is almost an audible noise for a little while before it fails totally. So be sure to get it checked out if you hear a strange noise coming from under the hood. It might not seem like it right away, but it could potentially save you a lot of money in the long run.

That concludes our look at the causes for why your transmission is overheating. As always, for my members, if you have any questions, or need some diagnostic assistance, stop by at www.mycheckenginelight.net and see me!

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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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