Troubleshooting Automatic Transmission Problems
Automatic transmissions are seen as something of a dark art in the automotive industry. Generally non-transmission specialists won’t even hazard a guess as to what might be wrong with your transmission, let alone attempt to repair it. With this guide I aim to give you a basic understanding of what particular symptoms mean, and how serious a problem you might be facing.
Please be aware, even armed with the knowledge you find in this article, there’s not a whole lot the average driver can do from home without the proper tools. But it doesn’t hurt to know what’s going on.
A Bit of Terminology
Here are a few terms we’ll be using.
Fault Mode, Failsafe, Limp Mode: Many terms that mean your car's onboard computer is expressing concern about the transmission for some reason. Generally it causes some kind of indication on the dashboard, such as a flashing cog or the dreaded “PRNDS” light. Practically speaking it will often result in your transmission limiting itself to a single forward gear, such as 2nd or 3rd.
"Flaring" or "Slipping" into Gear: When an auto "flares" on a gear change, it has successfully disengaged the previously selected gear, but for whatever reason, it hasn't engaged the next gear’s clutch or clutches in time. This most often results in a brief loss of power as the rev counter on your car wheels around toward the red before the clutch finally applies and normal drive resumes.
Thudding into Gear: This one is fairly self-explanatory. If, during a gear change, there’s a disconcerting thud or bang from somewhere in the bowels of your car, usually accompanied by a jerk as the transmission engages the selected gear, that’s what is meant by “thudding” into gear.
Now you have some terminology; let’s do some troubleshooting.
Fault Mode, Failsafe, Limp Mode, Get-Home Mode, Etc.
Fault mode is the most obvious place to start, especially in newer cars as they are far more sensitive to transmission faults than older ones, and so are more likely to go into fault mode if there’s something amiss. You may find that turning the engine off and back on (also known as cycling the ignition) will clear fault mode. If that happens, there’s a small chance you could be lucky and the fault was just an aberration. More likely, however, the fault will come back at some point in the future, and more often than not the amount of time you get before the fault returns will get shorter and shorter until it is hanging around all the time.
Unfortunately, fault mode is kind of a catch-all symptom that, on the surface, doesn’t tell us anything useful. Cars—especially older vehicles—have very limited ways of informing the driver that there is an issue with the transmission. In many cases there is just this one. To give you an idea of the range of things that can cause fault mode, here is a short (and by no means comprehensive) list of things I have known to cause a transmission to go into fault mode;
- Severe internal transmission failure
- Faulty engine RPM sensor
- Blocked air flow
- Faulty wheel speed sensor
- Incorrectly sized tyre on one wheel
As you can see, fault mode by itself is no help. What you need to do in this instance is get your vehicle scanned to see what fault codes are being presented. This will likely require you to take your car to a garage, as the diagnostic scanners you can buy on eBay for the price of a couple of movie tickets probably won’t be able to read the codes you need to see.
Thudding and Delayed Engagement
There are two kinds of delayed engagement: one causes the aforementioned thudding into gear, the other being more of a driver perception—it feels to be taking too long to change. If the latter, there is probably nothing wrong with the transmission, it’s likely just the way that particular transmission drives. If, however, you’ve noticed the transmission change its behaviour, and you know that the gear changes are delayed, the cause is a control issue. That is, the computer controlling the transmission is changing gear at the time it feels is correct. In such cases you might need a software update or reset (dealers should be able to handle that) or perhaps a replacement transmission control module.
Now, if the delayed engagement results in a thud or bang when the gear finally engages, the transmission is not behaving the way it should. The control module judges the speed of the various clutches in the transmission and decides when to change gear. When the gear change is delayed, the selected gear clutches may be turning too fast by the time they finally engage. Imagine trying to jump on to a bus as it drives slowly past you. If it’s slow enough you can comfortably hop on, but if it’s speeding by and you grab the handrail, you’re going to have your arm jerked horribly as you’re taken along.
One of the more common causes of this kind of fault is wear-and-tear debris in the transmission fluid clogging up valves. The valves that direct the flow of oil stick because of the debris, so that the oil has to build up pressure until it is able to push the valve free, by which time the gear change is too late to go smoothly. In these cases the fix can sometimes (though admittedly not often) be remedied with a simple transmission fluid change. There are also additives that can be added to the transmission to break down the debris clogging the valves. If those fail, however, it’s time to get a specialist to look at taking bits out of your transmission and cleaning them up (or replacing them in worst case scenarios).
Flaring and Slipping
Slipping is a bad sign in automatic transmissions. If you experience a flaring or slipping of your transmission, stop. Do as little driving as it takes to get your vehicle to a transmission specialist as possible. The chances are the problem will result in the transmission needing to come out regardless, but continuing to drive a slipping automatic transmission can severely damage the internal components, not to mention stranding you when the transmission finally gives up the ghost.
Like thudding, slipping is caused by the clutches that are due to engage next doing so too late. Unlike thudding, however, they don’t engage properly, and the lack of a proper application causes the clutch to slip, sometimes momentarily before engaging properly (“flaring”), and sometimes indefinitely as the clutch never properly applies.
When an automatic transmission clutch slips, it wears away more of the friction materials involved. The debris from that wear contaminates the transmission fluid, clogging up valves and blocking the filter. Eventually, the filter will be so restricted that there will be insufficient oil flow to properly apply the clutches which causes… you guessed it, more slipping. From this point it’s a very short leap to no drive at all as more debris is created, further blocking the filter.
Like thudding, slipping can be caused by sticking valves, though it is more often caused by issues with the clutch itself.
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What to Do If You Are Concerned About Your Transmission
With any automatic transmission fault, the first port of call should be to check the transmission fluid (read up on your particular transmission for the proper procedure). First, you want to make sure you have enough. Oil in an automatic transmission is not simply a lubricant; it is the hydraulic fluid that is needed to apply the clutches. If you find your transmission is short on fluid, be sure to top it up before driving the vehicle at all. Then you’ll want to find out why it was short, and repair any leaks. If there is enough fluid, check the state of that fluid. If it smells burnt, it’s time to go see a transmission specialist.
If the fluid is fine, the next course of action is to have your vehicle scanned for fault codes. Obviously the transmission itself needs to be scanned, but don’t limit your inspection to just that system. In particular, speed-sensor-related faults, air-flow issues, and voltage-related problems are known to affect automatic transmission performance.
Finally, know how your vehicle works. In my experience working with automatic transmissions, I’ve seen my fair share of customers complaining of a loss of first gear only to be told they’d accidentally switched their transmission over to “Snow Mode,” which forces the vehicle to set off in second gear!
And, of course, look after your transmission. Don’t ignore signs of a problem emerging, because they will almost always get worse. And if your transmission has a recommended service interval, be sure to observe it.
Questions & Answers
© 2016 John Bullock