Troubleshooting Automatic Transmission Problems

Updated on August 29, 2016
beagrie profile image

John is a fervent writer, avid gamer, and guitar lover. He earns his sandwiches fixing automatic transmissions.

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.

Automatic transmissions are complicated beasts...
Automatic transmissions are complicated beasts... | Source

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.

Like many things in life, you only notice an automatic transmission when it goes wrong!
Like many things in life, you only notice an automatic transmission when it goes wrong! | Source

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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The dreaded "PRNDS" light. Commonly seen in Audis, Volkswagens, and Skodas.
The dreaded "PRNDS" light. Commonly seen in Audis, Volkswagens, and Skodas. | Source

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

    Comments

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      • profile image

        John 

        8 weeks ago

        i have f250 2008 6.4 iwas leaving and befor going into 2nd it started jercking bad finly it went into second I get home get back in next day it didn't want to go into second but did and hard by time I get a half mile nothinh no gears at all to off turn back on repeat all over again

      • profile image

        Aaron 

        2 months ago

        Hi mate,

        got an issue with a ZF 6HP26 regarding TC control I'm hoping you can help with.

        The car belongs to a customer of mine, basically when driving from say idle engine speed the TC won't fully engage until about 1800rpm, gear shifts won't happen until about 2300rpm so very delayed, when braking the revs will slowly come down with vehicle speed until about 1300rpm and then the revs will just drop to idle so TC fully disengaged, and won't fully engage again until 1800rpm as before, on thing I've noticed when giving the car some stick then braking the TC will be correct momentarily and come down to 1000rpm no issues but then as you attempt to drive normally again will go back to how it was.

        Now the box has been serviced so new ZF sump and fluid along with new bridge seals and tube seals for the valve body, no different, TC remanufactured, no different, valve body rebuilt with a seperater plate and a sonnax zip kit plus a new EDS6 TCC solenoid, no different, box was removed and the following parts replaced, front TC seal on the pump, pump outer o ring and inner o ring, New stator bushing, New seal rings on the rear of pump and input shaft, New e clutch intermediate bushing and one e clutch friction replaced due to wear on the outer edge, all other bushings and clutches related to A and B were checked and fine, I didn't removed the central support for C and D as didn't see the relevance but after all that no different, valve body with mechatronic sent of for testing and remaining 6 solenoids tested at the same time, all checked out fine, gearbox heat exchanger replaced and no different.

        All I've got left to try is a software update I think or remove the box again and check the rear half which I'm not keen on doing again atm.

        Before I thought it might be to do with worn bores on the wk-v or wd-v but with it testing fine was at a loss with that.

        I'm certain it's a pressure related issue but I'm running out of ideas, with it being momentarily ok after some hard driving it's starting to make me think it's oil pump related or the extended increased pump speed has increased internal pressure overcoming an internal pressure leak but I'm just at a loos as to where it could be.

        I hope you can be of some help here as other trans specialists I have spoken to are no further along then me at this point.

        Cheers for your time

      • beagrie profile imageAUTHOR

        John Bullock 

        6 months ago from Yorkshire, England

        Hello Lance,

        Brown oil in an automatic transmission is *very* bad. It usually means water contamination, which always means a complete overhaul of the transmission. Given the way your transmission went all at once, I would hazard a guess that you're going to be looking at a full overhaul regardless, unfortunately.

      • profile image

        Lance 

        6 months ago

        We were at a stoplight turning to the right car. Was driving perfectly fine pressed on the gas it went a little ways then all of the sudden we had no gears the car was still running just wouldn't go anywhere and we heard a zing sound like it was in neutral. The car is!a 2007 Ford Taurus with only 100,000 miles on it, and the fluid in it is more of a dirty oil color instead of red it has a red tint to it but more brown

      • beagrie profile imageAUTHOR

        John Bullock 

        7 months ago from Yorkshire, England

        Hello Norman,

        If your X5 is going into failsafe mode, you need to get it hooked up to a diagnostic machine to find out why. Failsafe mode in and of itself doesn't give you much information. It could be a problem with the transmission, but it could also be something innocuous like a faulty wheel speed sensor. Without getting the car scanned for fault codes there's no point in guessing.

      • profile image

        norman anderson 

        7 months ago

        my x5 is having this sharp jug in the transmission ,

        on the dash board it come with transmission fail save peogram

      • profile image

        Scott Slater 

        22 months ago

        I also noticed that as it had been overheating, right before I shut it down to cool. The 'D4' indicator had flashed a few times.

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