Your diesel truck is built to last. With its powerful engine, reliable steering, and sturdy frame, you can trust that not only will you get your shipments where they need to go, but you will also stay safe in the process. But your diesel truck will only last if you take care of it, and one of the most surefire ways to cut the life of your truck short is by leaving transmission issues unchecked. Not only do the problems with your truck’s transmission worsen the longer that they’re ignored, but they can also interfere with other integral parts of your diesel truck’s performance.

Lots of diesel mechanics have their own way of figuring out if a truck’s transmission is going sideways, so it can be hard to tell which ones are worth listening to or not. This is why we have compiled a list of 5 simple signs to help you decide if you need to get your tractor trailer’s transmission looked at by a diesel mechanic at a semitruck automotive service shop.

1: Unusual Sounds

Have you been driving down the highway recently and noticed your diesel truck has been making a humming, whining, or clunking noise while its running? This is one of the easiest detectable signs of transmission trouble that will kill your semi. Whining or humming usually means that there’s a problem with the transmission fluid pump which is usually caused by a low level of fluid or a failing pump. But this sign is also commonly confused with carrier bearings and wheel bearings, so make sure you take your tractor trailer over to a semitruck automotive service center to be inspected by a diesel mechanic who can check all those bearings before jumping to conclusions about your transmission.

2: Difficulty Shifting Gears

If you find that your diesel truck’s gears are getting stuck or taking too long to shift, it’s not just a random quirk with the clutch. It means that you need to take your tractor trailer over to a diesel mechanic for service right away. Slow shifts can be a sign of unbalanced transmission pressures or a worn-down part in your diesel truck. This is an issue that needs to be resolved right away since harsh shifts can wear out clutches and bands and lead to slipping. Eventually, this slipping leads to your transmission not going into gear at all which means that your tractor trailer will no longer be drivable.

If you want to try fixing the issue on your own first, we recommend changing your transmission filter to see if it is clogged. Often, a dirty filter can slow down gear shifts, but if that doesn’t fix the problem then you may need to take your diesel truck to a semitruck automotive service center to be checked for any parts that need to be replaced.

3: Vibrating

If you find that your diesel truck starts to vibrate or shake unexpectedly when you are changing gears, particularly on bumpy and uneven roads, then you need to get your semi’s transmission inspected by a diesel mechanic. This is another major sign that your transmission may also be affecting other components of your tractor trailer. To get this fixed, you will need to have an ASE-certified mechanic recalibrate your clutch or even replace it entirely.

Vibrating transmission also may mean that there is a problem with your shocks. They’re part of your diesel truck’s suspension and help to stabilize the tractor trailer when turning, braking, accelerating and driving over uneven surfaces— something we know all truckers need when they’re out on country roads making shipments or doing transport jobs. It’s important to keep your shocks in prime condition and replace them when needed because, over time, these shocks can wear out and are usually a big reason your diesel truck may be experiencing transmission vibration.

We recommend taking your tractor trailer to a semitruck automotive service center to be further inspected by a diesel mechanic for any suspension issues that may be causing your transmission to vibrate.

4: Odd Smells

Yes, you can actually tell what shape your transmission is in just by the smell of it. While this is less common in tractor trailers, a strong, burning scent usually means that your transmission fluid is beginning to overheat and break down. This can be due to the age of your transmission fluid or that your transmission itself is not being properly lubricated by the fluid that helps it change gears. But if your diesel truck’s transmission is burning too hot, it will lead to an increase in friction and corrosive activity in the engine.

Be sure to take your diesel truck to an ASE-certified mechanic right away if you notice this smell. If left unresolved, the transmission could eventually damage itself enough that it will break down completely.

5: Gear Slipping

Another tell-tale sign of transmission trouble is when your gears begin to shift while you are driving without manually moving them or changing speed. There are a couple of issues that can lead to gear slipping. One of them, like number four, is low transmission fluid, but it could also be an issue with your engine’s internal combustion part or worn-out bands.

So, you can see how it can be tricky to find the real cause to why your diesel truck is slipping gears, which is why we recommend turning it over to a diesel mechanic to run a diagnostics test on your truck for you.

Trusted Knoxville, TN Mechanics

As you can see, there are lots of reasons why your tractor trailer may be experiencing transmission troubles so you will want to be sure that you are treating the right cause. At CDS, our ASE-certified team of diesel mechanics are experts in diagnosing and treating all semitruck automotive issues both big and small. Equipped with the latest technology in the field, our knowledgeable team has the know-how and the muscle to get your diesel truck fixed the right way and always on the first try— because we believe in quality service that gives our clients peace of mind when they’re out on the road. There’s never a bad time for a preventative maintenance inspection on your diesel truck or tractor trailer, so contact us at to make an appointment or stop in our Knoxville location today, and we promise that we will have you taken care of and back on the road in no time.