How Does a GMC Exhaust Brake Work?

The GMC exhaust brake is a handy feature found on many diesel trucks. It assists the regular brakes and helps maintain control when going downhill. Let’s take a closer look at how this system operates.

What is a GMC Exhaust Brake?

GMC Exhaust Brake

A GMC exhaust brake is a system on diesel trucks and SUVs made by GMC that helps slow the vehicle down without wearing out the regular brakes too quickly. It works by using the engine itself to help slow things down.

GMC exhaust brakes are found on many of their heavy duty diesel trucks like the Sierra and Yukon, as well as some of the bigger SUVs like the Yukon XL. The main goal is to assist the regular wheel brakes when going downhill or slowing from highway speeds with a heavy load. This spreads the braking workload between the engine and the brakes themselves.

How Does a GMC Exhaust Brake Work?

GMC Exhaust Brake

Alright, so how does a GMC exhaust brake actually do its thing? Well, when you turn it on, it closes a valve located in the exhaust pipe right before the truck’s catalytic converter. This valve is usually called an exhaust brake butterfly valve.

When the valve closes, it traps some of the exhaust gases in the engine that would normally be pushed out during the exhaust stroke. That extra resistance slows the engine down because the pistons have to work harder to push the gases out.

A slower turning engine then causes the drive wheels to slow down since they’re connected through the transmission. The valve is controlled either manually with a switch or automatically by the vehicle’s computer system based on how hard the brake pedal is pressed.

When to Use It

So when should you use your truck’s GMC exhaust brake? Mainly when you’re going downhill with a heavy payload in the bed or behind the truck as a trailer. Steep downgrades on mountain roads are a perfect time to rely on the exhaust brake to help regulate your speed safely.

It also comes in handy for slowing down from highway cruising speeds without constantly riding the regular brakes. In this case, the exhaust brake lets the truck’s momentum do more of the work as it coasts downhill in a lower gear. This is less stressful on the wheel brakes over long distances.

GMC Exhaust Brake

Manual vs Automatic Systems

Some older GMC exhaust brake systems can only be turned on and off manually with a switch located near the gear selector. The driver has to remember to flip it on when needed.

However, most newer diesel trucks have exhaust braking controlled automatically based on inputs from various vehicle sensors. The electronic control module monitors factors like road grade, vehicle load, transmission gear, and brake pedal pressure. It then decides when conditions require exhaust braking assistance and activates the butterfly valve accordingly.

This automatic setup provides exhaust braking functionality whether the truck has a manual or automatic transmission. The driver simply brakes as normal without worrying about the extra switch. The computer handles it all behind the scenes.

GMC Exhaust Brake

Maintenance Tips

To keep the GMC exhaust brake working properly as the truck ages, it’s a good idea to follow the maintenance schedule listed in the owner’s manual. One thing to check occasionally is the exhaust brake butterfly valve itself. Make sure it’s opening and closing smoothly when commanded.

Listen for any strange clunks, bangs or other unusual noises from the exhaust system that could indicate a problem. Regularly replacing engine air and fuel filters also helps keep the valve mechanism running clean and sludge-free. With simple upkeep like this, the GMC exhaust brake should provide reliable service for many years!

Common Problems

While generally very reliable, there are a few issues that can come up with GMC exhaust brakes over time. One is carbon buildup on the butterfly valve from years of exhaust residue. This can cause the valve to stick occasionally instead of opening and closing smoothly.

The fix is to remove the valve and have it professionally cleaned of carbon deposits. You may also need to replace the valve if it becomes warped or pitted from extreme heat cycling. Check owner’s manual service intervals and have a mechanic inspect it during major tune-ups.

GMC Exhaust Brake

Another potential problem is faulty wiring to the electronic control module. Glitches here can cause the exhaust brake to activate at the wrong times or not work at all. Have a diagnostic scan run to check for trouble codes pointing to wiring faults. Corroded connectors may need cleaning or replacement.

Finally, exhaust leaks can undermine the brake’s ability to trap exhaust gases. Listen for hissing noises and inspect clamps, pipes and welds for cracks annually. A leak lets precious exhaust energy escape instead of being reused to slow the engine. Quick repair prevents worse damage down the line.

With preventative maintenance and quick fixes, these issues need not ruin your day. The GMC exhaust brake just keeps on doing its job to save wear on the regular brakes downhill.

Aftermarket Upgrades

GMC Exhaust Brake

For serious towing applications, some diesel enthusiasts upgrade their truck’s stock GMC exhaust brake for added capability. Larger butterfly valves flow more exhaust volume, developing extra braking torque.

Custom tuning allows activating the brake lower in the rpm range too. Exhaust-brake-specific transmissions from companies like Allison provide lower first gear ratios to really tap the braking potential.

For the hardcore towing crowd, these modifications squeeze out that last bit of safety and brake life extending ability on long mountain descents fully loaded. But for most, the factory system does an excellent job already without modification.

Final Thought

The GMC exhaust brake is a smart addition to any heavy duty truck. It spreads the braking workload between the engine and wheels, saving wear over the long haul. With some basic preventative maintenance checks, yours should provide years of reliable downhill braking assistance. Don’t be afraid to use it – that’s what it’s there for! As always, safe travels in your GMC.


Does the exhaust brake work in all gears?

On most trucks, the exhaust brake will be active in gears 5 through 1 when descending a grade. This provides braking assistance no matter what gear the truck is in.

Can I use it for normal city driving?

The exhaust brake is really intended for use during downhill driving or heavy braking situations. In regular city driving it may not engage at all. It’s best left alone during normal around town cruising.

Will it damage the engine to use it too much?

No, the exhaust brake is designed to be used frequently with no adverse effects. As long as it’s operated as intended during braking or downhill driving, it won’t cause premature engine wear.

How do I know if mine is working?

You may notice the engine getting louder or feeling more resistant when accelerating when the exhaust brake is on. There may also be a warning light on the dash. Most importantly, pay attention to how smoothly you can control speed going downhill – it’s working if braking feels easier.

Can two trucks’ exhaust brakes interfere?

No, each truck’s exhaust brake is independent and self-contained to that individual vehicle. There is no way for one truck’s system to interact with or impact another nearby truck.

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