Exhaust Brake

[VerMonsterRV]

So, since this is the first truck I have ever owned that has an exhaust brake I have a question. I use the brake frequently, but especially when I am at higher RPM I notice the throttle pedal automatically depress while exhaust braking. First, is this normal and if so does anyone know why it is doing this?

 

[Vegard_S]

So, since this is the first truck I have ever owned that has an exhaust brake I have a question. I use the brake frequently, but especially when I am at higher RPM I notice the throttle pedal automatically depress while exhaust braking. First, is this normal and if so does anyone know why it is doing this?

This is totally normal I have the same behaviour in my Unimog

The throttle pedal is mechanically connected to the valve / brake in the exhaust line. When you activate the exhaust brake, by stepping on the button in the floor, the air-sylinder that pushes on the valve also affect the throttle pedal (although the linkage further over to the diesel pump is still at a stand still). Normally on these trucks you stop the engine by pulling the throttle pedal backwards, what happens then is that you close off the fuel feed from the diesel pump, and at the same time partially close the valve in the exhaust line.

 

[Neil]

When I press the button, my throttle pedal is instantly force into the upright position and is rock hard.

This is normal

Neil

 

[Geo.Lander]

From LN2 forum it states that simply removing your foot from the gas engages the engine brake also:-
"The foot actuation valve acts on the shut-off cylinder of the injection pump and the actuation cylinder of the engine brake at the same time.
There is a supply at the actuating valve, when actuated the compressed air then passes through the separation point on the bulkhead and via an air line in the left frame side member to the exhaust manifold, there is a T-piece, one line goes from the T-piece to the engine brake and the second still works a little backwards in the frame, then runs over the transmission bell housing on the right side of the engine to the parking cylinder of the injection pump. The cylinder sits in the throttle linkage, when actuated it pushes the regulator of the injection pump to zero delivery."

 

[DiploStrat]

Having crossed the Sierra Nevada and the Rockies before I knew that that botton was an exhaust brake, I can testify - you want the exhaust brake. A 5 litre diesel has NO real retarding effect on 9k kilos of vehicle!

 

[VerMonsterRV]

Thanks again, just came across a great article from Banks about diesel engines and exhaust brakes https://official.bankspower.com/tech_article/how-an-exhaust-brake-works/. Never realized fuel would be needed for braking.

 

[Neil]

Jon

Very interesting article.

I am pretty sure that my exhaust brake completely shuts off the fuel being pumped to the injectors.

I think this because when stationary in neutral you use the button to stall the engine , which it does immediately.

Also Mercedes state that it should never be used on ice or snow as it could lock up the wheels , cause a skid and turn the engine off and kill the power steering .

In these circumstances going down hill on a snowy mountain pass I think you would need to start your next laundry with a spoon

Neil

 

[charlieaarons]

Living in Alaska I can say that you can use an exhaust brake on snow and ice - carefully! I drove it to work every day for a winter.
The vehicle must be in 4wd to spread the load to all 4 tires. And if it is a 2 stage brake (my first stage is a flap in the exhaust beyond turbo, second and more powerful is Jake brake type mechanism in cylinder head involving extra exhaust valve), use only the first stage unless you have confidence in the surface.
I had some portal work done at a dedicated Unimog dealership in Karlsruhe in 2017. We told them to replace brake pads if worn 50% or more, after 94,000 miles. They said they were still good. I use the exhaust brake obsessively.
Bottom line: on snow/ice always use 4wd, carefully use flapper brake if indicated, don’t use Jake type brakes.
Just my experience.

 

[Neil]

My exhaust brake is quite crude.

There are no stages, its on or off.

Its very good. In a lower gear like 3rd the retarding effect is really dramatic

I love , I use it all the time . I cannot imagine driving a truck of thus size without it.

On ice or snow in 4th gear or below it would definitely lock the back wheels and stall my engine, which would be scary

Neil

 

[charlieaarons]

On ice or snow in 4th gear or below it would definitely lock the back wheels and stall my engine, which would be scary

Neil

Again, I repeat myself, if using exhaust brake on snow or ice, put it in 4wd. Why not in 4wd on snow or ice? That’s begging for trouble.

 

[Neil]

Again, I repeat myself, if using exhaust brake on snow or ice, put it in 4wd. Why not in 4wd on snow or ice? That’s begging for trouble.

Of course I would be in 4wd on ice and snow. I didnt suggest otherwise

However, It would probably still lock up the wheels ( if in a lower gear ) and cause a stall . It stalls the same in 2wd and 4wd.

Neil

 

[VerMonsterRV]

So now I see what the small pneumatic cylinder does near the injector pump. Makes sense it shuts the fuel off when using the exhaust brake/shutoff switch. Since my throttle pedal sinks a bit when pressing the switch I am now wondering if my throttle pedal linkage needs adjusting (maybe I have even more power lurking under the cab ). I'll take a look at things when I finally get around to putting in the hosing for our engine pre-heat, really need to get that project finished as temps in the morning here are well below freezing.

 

[mxpxer7]

Hoping to piggy back on this thread...

I have a 94' Mercedes SK1824 (just bought it). There's a circular button behind my left foot on the floor. I think this is the jake brake. I drove the truck from salt lake city, UT to Bend, Or this past weekend. I had no idea how to use the jake brake. I was pressing and holding the round button with my heel. I see now that's probably not how it's done lol.

So just tap it, it should turn on and that's that? Is there there a way to know if it's on or off? Is it press for on, press again for off? Sorry, very newb questions.

 

[Neil]

Hi.
It's unlikely to be a Jake Brake.. A jake brake decompressed the engine. Its more likely to be an exhaust brake which cuts off the exhaust pathway and increases back pressure in the engine.

They are operated by holding your heel on the button for the entire time the braking is required. Sometimes this can be long periods.
I have had my heel on the button for up to 30 minutes at a time without issue.

The lower the gear you are in the more braking is applied. In the higher gears very little retardation is applied

So if you were coming down a long decent you can select a lower gear, apply the exhaust brake and the engine will hold the vehicle at a comfortable manageable speed for the entire decent.

It is probably Pneumatically operated and the best bit of kit you can have

Neil

 

[Joe917]

You need to hold it down with your heel as Neil says. You should also be descending in the same gear you would need to ascend the hill and use the exhaust brake all the way. Descending long steep hills using the service brakes is asking for trouble. If the exhaust brake can't hold your speed down use your brakes to slow enough to drop a gear and get back on the exhaust brake. Drum brakes overheat quickly and loose effectiveness.