Originally Posted by BandJCarm
I am NOT a gear head. Let me say that up front.
But......everything I read in manual and on Duramax forum says that the LML Duramax does not have a true engine brake at all.........just a transmission brake. The manual simply says both EB and TH can be engaged separately to enhance each other. The guys on the forum say that the EB (exhaust brake, not engine brake) downshifts, and the TH mode 'make the downshifting more aggressive'........which is all I could imagine it was doing.........
That may be enough, and it may be noticeable.........just haven't tried it under heavy load yet (don't have my trailer yet).
And I fear I've 'hijacked' a Ford discussion thread..........and I'm sorry. I thought it was a universal discussion but apparently it's not. My bad.
I'll try to clear things up for you this best I can.
T/H changes shift points and pressures in the transmission. It also locks the torque converter up in all gears after 2nd gear.
Engine brakes are not on our engines. But they are available for the Cummins in the aftermarket world. An engine brake (Jake Brake) uses solenoids to open the exhaust valve during the compression stroke.
Exhaust brakes on our trucks are also called a turbo brake and a VGT brake. They use the turbo veins to create exhaust back pressure to slow the engine down.
The exhaust brake and T/H mode are separate systems, but when they are both engaged they do work together. The exhaust brake slows the engine down, which causes the transmission to down shift more. And the more it down shifts, the higher the engine RPM's are. And the higher the engine RPM's are, the better the exhaust brake holds back.
As far as the OP.
The reason for letting the engine idle after running it hard is because the turbo is doing thousands of RPM's, and when you shut the truck off, it's not circulating oil anymore. Under full boost the turbo is spinning up to 30,000 RPM's. Bearings don't last very long at that speed with no oil circulation.
My truck doesn't have an exhaust brake switch. Mine is achieved through EFI Live tuning. Whenever the T/H is engaged and I have my tuning on the tow tune, it turns the exhaust brake on. The only time I don't like it is when I am in heavy traffic. If I could turn it off in heavy traffic, I would. The turbo has a lag going from closed veins to open. All other times it works great!