Kenworth T680 W Automatic

I am confused on the automatics and going downhill fully loaded. (41k)

At the crest of the hill I slow down 50-55. (Not major downgrade) I switch down to 9th gear. The rpm goes up to 16,000 and roars. I start firmly pressing the break on and then release for 5 seconds. I start gaining momentum. I have the jag brake on full and here is where I am confused. (Jag brakes does not seem to help)

Should I have shifted down to 8? Because 9th and applying brakes is not cutting it. I am not smoking the brakes but I can smell them when I stop. (That's if I stopped right away)

Side note. My thoughts on the jag brakes from a previous truck .... They slowed the truck down. But not so much here. I just brought the truck to dealer and said they not work. They tested and said they work. Feeling dumb😠

You can feel the Jake brake come on in an automatic the same as a manual. I'd be concerned if I had to use the service brakes on an ordinary hill. Is it possible to get a different shop to check it?

I am confused on the automatics and going downhill fully loaded. (41k)

At the crest of the hill I slow down 50-55. (Not major downgrade) I switch down to 9th gear. The rpm goes up to 16,000 and roars. I start firmly pressing the break on and then release for 5 seconds. I start gaining momentum. I have the jag brake on full and here is where I am confused. (Jag brakes does not seem to help)

Should I have shifted down to 8? Because 9th and applying brakes is not cutting it. I am not smoking the brakes but I can smell them when I stop. (That's if I stopped right away)

Side note. My thoughts on the jag brakes from a previous truck .... They slowed the truck down. But not so much here. I just brought the truck to dealer and said they not work. They tested and said they work. Feeling dumb😠

I trained on two T680 manual 8-speeds. I now have a 8-speed Freightliner.

The Kenworths had significantly more power when accelerating. They also jaked harder. My Freightliner will gain speed with a full load in 7th gear at 55 miles per hour, 1550 RPM on a 5% slope. I have to keep just a couple pounds pressure on the brakes every now and then to keep the speed to 55-56 and RPM to less than 1600-1650.

Both Kenworths I drove, with similar loads, would have (and did) slow in the same situation.

I believe it is a fact that the more power the engine has, the more power the jakes have. It is an engine brake after all.

I think there's two levels of braking.

If you use cruise control, when you start down a hill, the Jake will kick in automatically. But if you pull the lever also, you'll get a real brake effect that will definitely slow you.

If you're just on the accelerator, you obviously let off. Then pulling the Jake lever gets you a "lighter" less powerful setting in the Jake.

That's an impression I get.

In my T680 automatic, the jakes are much lighter while on cruise. If I take it off cruise and let it downshift to 9th, the higher power jakes will kick in.

I believe that the jakes in these trucks arent quite powerful enough. If I have a heavy load, I often have to turn them on high and still have to stab brake sometimes.

I've never tried downshifting to 8th to see if it helps.

Whatever you do, don't ride the brakes all the way down. Just brake long enough to bring your speed down to a safe level, then let it build up speed again.

Chickie wrote:

I believe that the jakes in these trucks arent quite powerful enough. If I have a heavy load, I often have to turn them on high and still have to stab brake sometimes.

Depending on the steepness of the grade, that's fairly normal to use the service brake as an assist to the engine brake (Jake). I have always used the two braking systems in tandem (unless in the snow and/or rain).

Downshifting will definitely help. Nice thing about those automatics is you can downshift easily even when going down a steep downgrade. Just a click of a button does it. When driving an automatic, I rarely have to use my service breaks on a steep grade, mostly while the truck is downshifting as it goes to neutral for a moment and you will start to gain speed otherwise.

Absolutely, Mark. If you're smelling the brakes and gaining too much speed downhill, trying a lower gear like 8th might help control your speed better with less brake use. The Jake Brake's effectiveness can vary between trucks, so even if the dealer says it's working, it might not feel as strong as you're used to. Consider asking them if there's a way to adjust it for more power. Stay safe and keep experimenting with gear settings to find what works best for your Kenworth T680.

I am confused on the automatics and going downhill fully loaded. (41k)

At the crest of the hill I slow down 50-55. (Not major downgrade) I switch down to 9th gear. The rpm goes up to 16,000 and roars. I start firmly pressing the break on and then release for 5 seconds. I start gaining momentum. I have the jag brake on full and here is where I am confused. (Jag brakes does not seem to help)

Should I have shifted down to 8? Because 9th and applying brakes is not cutting it. I am not smoking the brakes but I can smell them when I stop. (That's if I stopped right away)

Side note. My thoughts on the jag brakes from a previous truck .... They slowed the truck down. But not so much here. I just brought the truck to dealer and said they not work. They tested and said they work. Feeling dumb😠