Automatic transmission with Jake's going down the hill

That’s why I asked him about manifold pressure. He should build 30psi easily on stage 3. Out here on the flats I never use stage 3. It’s overkill. Stage 2 will bring you to a stop without having to use much brake.

 

The x15 is the the improved and more efficient version of the older ISX Cummins....

 

Well every time we get on the topic of jakes it turns into a cat fight on here. Usually someone says unless they’re official Jacob’s brakes they should be called engine brake, and the automatic transmission is technically an auto shifting transmission.

Anyway, my point is, different trucks are different. I ran a Cascadia that had an amazing jake. I would hit my brake pedal just enough to turn my brake lights on. The Western Star I’m in I literally didn’t even think they worked at first they were so much weaker.
All that being said, I’ve never ran one that stage three came even close to whipping my head forward.

 

Right. Who was the member that was running heavy to the mines in Nevada that had his Jakes cut out at something like 20 MPH? Factory programmed to cut out at low speeds. IIRC, the dealer changed the settings and Jakes stayed on.

 

The 2017 and 2022 I owned and the 2026 I’m in now are all the same. In the 2017 and 2022 the only time I used stage 3 was out west defending an actual grade. I’ve been driving this 2026 and I haven’t used stage 3. It simply isn’t needed for normal driving. Stage 2 is all 6 cylinders. I don’t know if there’s anything to it but I feel the less you use stage 3 the less you’re making the actuators work when the turbo kicks in.

 

So many people just get in a truck and drive without programming anything. Just like having to hit the brake pedal to kick the engine brake on. That would drive me nuts.

 

I'd guess the engine brake isn't even engaging at 20mph. It's all in the programming.. we have the same setup at work and at somewhere around 20 or 30 mph the Jake's aren't even engaging it just down shifts faster.
I'd leave it in auto.
Our 12 speed autos have 4 Jake settings, 1,2,3 and max. Max is just level 3 but it down shifts to keep rpms high. And holy cow does it slow down fast and will throw you forward even with a heavy load.

 

Ok it doesn't look like anyone has given you the answer that's likely the issue for you from my experience in the W990 with the X15 motor and Eaton's transmissions..... You need to have shop check your default "kick-in" speed for the Jake to initiate.....

Our 3 Kenworth trucks 2025-2026 the factory setting is 35 mph ..... Which means if you're not going at least 35 mph to start the Jake won't kick in. I had exactly same problem till we changed it. I have mine set to 10 mph now because I drive a lot of mountain passes where safe speed on the windy 2 lane highways with 5-12% grades and 105,000 GVW is maybe 15 mph .....

Once we changed that setting it all works like a charm with one other modification...... In the medium and high jakes in "Automatic" mode it will think you want to stop and start dropping gears faster than you want .. so I run almost all my downhills in "Manual" so I decide what gear works best with the Jake.

Hopefully that helps

 

Take it to the shop and tell them to turn the jakes up to the max (may also be known as an adjustment of the jakes).

Tell them the jakes are not slowing you down descending a grade and that this is a safety issue. If they give you any crap tell them you'll call Safety.

 

FWIW, I drive an International LT with an X15 and a 12-speed auto, and I’ve found that I need to set the cruise for five mph below the speed I want to maintain going down hills.