When you engage the cruise control on a car, and then the road becomes downhill, the car usually shifts down and uses engine braking to maintain speed, correct?
So if there’s a sign that says “Engine Braking Prohibited,” am I technically breaking the law if I use cruise control? Am I only breaking the letter of the law but not the spirit, or both?
I thought those signs were for big trucks with loud compression release engine brakes, not passenger vehicles.
Of course they are.
Those signs are specifically for engine braking systems like Jake Brakes. The reason those systems are prohibited in many areas is because of how much noise they make.
They can’t call them Jake Brakes on a sign because that’s a brand name (even though they are a bit like Kleenex or Xerox, everyone calls them that even when they aren’t that brand). In fact, if a township actually puts up a sign banning Jake Brakes, the Jacobs Vehicle Systems company will probably sue them for unfairly discriminating against their company and brand.
So, since they can’t call them Jake Brakes, even though that’s the most convenient quickie term that everyone would recognize, they have to some up with some other way to say “really loud diesel engine compression braking systems are prohibited” in a way that actually fits on the sign. Hence, “engine braking prohibited” even though the actual ordinance probably does not ban engine braking in general.
As far as cruise control systems go, in my experience they don’t downshift. They just cut off the flow of fuel and remain in their current gear. On a steep enough hill, if cutting off the fuel completely isn’t enough to keep the car slowed to the cruise control system’s setpoint, the car will speed up. There may be some vehicles out there that downshift to slow down, but none of mine ever have.
My Ford Mondeo (Fusion) changes down and when I mentioned it on a motoring forum, several people were surprised (and even disbelieving). Maybe it’s a Ford thing?
In any case; how would anyone not actually in the car, know that it had downshifted?
Hasn’t happened in California as far as I know. The anti-compression-braking signs signs I know about specifically say Truckers: Easy on the Jake Brake.
I found out I own one that will (downshift to slow while under cruise control). I didn’t know this when I bought it.
I have a Ram Diesel pickup truck with an exhaust brake (EB). The EB has two settings, one is “slow down as much as possible”, and the other is “hold the current speed when the driver lets off the gas”. The transmission has a regular mode, and a tow/haul mode. When set in tow/haul, releasing the gas pedal will cause the tranny to downshift while slowing down.
If I set the cruise control to, say, 70 in the mountains, while in tow/haul mode, and the EB in the hold current speed setting, it has the effect of “locking” the truck’s speed at 70 mph. It’s uncanny how closely it holds it. I’ve pulled trailers upwards of 8000 lbs thru some of the Colorado passes and the truck stays within 1mph of the set speed. Uphill or down, it doesn’t care. I assume that there are trailer weights and slopes that would overcome this, but I haven’t encountered them yet. I can go for hours with my feet flat on the floor.
Missed the edit window: Should have added…
As far as the signs prohibiting engine braking, I ignore them since there’s very little extra noise when the exhaust brake is slowing the truck. I assume I’m violating the letter, but not the spirit of the law.
My town had a sign that said “No Jake brakes” and just to see what would happen I took a picture of it and sent it to Jacobson along with location information.
About 2 months later the sign was removed and replace with one that said “Noise Ordinance Strictly Enforced”. So apparently Jacobson will try and protect their trademark.
OK thanks. I didn’t realize (until I looked up on wikipedia) that on diesel engines, engine braking involved more than just downshifting.
Still seems like an unnecessarily confusing sign though. How was I supposed to know that the sign didn’t apply to cars?
My current rental (Toyota Corolla) does downshift, I confirmed by looking at the tachometer. I guess I shouldn’t have assumed it was universal. (I never owned a car with a functional cruise control system, except my current Chevy Volt which of course uses regenerative braking rather than engine braking.)
My Impala uses the brakes to slow down when the adaptive cruise control is engaged. It will keep a set (“near, medium, or far”) distance from the vehicle in front regardless of its speed up to the speed set on the cruise control. I still have to steer. It doesn’t work well on twisty, turny roads.
95% of the population doesn’t even know what “engine braking” is. 4% of the population knows what engine braking is and knows that those signs only apply to diesel compression release brakes. You’re in the 1% that knows the former and not the latter 
Had a manual with a busted muffler.
Gasoline engine, but when I downshifted and engine braked down a hill, it was similar to (but not quite as loud as) a diesel semi.
Having lived most of my life within half a mile or less of an interstate, the distinctive sound of “BRAPPPPPPPPP!” is tied to many childhood memories.
I was within half a mile of an interstate for most of my childhood too, so it’s the same for me.
If you have never heard Jake Brakes before, here are a couple of examples:
https://www.youtube.com/watch?v=X2dMiSOmGTsI’ve sometimes wondered whether people who live near stops are irritated by the louder whirring noise of ordinary passenger cars that use downshifting for ordinary engine braking (not the compression kind on big trucks). I do this downshifting. Indeed, my car’s instruction manual says to. But it clearly does make a little extra noise.
The signs have been explained. Best point was indeed the presumably small % of people who pay any attention to those signs but don’t know they don’t apply to cars.
As far as cruise control downhill, my cars have not only a tachometer but a display showing when regenerative braking (IC not a real hybrid, but the alternator applies braking force to charge the battery) kicks in. When it does on a down slope I switch cruise control back to ‘ready’ till I reach the bottom of the hill. If I’m in ‘eco pro’ mode in one of may cars, the automatic transmission goes to neutral and the car rolls free. The other car is an automated manual (DCT) and doesn’t go to neutral but I can paddle shift it to top gear if not there already. I don’t apply brakes on any normal major highway down grade except if other cars are in the way, or the road is slick, or exceptional cases where cops are known to give tickets for speeding on down grades. In good conditions traffic allowing I let the car coast as fast as it wants to, and on major roads that won’t exceed the cars’ ability to take corners. On some smaller steeper, windier roads engine braking might be necessary or even friction braking.
What year is your truck?
I’d love to have it but my 1999 does not.
I’ve only driven old, or cheap, or cheap old cars/trucks, so I’ve never driven anything with braking connected to the cruise control, although I know it exists. But anyway…
My current car is a small cheap GM model. In this model year, the relationship between the gears and the engine control system was updated. The old model jerked quite badly when down shifting. There was a large engine-braking effect at the point of down-shifting because the engine speed didn’t match the gearbox speed.
The model that I am driving now gives the accelerator a squirt when down-shifting, so down-shifting is much smoother. But that means I can’t use it for engine braking in manual-mode. If I force it to shift down, it squirts the accelerator and the car speeds up
This thread is the first time I’ve heard of cruise control on a mass-produced unmodified passenger car automatically downshifting to control speed while descending a grade. My current car (Infiniti Q50) displays the cruise control’s target speed on the dashboard; if the actual speed exceed this by more than a few MPH (as when descending a grade or deliberately putting your foot on the accelerator to hasten a pass), the target speed starts flashing to get your attention, but it won’t downshift or brake to reduce your speed.
I thought “Jake brake” was a nickname of the Jacobs brake system. Can they enforce trademark on the nickname? Keeping it to the auto-world, is “Chevy” protected?