Engine Brake / J Brake

utahclaimjumper said:

  A properly set up Jacobs "brake" can make as much horsepower slowing you down as the engine is able to make speeding you up.>>>Dan

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Actually most electronic controlled diesels make more retarding horsepower than rated horsepower. I own a 6NZ cat that can be set at 435, 475, or 550 rated horsepower but jake brake retarding is rated at 600 horsepower. Mechanical diesels can be all over the place depending on manufacturer but, were nearly always under rated horsepower. My first jake brake was in a 1963 peterbilt with a small bore Cummings 335 hp engine. Retarding horsepower was 180 horsepower. This was in 1965 and I was running a trip a week from Denver to LA and back. At the bottom of every mountain pass or down grade, I said "Thank You Mr. Cummings for your invention". Odd thing his own engine company refused to develope his idea and he had to go to Jacobs.

Thats "Cummins" engines. Dang auto correct.

So those of you that say you keep the EB on all the time - do you have a 2 stage and are talking about leaving
the low setting on all the time?

IF so,  IF you had only one setting - on or off - would you still leave it on ALL the time?  Just curious.

Ours is either on or off no low high setting and we leave it on all the time except when the road may be slick. In my previous post you will notice I said I learned to feather the accelerator pedal so the J-brake does not engage until I want it to engage. It can be very annoying when in stop and go traffic it engages and disengages. I have driven trucks that also had it. Most of the so called J-brakes in the MH's are exhaust retarders. The original J-brake was incorporated into the Cummins head. Someone may correct me but if I remember right it came out on the 190 or was it the 185 Cummins.

With my exhaust brake (1 setting) I "generally leave it on all the time, but sometimes turn it off in town or on flat roads.

Most of the so called J-brakes in the MH's are exhaust retarders.

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Motorhomes that claim to have a Jake brake usually have it. The 'exhaust retarders' on motorhomes are often referred to as 'PacBrake', and I've never heard them referred to as 'J-brake' or Jake Brake.

One misnomer is that Exhaust/Pac Brakes are sometimes called 'engine brakes' which, of course, they're not.

Some trivia ... when a CAT mechanic had the rocker covers off the 3196 engines in our boat, he showed me where the Jake Brake solenoids would be mounted in the over-the-road (truck/tractor) version of the same engines.

Tom said:

Motorhomes that claim to have a Jake brake usually have it.

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A don't disagree with you Tom, but I have heard many RVer'  and diesel truck owner refer to an exhaust retarder as a Jake Brake.

This is an opportunity to educate the misinformed.

My Jake Brake is on high 100% of the time. However, I had Allison reprogram mine so that it does not downshift. That was by far the best mod I have done to our coach.

From the factory the Jake is programmed to downshift to the next lower gear as soon as it can without over-revving the engine in that lower gear. In other words, it is programmed to provide 100% braking effort 100% of the time. But what if you want just a little help slowing for an exit ramp, or driving in traffic at 20-30 mph? The Jake would be nice, but not at 100% braking effort. By programming it so it does not downshift, the Jake engages in whatever gear I happen to be in. As you slow down, the transmission will gently downshift to the next lower gear at 1,000 rpm and the brake remains engaged. At 1,000 rpm in second gear (15 mph) the brake disengages.

When driving in the mountains, you can downshift as necessary to get the braking effort you need. In our coach, I have never had to shift down lower than 5th gear on interstate grades. On the US routes, where the grades are steeper and the speed limits lower, 4th gear gets us down the steep ones without touching the service brakes.

City ordinances prohibiting the use of engine brakes are predicated on the noise they typically generate. However, in most motor homes the Jake brake is well muffled to the point where it does not make any noise. Consequently I pay no attention to those signs and use the engine brake all the time.

Freightliner will freely admit that they program the Jake the way they do because they have no confidence in the ability of the typical motor home driver to know how to properly use an engine brake. So they program it to deliver all the braking it can, all the time. However, if you take the time to learn how to drive your coach properly, this is a terrific mod. It will cost about $60 and take about 30 minutes at any Allison shop.

If you are the type of driver that just puts the transmission in drive and rolls, then this mod is probably not for you.

So those of you that say you keep the EB on all the time - do you have a 2 stage and are talking about leaving
the low setting on all the time?

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Use of different setting with a multi-speed engine brake (there are both 2 & 3 stage models) is largely dependent on personal preference and the terrain expected.  I mostly used the "low" (reduced braking) setting, but where grades were steeper I would run with the higher stetting.

As others point out, the amount of braking force can vary a lot, whether single speed or multi-speed, and whether a built-in Jacobs engine brake, or an external exhaust brake or transmission retarder. Engine size has a lot to do with it as well.

There are also control differences. An engine or exhaust brake can often be programmed to operate in different ways. The most systems, the EB auto-engages as soon as the accelerator peddle goes to zero accel, but some can be set to not engage until the driver also taps the service brake pedal. That semi-automatic mode is very convenient for rolling hills and stop & go traffic.

The bottom line is that you can and should use it in whatever way you find helpful & convenient. By all means use the exhaust or engine brake to help slow the vehicle whenever practical. It saves on service brake wear & tear and helps slow the vehicle quicker. If you want to manually engage & disengage, that's fine. If you find the automatic or semi-automatic modes convenient, then do that.

This applies to me via my owners manual and may or may not be the same for any of you.

When all else fails -read the manual.

First off is, in my case they call it an Exhaust Brake.

It also states:
Several operating conditions must be true to activate the exhaust brake:

1. The exhaust brake switch must be in the ON position.
2. The operator's foot must be off the accelerator pedal (pedal at low-idle speed position).
3. The engine speed must be above 1000 rpm.

This might explain why one poster stated that it let loose at 10-20mph - maybe the RPM's dropped below 1000.

It also states:
Automatic Exhaust Brake Activation in Cruise Control

This feature allows the ECM to use the exhaust brake to
maintain set cruise control speed in downhill motoring con-
ditions. If this feature is enabled, the exhaust brake auto-
matically engages anytime the cruise control is active and
the vehicle speed exceeds the set cruise control speed by
more than a programmed amount.  The exhaust brake will
disengage when the vehicle speed decreases back to the
set cruise control speed.

I will have to do some mountain driving to see if this feature is enabled.

Going UP a mountain I can't see using the cruise control, as I usually follow the slow trucks going uphill too. 
So IF I set the cruise as I come to the top of the hill,
I will be curious to see if at that point the coach won't go but a few mph faster then what I set it at, going downhill.

Now I know everyone is different and so this question might have a lot of different answers.

How fast do you tend to drive going DOWN a mountain (yes I know they are all different) lets say 7% grade?

I have already said (in other threads) that I usually drive at 55-60mph because my trailer manual states Max Speed 55mph.
So most all traffic passes me and most of you would too.  I don't want to go so slow (downhill) as to
make all the Semi-Trucks have to go around me - but I know I can't please everyone and some truck drivers
have a deadline and drive way faster than what is safe.  IMO.

With the assistance of the light on the dash indicating the exhaust brake is engaged, I've trained myself to find that spot on the accelerator to avoid having it come on when I don't want it to (just slight pressure on the pedal). 

Down hill speed is really dependent upon the gear ratio, transmission, and weight.  I find mine will crawl nicely down 7% at about 30 mph in 2nd gear (if I remember right) at a comfortable RPM and exhaust brake engaged - I don't have to use the brake.  If I grab the next gear, it requires braking, which is OK by the way if the road conditions are right (mild curves, etc).

How fast do you tend to drive going DOWN a mountain (yes I know they are all different) lets say 7% grade?

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That depends. In my Beaver, I'd kick in the second stage of the compression brake and let it run (if it was a fairly straight shot -- tight curves are another story), with it mostly holding under 60 without using service brakes. My Ventana with the VGT (Variable Geometry Turbo) doesn't supply nearly the amount of braking power the Beaver did, so on a grade that steep I'll probably try to keep it under 60, which means using the service brakes briefly, every little while (at 7% it could soon get over 80-85 mph -- too much). Or I may elect to slow to 35-40 where it'll shift to a lower gear enough to keep the speed down better. Or on occasion I may have to get under 30. So much depends on wind, how long the steep run is, what the curves are, what traffic is doing, how many lanes each way, etc.

No faster than I feel I can control the vehicle should the retarder fail or a mechanical failure should occur. I've seen jake brake fuses blow, switches short out, transmissions fail, pinion nuts back off, drive lines fail and ect. on vehicles going down a mountain. There is probably more stress on the drive line going down the hill than up. These occurrences are rare but, they happen more often than you would think. One reason they have runaway ramps.

How fast do I descend mountain grades? That's very circumstantial, but on the grades we have around here (most are 6% and many miles long) I'm never above 50 mph - usually closer to 40-45, but it largely depends on how curvy the road is, how wet it is and traffic conditions.

For what it's worth, I do exactly the opposite of how you thought you might use your cruise control. Mine is almost always on when I'm climbing a grade, and it's usually disengaged when I'm descending a grade. I suppose it wouldn't hurt anything to leave it on - I'd just dial the speed down a bit - but it's not something I need on a long descent. I find it very easy to control downhill speed with just the Jake's two settings.

A question for A Traveler... Do you regularly downshift (manually) when descending grades? We have some pretty long grades around here, and we're on them quite a bit. With a two stage Jake, I've never felt the need to downshift - even on a 9% grade while towing. The Allison always seems to choose the right gear for my speed and conditions. Just curious

Kev

gwcowgill said:

Ours is either on or off no low high setting and we leave it on all the time except when the road may be slick. In my previous post you will notice I said I learned to feather the accelerator pedal so the J-brake does not engage until I want it to engage. It can be very annoying when in stop and go traffic it engages and disengages. I have driven trucks that also had it. Most of the so called J-brakes in the MH's are exhaust retarders. The original J-brake was incorporated into the Cummins head. Someone may correct me but if I remember right it came out on the 190 or was it the 185 Cummins.

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I too have never heard the term "J-brake." The original Jake brakes as marketed by Jacobs Tool and Die Company were aimed at the Cummins diesel engine market. The Jake brakes were not incorporated "into the Cummins head" but were a separate cast housing attached on top of the rocker box housing.This separate housing channeled pressurized engine oil into the slave pistons which pumped the oil to a higher pressure, causing the exhaust valves to pop open at TDC. The electric solenoid valves were used to control the oil flow to the slave pistons.

We used to have Transmission Retarders on our old Detroit Diesel fire trucks, and they were about as effective as a parachute

Kevin Means said:

...A question for A Traveler... Do you regularly downshift (manually) when descending grades? We have some pretty long grades around here, and we're on them quite a bit. With a two stage Jake, I've never felt the need to downshift - even on a 9% grade while towing. The Allison always seems to choose the right gear for my speed and conditions. Just curious

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Kevin, the answer is yes, I will shift down manually if I need to. On the interstate grades in the Appalachian Range, I have only had to go down to 5th on a handful of the grades. On most interstate grades, 6th gear is perfect. I can ride the Jake Brake down the hill at 65 mph and not touch the service brakes. I have not yet driven in the Rockies and I suspect that some of those grades are steeper than we have back east. We're looking forward to traveling in that part of the country!

In reading the various motor home Internet forums, I have found that I take a somewhat different approach to driving our motor home than most drivers. I genuinely enjoy driving our coach. And when I'm driving, all I do is drive. I don't listen to music, only occasionally listen to talk radio, I pay close attention to the road and I'm always up to speed on my situational awareness and what I'm doing. I am a commercial pilot and I approach driving the motor home in much the same way that I fly the King Airs. 100% attention to what I'm doing 100% of the time. I can describe it by saying I'm an "active" driver as opposed to a "passive" driver. I stay involved in my driving, so doing a little manual shifting is just part of the process for me...and I enjoy it.

I understand - thanks. By the way, I recently retired from the San Diego Police Dept as Chief Pilot of the Air Support Unit. After 25 years of listening to 11 police frequencies and ATC simultaneously, it's really nice just listening to the road noise.

Kev

Kevin Means said:

How fast do I descend mountain grades? That's very circumstantial, but on the grades we have around here (most are 6% and many miles long) I'm never above 50 mph - usually closer to 40-45, but it largely depends on how curvy the road is, how wet it is and traffic conditions.

For what it's worth, I do exactly the opposite of how you thought you might use your cruise control. Mine is almost always on when I'm climbing a grade, and it's usually disengaged when I'm descending a grade. I suppose it wouldn't hurt anything to leave it on - I'd just dial the speed down a bit - but it's not something I need on a long descent. I find it very easy to control downhill speed with just the Jake's two settings.

Kev

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My thinking on turning the Cruise OFF going uphill is that I usually follow Semi-Trucks going up and they (depending on how heavy) might be
doing 25-40mph and I see no reason to pass them. Besides it cost more fuel to go uphill faster, and I'm rarely in a hurry.

Turning the Cruise ON going downhill is what is described in the engine manual.
I'm sure it will take some time to figure out what works for me.  Having a single stage exhaust brake - will I have to downshift to 5th or 4th
or maybe into 3rd gear going downhill?  IDK.  But I assume there is only one way to find out.

I do think that 45mph might be a good speed to descend a 6% grade.  Again if I'm following trucks that MAY help me gauge a speed better,
but there are too many variables to guess what will be - when each hill is different. 

Thank you for your input and I will keep it in mind once I start driving it again this coming Spring.