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Old 02-12-2009, 02:07 PM   #1
prarieschooner
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Default Jacobs Brake?

I took the '82 to get the tires put on yesterday. I could use a little help understanding the Jacobs Brake operation, when to use it and why.
Steve
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Old 02-12-2009, 02:55 PM   #2
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Good morning Steve! I have a "Jake" brake on mine, also. I use it, obviously, on steep declines. I put the trasmission in a lower gear (3rd, 4th depending on the length and grade percentage) and then just flip the switch and coast. You'll feel and hear the "Jake" take effect. On really steep, long grades you may have to use the brakes to scrub some speed off periodically, but it works pretty well, and certainly reduces the need for braking 70-80%. To give an example, you may be familiar with Sherwin Grade (between Bishop and Mammoth Mountain) on HWY 395. It's about 6-7%, and about 7 miles long. At the beginning of the grade I put the coach in 3rd gear, and turn on the "Jake" brake. Typically this takes me down to around 30-35 mph. I can hold it under 45 for about 3 miles before having to scrub some speed off again. Usually have to use the brakes twice on that downhill. Best thing with these big coaches is to just take your time and preserve the brake lining. Without the "Jake" brake you would be on the brakes for the entire 7 miles, or in 1st gear the entire time, and still using the brakes a lot.

Another time when I have used it is when I have had to make what I would call a "panic" stop where just braking would not stop me in the required distance. I hit the brakes, and flip the "Jake" switch and it is amazing how fast you can stop the coach!

So, what kind of tires did you purchase?
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Old 02-12-2009, 04:47 PM   #3
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Clarke gave an excellent description of the use of the Jake Brake. Diesels not equipt with an exhaust brake or a Jake brake have significantly less inheritant compression braking than gasoline engines. Let off a gasoline engine and you can feel it slowing you down due to the butterfly or the throttle plate closing off the air intake into the cylinders. Let off a diesel engine, particularly a two-stroke diesel engine, and you don't feel as much slowing affect since there is no butterfly/throttle plate to close. As a matter of fact, the Detroit Diesel two strokes have a low pressure blower that is gear driven off the engine that continues to pump air into the cylinders even though the fuel is shut off to the cylinders. A true Jake Brake (engine compression brake) provides engine braking by turning the engine into a giant air compressor. A Jake Brake works by closing the exhaust valves on all or certain cylinders. It is important to note that the higher the engine rpm, as long as you are within the engines maximum speed range, the more braking power the Jake has. Therefore you want to gear down to keep the engine up close to redline to maximize the effectiveness of the Jake. It will help at lower rpms but typically, on an engine that redline's at say 2100 rpm, most of the effectiveness is above 1800 rpm.

EDIT:
The Jake brake on a four-stroke diesel engine has more braking capacity than a two-stroke diesel engine. Example: Series 60 (4 stroke) Jake has about 550 braking HP and an 8V92 (2 stroke) Jake has about 260 braking HP.

Using just the service brakes on a long grade is a recipe for disaster in a heavy vehicle as you will have to stay on the brakes to the point that they overheat and become ineffective. Use of gearing down the transmission alone may get you to the bottom safely but you might have to go down in 1st gear without the Jake to maintain a constant controlled speed as opposed to maintaining a constant controlled speed in 2nd or 3rd with the Jake Brake.

Some newer coaches have two speed Jake Brakes. With those you can switch between low and high to vary the number of cylinders that are aiding the braking effort.

ALWAYS be safe and obey the truck speed limit on long declines. It doesn't matter how fast you get to the bottom, it matters how safely you get to the bottom of the mountain. Avoid use of the Jake brake on wet roads.
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Old 02-13-2009, 12:32 PM   #4
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Hi Steve,

I was visiting a bus repair garage in mineapolis yesterday that I have work done on my coach and we were discussing the use of the jake brake. The manager is a very good friend of mine and he says to leave the jake on all of time. I never turn mine off except when I come to a comunity where they restrict the use of them. This garage is primarly a detroit place and they now them well.
I have an 88 coach and my last newell was a 78. Michael, I had a two stage jake on my 78 which had a cummins in it. It would run 4 cylinders on low and all 8 on high. It worked well but I rarley used low.

Michael and Clarke are right jakes save your brake and there is no question that they slow you down when in use and they are a nessity in the mountains or steep or long hills.

Besides all that I love the sound of them.

Happy travels, Wally
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Old 02-13-2009, 02:15 PM   #5
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Wally, thanks for the information. I wasn't aware you could leave the Jake on all the time. Does it only activate on zero throttle?
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Old 02-13-2009, 02:47 PM   #6
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Yes, most are set up to act on zero throttle. There is supposed to be a "sweet" spot where you are just beyond zero, where you are off the throttle, but not engaging the jakes.

It is a very small spot in my Newell.

There are many different opinions about running with the Jakes on all the time. The one downside for running with the all the time is that you do tend to glaze your brake pads because you don't ever really get on them that hard. On the other hand, they really do help in a panic stop, and who will have time to activate them in a true panic stop.

I usually run without them on the interstate for two reasons. One, they crosstalk with my cruise control. Two, if I'm not on cruise, I don't like the brakes coming on just because I am out of the throttle. If I could adjust in a bigger sweet spot, maybe I would rethink that.

There is a way to wire them into the brake pedal, and not the throttle, so that they activate on brake pedal only.

I do run them around town and in any kind of heavy traffic.

And to prevent the glaze on the brake pads. I occasionally do a couple of intentional hard stops. It really makes a difference in the overall performance of my braking system. If the coach gets to a point where it seems hard to stop from 15 mph to zero, it is time to deglaze the pads with a couple of 60 to zero hard stops. Make sure everything is tied down :-)

One more thing, as others have said, you don't want them on in slick conditions, because they can break your rear tires loose.

Speaking of brakes, here is some trivia that I was surprised to learn. The major brakes on the rear. Just opposite from a car. The brake pedal activates a two stage air valve, and the rears come on first, and then the fronts. Made sense after I thought about it. In a car, application of the brakes shifts a lot of weight to the front, and takes it off the rear, so the heavy braking is done by the front. That is not nearly so pronounced in a bus, the weight is in the rear, on twice as much rubber, so the primary braking is on the drive tires.
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Old 02-13-2009, 03:13 PM   #7
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Richard, thanks for the illumination. Makes sense to have it on around town, and in heavy traffic. As I have said, and others have said, this forum is invaluable. Each topic discussed is like attending a mini seminar.
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Old 02-13-2009, 03:32 PM   #8
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That answers most of my questions about the Jake Brake. From what I read it would be alright to have it on all of the time and no problem with the Diesel Engine. It would be better to use it as needed when the Road Conditions allow and it will not effect Cruise Control, although my Cruise isn't working just yet (another search then questions later).
We now have 6 new GOODYEAR G395LHS tires on the Coach, I didn't have the $$$ for the Michelin and probably wouldn't have sprung for them right now anyway. The BRIDGESTONE tires that were on the coach were weathered badly, had flat spots and dated 1997 so I didn't want to travel very far to get the new tires making it necessary to get them here in Yuma. The only real Truck Tire Shop here in Yuma that can spin balance Tires this size (11R22.5) is Purcell Tires.
I will have to let you know what I think about them after we get a few thousand miles but I can tell you that they ride much better than the tires that haven't moved in a few years and it sure makes me feel better about new rubber! These are a combination Regional and Long Haul type tire that are good for all positions.
Steve
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Old 02-13-2009, 03:58 PM   #9
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Congrats, Steve, on the new rubber! Do you know what make your cruise control is?
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Old 02-13-2009, 04:04 PM   #10
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How does one know?
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Old 02-13-2009, 04:28 PM   #11
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Ok guys. I got lost at Richard's comment about slick conditions potentially "breaking the rear tires loose". I had never heard that before and am not certain what it means.

Richard, could you expand on that a little?

Larry P.
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Old 02-13-2009, 05:29 PM   #12
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Good question, Steve.....might have to crawl in the engine compartment, clean off the unit and see if there is a brand name on it. A lot of them are Bendix, but mine wasn't. My mechanic had to remove the unit to get a look at it in order to determine who made it, and then found out they were no longer in business, hence my move to Cruise King.
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Old 02-13-2009, 05:44 PM   #13
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Sorry, it's that southern good ole boy talk. Breaking the tires loose means sliding them, a skid.

If you have the jakes on, the come on when you let off the throttle. They are not like regular brakes in that they are either on or off, the amount of braking is proportional to the engine rpm. So, if the road is slick, and you let off the throttle, it's like a very hard application of rear braking. The tires can start to slip on the pavement.

Unlike normal braking where you would simply let up on the brake pedal if this started happening, the only way to get the jakes to release is to feed it some throttle. It's not what one would do instinctively in a skid with a 20 ton vehicle. Plus if it's skidding I don't think I'll have the presence of mind to hunt for the jake brake toggle switch either.

I am not saying don't run with them, just be aware that descending a icy or snowy grade with jakes could have a behavior you need to be aware of.

One more thing, my jakes do interfere with my cruise. If the bus starts down a grade the cruise will back all the way out of the throttle and I feel the jakes come on. Somewhat aggravating to scrub off that speed that will help you up the other side. Not a big deal in flatlands, but definitely aggravating in rolling hills.

Hope this helps with the explanation.
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Old 02-13-2009, 05:46 PM   #14
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on our coach the jake was wired in to low only. i located the purple wire coming out of the valve cover and with the help of my wife and a mirror on a stick found the other wire on the other valve cover put in a two position switch and some wire to the back and now high-low and the original switch is on-off Brian.
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Old 02-13-2009, 11:00 PM   #15
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anyone know if the jake on the 1990 is a two stage wired to be a single stage as well?

i will have to look.

tom
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Old 02-13-2009, 11:21 PM   #16
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My 1992 has a single stage Jake but does operate all 8 cylinders.
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Old 02-14-2009, 12:59 AM   #17
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from what i understand detroit diesel sent them out with jakes on both banks but for safety reasons (skidding) they hooked up low only. Brian
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Old 02-14-2009, 01:02 AM   #18
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so low is one side hooked up and high is all 8 cylinders? where does the wire come out and i will go check mine.

thanks

tom
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Old 02-14-2009, 01:20 AM   #19
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My 93 is like Michael's. It is hooked to low only on all 8 cylanders. Thanks! Marc
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Old 02-14-2009, 01:34 AM   #20
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Our jake was hooked up on the drivers side the wire is on the back side of the valve cover about midway the jake only runs one bank on low both on high if you have all 8 working on a 7-8 percent grade you have to use throttle on high! slows down to about 35 in drive. Brian
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