Wierd Jake Question

[Lin]

We are having a great trip up Hwy 395, Ca.  This has got to be one of the most scenic roads in the country.  All has been going well, but I do have a question about the operation of the Jake Brakes.  We are sometimes coming down some serious grades on winding roads.  They are on roads that are above 7000 feet, although I do not think that that affects this.  The Jakes work great in 3rd and 4th in their respective rpm range.  The issue is in 1st and 2ndd which is often required.  I have been manually shifting into the lower gears to try to keep the rpms up and speed down while descending.  I am also using the Jakes.  However, the transmission is apparently not staying in lockup anyway.  the funny thing is that if I turn on both stages of the Jakes, they pulse on and off (this is only in 1st and 2nd).  The engine rpms stay at about 800-900, and I begin picking up speed.  If I only put on one stage of the Jakes, they stay on, the engine rpms also remain at 800-900, and the bus maintains a nice slow speed regardless of the the grade.  What's going on?

[bevans6]

I will bet that you don't have enough oil pressure at 800 rpm to operate both pairs of Jakes.  Jakes work best at over 1200 rpm, and around 1600 - 1800 is perfect. Jakes work by opening a valve ( the solenoid) that allows oil pressure to flow into a device that opens the exhaust valve around top dead center, letting all the combustion pressure go down the exhaust before it has a chance to push the piston back down.  With low or marginal oil pressure, the chamber that pressurizes to open the exhaust valve early doesn't work right.

Presuming an 8V71 and two stage Jakes, one side has four chambers and two solenoids to work, and adding the other side doubles that.  You might just not have enough oil pressure and volume to run both sides at once, but one side manages to work.

Brian

[Lin]

Thanks for the diagnosis, Brian.  Short of altering the Allison to stay in lock up whenever the Jakes are engaged, is there any way I can use them now that would work better than what I am doing?  If a rev the engine while coasting down a grade, it seems to lock in for a moment but then drop right out again and return to 800-900 rpms.

[bevans6]

I don't have any experience with an allison, but i would think it's supposed to stay in lock-up.  jakes can't do much if the trans isn't in lockup.  If that is the problem, the whole idea of the oil pressure is probably moot. 

[luvrbus]

Your main problem is you don't have the 3 element torque converter your is 1C ,2C,2L,3L,and 4L if you don't want the Jakes to disengage at 800rpms by pass your buffer switch then they stay on forever still doesn't stop the transmission from free wheeling in 1st and 2nd in the converter mode,the governor on the transmission controls that part you have to bypass that part which to me is not a good idea

good luck

[TomC]

Most Allisons-even with them pulled into 1st gear-will only stay in torque converter lockup till about 20mph.  Once the rpm gets down to about 8-900, the Jakes aren't doing much then.  The higher the rpm the more braking you're going to get. On a 71 series, you can allow the engine to go to 2450rpm (which is about the speed that the Allison will automatically upshift to protect the engine).  On a 92 series I would go no higher then 2300rpm.  Try keeping in 1st gear as much as possible and control the Jake with your gas pedal.  Good Luck, TomC

[bevans6]

The way I read it, he is at a speed where the transmission should be in lockup and the rpm's nice and high and the Jake's working fine but it comes out of lockup prematurely, in his view, and the engine slows down to 800 rpm being pushed by the converter.

Brian

[loosenut]

My jakes don't operate below 20 mph no matter the gear or switch setting.  I was told that  jakes were installed that way.

It is quite annoying to use them close to 20mph as they shut off when I drop below 20 then as the bus starts to roll past 20 a thump as they kick-in and I start to roll then thump etc. 

I have a 6v92 so the set up may be different.

Mike

[Lin]

We are in Carson City now, and at a wifi equipped campground.  This is more complex than I realized.  I have a dash light the comes on when the Jakes engage, not merely when the switch is turned on.  Also, since I have the Jakes wired through a large solenoid relay, I can often hear the relay snap on and off.  If they are not really designed to operate below 20 mph, I can't expect more than I am getting, but it would be nice to have all the braking one could get when coming down high grade switchback roads.  Again, the confusing thing is that the Jakes pulse on and off if either 1st or 2nd if both stages are on, and I will start to pick up speed.  But they stay on if only one  stage is turned on, and it holds the speed.  In both cases, the rpms are way lower than they would be if the transmission was in lockup (to my understanding anyway).  I am content to use it this way if it is not doing harm, but am curious about what is going on here.  Thanks for all the help.

[luvrbus]

I told you not to waste the wire LOL,they are wired right Don knows what he is doing

travel safe

[Lin]

I could have used that extra wire to tie to an anchor to toss out the back for extra braking.  Hmm, I smell patent!

[belfert]

I have a newer bus with electronics and a 5 speed automatic.  When my Jakes come on the transmission automatically downshifts a gear.  I find that the Jakes are far more effective in 2nd or 3rd gear than in 4th gear.    I have to be sure to downshift the transmission as I crest the hill or the RPMs will get too high and the transmission will automatically upshift making the Jakes less effective.

I have never been in a situation where I have needed to go less than 20 MPH downhill so I don't know how my Jakes perform at that speed.  I almost always stick to interstates were the grades are less than other routes.  I know there is a general rule not to go down faster than you went up, but I don't think I've ever been below 30 MPH on the way up yet.