Author Topic: What does a jakes brake do?  (Read 2357 times)

Offline TrevorH

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What does a jakes brake do?
« on: February 09, 2008, 11:27:23 AM »
On the bus I just purchased there are 2 switchs that I cant figure out.  The first being a jakes brake.  It is a 3 position switch.  What does this do?  Second is a high idle.  I know what it is supposed to do but when I switch it, it does nothing.  Where do I start looking for issues?
1987 MCI 102A3 8V92TA 5 spd MT
Tucson, AZ


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Re: What does a jakes brake do?
« Reply #1 on: February 09, 2008, 11:47:01 AM »
The purpose of the Jake Brake is to close two of the four exhaust valves in your engine and utilize the resulting compression buildup in the engine to slow you down, providing your engine is at high enough R
PM's for it to do some good.  Don't expect much in the way of results below 1500 RPM's.

You speak of a 3 position guess is that center would be 'off', and one of the other two positions would switch in Half (2) of the cylinders....and the other position would switch in all four. Tracing out the wiring would be the only way to be sure.

The fact that you high idle switch does nothing leads me to believe that whoever installed, or had them installed, didn't concern himself about the high idle and elected not to buy the switch for the buffer screw replacement.  It's about a $250 item and not all owners go for it....I did...but I live in the mountains and I want all my functions.



Offline rv_safetyman

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Re: What does a jakes brake do?
« Reply #2 on: February 09, 2008, 11:48:21 AM »
There have been lots of threads on Jake Brakes.  If you put in the words "jake brake" you will get over four pages of threads.  You can do your own search of this board, or look at the following threads;


Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
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Offline Busted Knuckle

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Re: What does a jakes brake do?
« Reply #3 on: February 09, 2008, 12:45:26 PM »
FWIW with or w/o jakes the high idle switch will only work (a) when there is enough air pressure (b) the parking brake is set (c) transmission is in Neutral. All these are a must for a proper working system. Now on others I have seen several other things (rigged) that needed done to make them work (I'm having a brain fart @ the moment). Oh on one coach I worked on, the parking lights had to be on b4 anything worked !(I assume to place some load on the alt. but I wasn't the engineer that did it!) On another the A/C had to be on. You never know what the self engineering type will come up with, so some time ya just gotta play with things or get down and dirty and trace wires/systems on item at a time till ya find a fault or home brew design!
Also if the Jakes are aftermarket or home brew it also could effect the high idle as well as the other way around! Under normal circumstances it' is hard to test Jakes while sitting still!
;D  BK  ;D
Busted Knuckle aka Bryce Gaston
KY Lakeside Travel's Busted Knuckle Garage
Huntingdon, TN 12 minutes N of I-40 @ exit 108

;D Keep SMILING it makes people wonder what yer up to! ;D (at least thats what momma always told me! ;D)

Offline Sammy

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Re: What does a jakes brake do?
« Reply #4 on: February 09, 2008, 02:53:55 PM »
Trevor, congrats and good luck with your new toy.
Here's the place to go for Jake brake info -
They normally won't activate at idle speed,like BK has already said.
The basic fast idle circuit is: a electrically controlled (from dash switch and a relay) solenoid valve allowing air pressure to travel to a mechanical cylinder, mounted on governor cover of your engine, moving internal governor linkage to raise idle speed.The parking brake must be applied, there is a switch plumbed into the park brake air circuit for this too.
I suggest trying to get a copy of a service manual for your bus. It will have TONS of useful info and electrical diagrams too.Send e-mail or catch me in chat room, I'll help you as best as I can.
Sammy  8)