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Aberdeen MS | Jacobs brake (Jake for short) operates via an electric solenoid valve that uses oil pressure on top of the exhaust valves. When energized, like the rest said, it opens the exhaust valves at the top of the compression stroke basically turning the engine into an air compressor.
If you use the clutch to shift, yes, you can leave it on and down shift just as you are used to doing. If you float the gears, it requires a little technique keeping just enough pressure on the foot feed during the "float" to keep from energizing the Jake. Play around with it empty first, as you will come close to if not kill the engine the first couple times during the shifts. Provides for a good "pucker factor" if in a "close situation" loaded. BTDT more than once.....LOL.
After 1 million miles of OTR, getting extremely used to using the Jake in all conditions, I have killed the engine going down a grade on snow pack. *Note for Rich:.. 1 head is great in parades .... ya stop.... and let the ol gal "puff" away on one head, kind a rattles the carbon/soot out of the pipes a bit ....... run her up to 2100 ..... then give her all 3 heads. Ya can "poka dot" the crowd with soot... hehe.
After my little episode described above of killing the girl on a grade, I installed a toggle switch on the shifter handle that tied into the system after the master on/off but before the head selection switch. I use that switch to flip the jake on and off, works great when shifting. Flip the switch off ... only need your thumb ... shift... flip it back on. Just like using the splitter button. When I'm in conditions that are not conducive to Jake use (rotten weather, ice, heavy rain empty, etc) I can flip the master on the dash making sure I don't use the Jake.
I did that little trick of the shifter switch when I was in a "Vette dash" Pete. The master switch was about 5" outside my normal arm reach. I was always shifting myself in the seat to shut it off and the Cummins was just touchy enough that a little pressure on the foot feed I couldn't get it to "idle" down enough float shifting smoothly. Not that much of a problem with my Cat, but I've gotten so used to the shifter switch from before, I'm almost lost in a cab working a Jake without it.
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Jake brake use
