TESTING YOUR JAKE BRAKEDoes it seem like your Jake exhaust brake just isn't slowing you down like it used to? Here are some things to look at to help you determine if your exhaust brake is functioning.
First, when your exhaust brake is turned on, your gearshift display window should immediately indicate "2" meaning that second gear is preselected.   If this does not happen, check the fuse.   On my 98 36T, the fuse is located in the main front 12v electrical panel front panel (open the "hood" and remove the cover on the big black box on the driver's side of the coach - on the drivers side fuse board 2nd row down from top and 3rd position from left to right facing board, there is an wire coming out with a seperate fuse holder in the line.  This is the 5 amp fuse for the exhaust brake.   If this fuse is blown, some people have replaced this fuse with a 10 amp fuse and report no further problems.
Next, check to determine that your exhaust brake is actuating.   It may be "froze up" due to rust, lack of use or from short drives which cause the carbon deposits and moisture to cause it to seize.   You should be able to pull the exhaust brake actuating lever (although it will take quite a bit of force) and then it should smoothly snap back into the open position when you release it - of course, you would only do his when the engine is cool.   If it appears frozen in the open position, you can try freeing it up with GENTLE taps of a hammer and WD-40 spray, but be careful! As several others have already described, you can watch and see if it actuates from the under-bed access area while someone drives.   Note - you may still have a problem even if you see the arm move to close the valve, but this is probably rare.
If you exhaust brake is actuating, but it still doesn't seem like it is stopping you as it should, check your exhaust manifold for telltale black plumes indicating that you may have a warped exhaust manifold (which is covered by the Cummins Warranty).   You may also want to carefully check the exhaust manifold bolts and see if they are tight, but be careful so that you do not break them off by making too tight.   Check the Cummins book for proper torque values it is 35 lb-ft on mine.
Note that the exhaust brake was installed by Cummins and is therefor covered by your Cummins warranty.   Several members report that their dealership sometimes advises that the warranty period was only one year.   This may be true for commercial service, but it is not true for RV service.   Your Cummins engine in your RV is covered for a minimum of 5 years or 100,000 miles, whichever comes first.   Remind your dealer when they talk to Cummins to mention that this is an RV, not a commercial vehicle.
As a preventative maintenance measure, I routinely spray my exhaust brake actuating arm with WD-40.   Also, frequent exercise of the exhaust brake is necessary to prevent it from freezing up.   I always drive with my exhaust brake on - except on wet or slippery surfaces or when using the cruise control.