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Old 10-21-2021, 09:09 PM   #1
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Exhaust Brake

After coming home from our month long September trip, there were several items to fix on our MH. This one concerns the engine exhaust brake. I used it quite often going thru Rocky Mountain National Park. After we left the park I flipped on the switch to use it and nothing happened. After that going thru the rest of Colorado and their many high passes, I downshifted way down and took it real easy going downhill. I've looked at wiring diagrams and saw there's an inline fuse in that system somewhere, which I think might have blown from all of the usage. As of yet I haven't been able to locate that fuse. Anybody have a brilliant thought with this problem.
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Old 10-21-2021, 09:58 PM   #2
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Have you tried to manually move the brake linkage to see if it is stuck? It should be lubed with high-temp. grease if it is stuck.
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Old 10-21-2021, 10:02 PM   #3
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To be of any real help we're going to need all your coach and engine info .

Year , chassis , engine manufacturer and series .
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Old 10-22-2021, 07:11 PM   #4
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Exhaust Brake Problem

This is on my 98 Fleetwood Discovery, on a Freightliner chassis, with a 5.9 cummins engine.
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Old 10-22-2021, 08:16 PM   #5
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Quote:
Originally Posted by GraBros View Post
This is on my 98 Fleetwood Discovery, on a Freightliner chassis, with a 5.9 cummins engine.
Thanks for the additional information , I'll assume this is the ISB , 24 valve version of the 5.9L Cummins . 1998 was a change over year , from the 12 valve version of the engine .
There are several , requirements for exhaust brake operation .

Did you notice if the transmission , showed the beginning of the downshift program ; by displaying 2 on the control pad when the exhaust brake operation was called for?
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Old 10-22-2021, 08:31 PM   #6
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Exhaust brakes are hard to diagnose because you can't see if they're working until you drive the coach. You can go back and check the unit, most likely through a bedroom service access. It's a piston that pushes on the exhaust brake arm. As noted, it should be lubed once a year. PacBrake actually makes a specific lube.

While you're inspecting it, you can jump 12 volt power to it to see if it actually works. That will at least tell you if there is a problem at the unit.

Most coaches have an outside service bay under the driver's window. On all three of my DP's the fuses and relays for the exhaust/engine brake were located in that bay. There is also a transmission control module that looks like a black box 3" x 6". If you remove that lid, some may have fuses, but there will be relays in there. At least a place to start to look. Anything other than a fuse issue gets more complicated as the transmission has to tell the brake to operate.

As Skip426 said, was your transmission indicator showing 2nd or 4th gear when you turned on the exhaust brake?

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Old 10-22-2021, 10:28 PM   #7
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A page in this Freightliner schematic for 1998 chassis, list an inline 5 amp fuse for the exhaust brake switch , so the first place to test is for battery power to the exhaust brake switch with the key on.
Power through the switch and throttle pedal fully released , should trigger the trans downshift program and the trans shift pad should show either 2 or 4 as the target gear . When driving at 40 mph.

When you see the down shift , that indicates the fuse is good , the Cummins ECU sees the throttle position and you can move to the back of the coach , looking for an activation relay , the air supply solenoid , inspecting the air supply to the activator on the brake valve.

EDIT: Please note , Freightliner's schematics of this vintage leave a LOT to be desired .
Attached Files
File Type: pdf 1998 Freightlier with ISB schematics.pdf (1.42 MB, 6 views)
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Old 10-23-2021, 07:24 AM   #8
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On my 1997 Discovery with the Cummins ISB engine, it had a Jacobs Exhaust Brake, and NOT the Pac brake. The literature indicated not to lubricate it.

However, in my coach the brake was frozen in the open position because someone at Freightliner 'forgot' to connect the plug to the exhaust brake when the engine was installed in the chassis. Over the years of not working it froze.

I would guess 90% of the "exhaust brake" working in in the transmission downshifting to the lowest gear the computer thinks is safe. The actual exhaust brake itself only adds a small amount of additional braking.

So my question to the OP, was the transmission downshifting at all when going down the grades?
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