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Old 09-11-2019, 09:48 PM   #1
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Exhaust brake repair

1999 Coachmen Sportscoach 380MBS
Freightliner XC chassis
275hp 5.9l Cummins ISB
Allison MD3060

Situation:
I recently got my exhaust brake working. This is my first diesel pusher coach, so when I bought the motorhome almost two years ago, I knew there was some mechanism on the exhaust downstream from the turbo, and that the exhaust brake switch on the dash was ineffective. Beyond that, I had no idea what components were involved or how it was controlled. I did know, however, that it would be really useful for our planned roadtrip to AZ and CO!

Research:
I did some research on the brake and determined that it was a Jacobs brand exhaust brake, and that the inline exhaust piece with butterfly valve was very likely seized due to lack of use and maintenance by previous owners. I have extensive records going back about 15 of its 20 years and it was not mentioned once. I also downloaded lots of info from the Jacobs and Cummins Quickserve web sites, including Cumminsí application notes that gave me the correct Cummins part numbers for the brake, clamps, etc, and Cummins to Jacobs cross-reference tables which gave me the correct part numbers for ordering. Finally, I called Freightliner and requested installation and wiring information. They very helpfully emailed me a two-page functional description and five pages of wiring diagrams. I ordered the $643 brake, plus gaskets and clamps, based on all this information and before diagnosing or examining the needed parts. Risky, but I was trying to save time before our trip west. I ordered from Reliable Industries, who got the order out very quickly and only charged $21 for FedEx two-day from Louisiana to FL.

Repair:
I ran out of time during RV prep before leaving on our trip, but I had the chance to really dig into the diagnosis and repair with my son-in-law while we were visiting. (We canceled our CO leg) We propped open the bed/engine cover and he watched the engine brake mechanism while I drove. The ENG BRAKE light lit on the dash display, but no mechanism movement. I pulled the air cylinder off the butterfly lever arm with the brake still mounted and couldnít move the lever. Not a surprise, so I removed and replaced the brake assembly. This is a straightforward repair on my coach, with two stainless steel clamps and one air line and ready access under the bed/engine cover. On the air line, make sure to back up the fixed fitting with a wrench and do not apply any torque to the fitting. You donít want to snap a fitting off in your component and have to drill it out. If you ever do this job, make sure that you order a brake-to-turbo gasket. I also ordered new clamps, but the company sent two of one size, so I had to reuse the brake-turbo clamp. Given the age, I consider myself lucky there were no problems pulling the old clamp and reusing it. Also note to thoroughly scrape the old gasket off with a single edge razor since getting all the old gasket off is the only way for the clamp to fit properly over the joint and for the joint to seal properly. My helper pried the downstream exhaust pipe back to do a test fit of the brake, but it wouldnít fit. I had to remove the air cylinder in order to rotate the butterfly valve closed to get clearance. Removed the sticker paper from the gasket and stuck it to the brake mating surface. Fit, positioned, clamped, done. Reinstalled the air cylinder onto the bracket and the air line onto the cylinder fitting. Test drive? Still no function.

So, wiring problem, relay, air solenoid? Possibly all of the above?

From the two-page functional description sheet and the wiring diagrams from Freightliner, I knew that there was a Vehicle Interface Module with 4 chassis connectors, 6 relays (one for the brake) and 2 fuses, an additional Freightliner chassis fuse block with the exhaust brake fuse and another brake relay, and a dash control switch with 5 wires. The plan was to check fuses, then wiring, then relays. If that was all good, then Iíd have to try to get to the air solenoid over the front of the transmission. Very difficult spot. The VIM was found in the engine compartment, but everything there looked good. However, a lucky pull of the first fuse in what I guessed was the chassis fuse block under the dash yielded a blown fuse. Found ignition switched 12VDC to one side of the fuse and continuity to one of the exhaust brake wires on the dash switch on the other. Blind luck, there! Confirmed all the wire numbers and names on the dash switch with the diagram, and discovered a non-documented box labeled ďbrakeswitch.com.Ē Sketched a diagram of the wires on the dash switch and the brakeswitch.com box and determined that four of the leads from the brakeswitch box were used to insert the box between two of the leads from the chassis to the switch. One brakeswitch wire went to ground, the last to...? Given the name, brakeswitch, I checked to see what happened when the service brake pedal was depressed. Using an ohmmeter, I found that the brakeswitch.com box supplied continuity to the power lead from the fuse block and to the signal lead to the Cummins ECU when the brake pedal was just slightly depressed. So the brakeswitch box worked and I now understood its function. Sketched its wiring, too.

Replacing the fuse and hooking everything back up, the fuse blew immediately when I turned on the ignition switch and depressed the brake pedal. So, a wiring problem. Since my dash wiring is such a mess, I figured that the dash switch might be mis-wired. I pulled the switch from the dash, checked continuity across terminals, and found that the 12VDC was going straight to ground when the switch was turned on! That explained the blown fuse and the lack of operation! I also found inconsistent contact in the switch, so I took it apart and cleaned it. This is tedious, but do-able. Just be careful of springs and small parts escaping across the room. Sketched up the correct wiring connections per the Freightliner diagram and the internal switch contacts that I had worked out with an ohmmeter. Reinstalled the switch, wired it, and checked it out. No blown fuse!

Driving:
Took it out for a test drive, and with the exhaust brake dash switch on, a light tap of the brake pedal engaged the exhaust brake. Touching the throttle disengaged it. This was different operation than the Cummins and Jacobs operator manuals had described, but I wasnít surprised after working out the brakeswitch function with an ohmmeter. I got a chance to use the brake on eastbound I-10 on the long downhill into Benson, AZ. Pretty sweet. So much easier to manage downhill speed than gearing down to third and crawling down the hill. Iím guessing that the dash switch got flaky and the previous owner didnít really need it here in FL, so skipped the repair.

Summary:
Freightliner by phone, Cummins on Quickserve, and the Jacobs web site all had very useful information that allowed me to find the right parts to order, and to troubleshoot what the issue was. My parts total was about $750 and it took about six hours over two days to do the brake replacement and the wiring and switch troubleshooting and repair. I hate to think what a shop would have charged with my mess of wiring behind the dash.
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1999 Coachmen Sportscoach 380 MBS
Freightliner XC, Cummins 5.9 ISB 275 hp, Allison 3060
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Old 09-11-2019, 10:01 PM   #2
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Thanks for your write-up of your repairs. You are the 2nd person to repair your PacBrake and post your experience, thanks again.
I have a feeling my 1999 exhaust brake will require attention soon.
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Old 09-12-2019, 07:40 AM   #3
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Thanks for documenting your repair. Your system is totally different in operation than mine but it seems we have some common parts.
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Old 09-12-2019, 02:16 PM   #4
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Quote:
Originally Posted by Pandion View Post
1999 Coachmen Sportscoach 380MBS
Freightliner XC chassis
275hp 5.9l Cummins ISB
Allison MD3060



Driving:
Took it out for a test drive, and with the exhaust brake dash switch on, a light tap of the brake pedal engaged the exhaust brake. Touching the throttle disengaged it. This was different operation than the Cummins and Jacobs operator manuals had described, but I wasnít surprised after working out the brakeswitch function with an ohmmeter. I got a chance to use the brake on eastbound I-10 on the long downhill into Benson, AZ. Pretty sweet. So much easier to manage downhill speed than gearing down to third and crawling down the hill. Iím guessing that the dash switch got flaky and the previous owner didnít really need it here in FL, so skipped the repair.

Summary:
Freightliner by phone, Cummins on Quickserve, and the Jacobs web site all had very useful information that allowed me to find the right parts to order, and to troubleshoot what the issue was. My parts total was about $750 and it took about six hours over two days to do the brake replacement and the wiring and switch troubleshooting and repair. I hate to think what a shop would have charged with my mess of wiring behind the dash.
It was many years ago the the fellow who made that little black box past on to me what in the box. He had at that time closed shop and no Longer supplied the neat aftermarket exhaust brake control. I made my own from a simple relay and also added a foot switch to allow me to interrupt the cruise effect of the circuit so as ot to remove my hands from the wheel. On my blog I think there is a diagram of the brake contol and description.

The reason the originator of this modification made it was that he liked the coasting effect but really wanted something that would immediately activate the exhaust brake in an emergency stop. That is what it does. My foot switch mod simply allow you to bypass the feature and return to the standard Cummins/Fl operation. there is a link to the blog below in my signature.
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Old 09-12-2019, 03:57 PM   #5
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Congrats on an excellent repair, and a very informative write-up.
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