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Old 06-17-2016, 09:38 AM   #1
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2007 C7 pacbrake?

So, I had an injector failure, and a turbo gate valve failure. Nine years ok. However, while doing that the dudes at Cat removed a pacbrake. Did not know I had a pacbrake. I have a transmission retarder and a switch to control it. When the switch is on and you take your foot off the "gas" the tranny shifts into 2nd gear and the coach slows down. This was working when I took the coach in for repairs.

So those of you still operating a 300hp C7 can you educate me. Have I had both a pacbrake and tranny retarder all this time?

I have searched my records and manuals and can find no reference to the pacbrake operation.
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Old 06-17-2016, 09:51 AM   #2
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So, I had an injector failure, and a turbo gate valve failure. Nine years ok. However, while doing that the dudes at Cat removed a pacbrake. Did not know I had a pacbrake. I have a transmission retarder and a switch to control it. When the switch is on and you take your foot off the "gas" the tranny shifts into 2nd gear and the coach slows down. This was working when I took the coach in for repairs.

So those of you still operating a 300hp C7 can you educate me. Have I had both a pacbrake and tranny retarder all this time?

I have searched my records and manuals and can find no reference to the pacbrake operation.
Pac Brake only, the computer is telling the transmission to down shift when the Pac Brake engages, it's really not down shifting to "2nd" why it reads that I myself do not know, but I'm sure someone will chime in. I leave my Pac on all the time...thats what it's for!
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Old 06-17-2016, 09:56 AM   #3
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The transmission retarder , was an EXPENSIVE option , on high dollar coaches I'd be shocked if your coach has one.
The transmission program , that downshifts when the exhaust brake;( Pac Brake is a brand of exhaust brake) is active is standard to all Allison transmission on engines equipped with engine or exhaust brakes.
The shop would have had to remove and re-install the exhaust brake to replace the turbo, perhaps this is just a miss-understanding , of the wording of your bill.
You could look at the engine, exhaust manifold side , for an assembly next to the turbo that looks like the photo below, between the turbo and the muffler.
Only the outside cylinder will be visible the rest is hidden by the exhaust pipes.

My dash switch for the exhaust brake is also labeled " Retarder " but that's not what is on the motor , mine has an exhaust brake.
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Old 06-17-2016, 10:07 AM   #4
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Pac Brake only, the computer is telling the transmission to down shift when the Pac Brake engages, it's really not down shifting to "2nd" why it reads that I myself do not know,
The Allison shift pad will display 2nd, because that's the lowest gear it will down shift to while the exhaust brake is active. The actual gear the transmission is in while going down hill , is controlled by interaction between the trans and engine controllers , to make sure the engine is not over revved.
When the RPM drops below ( 2000 RPM in my case ) the trans will shift to the next lowest gear,; exhaust brakes work best at higher RPM.
In case the RPM gets too high ( 2750 RPM in my case ) the trans will up shift, so I need to watch the RPM and apply the service brakes before the upper limit is reached.
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Old 06-17-2016, 10:26 AM   #5
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The Allison shift pad will display 2nd, because that's the lowest gear it will down shift to while the exhaust brake is active. The actual gear the transmission is in while going down hill , is controlled by interaction between the trans and engine controllers , to make sure the engine is not over revved.
When the RPM drops below ( 2000 RPM in my case ) the trans will shift to the next lowest gear,; exhaust brakes work best at higher RPM.
In case the RPM gets too high ( 2750 RPM in my case ) the trans will up shift, so I need to watch the RPM and apply the service brakes before the upper limit is reached.
I figured something like that, when manually down shifted it reads correct.
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Old 06-18-2016, 04:07 AM   #6
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I was a PacBrake distributor for 20 years... a diesel engine has no ability to retard the speed of the vehicle as it goes down hill... an exhaust brake is like sticking a potato in the tail pipe... thus the piston comes up on exhaust gas in the cylinder and absorbs energy... to make this more efficient you need to lock the torque converter up with pressure and put the transmission in a lower gear... thus the features on the transmission...

Of course this is all done with electronics on the newer/modern engines/transmissions.. but the key here is to absorb the energy when traveling down a hill and add life to the brake when you need them...

I tow with a pickup/trailer at a combined weight of 23K lbs... and with the exhaust brake can go down hills like the Teton Pass west out of Teton National Park in 2nd or 3rd gear and never touch the brakes....

I'd go see why the shop didn't re-install the exhaust brake.... BTW as suggested above.. you should use it all the time...

Hope this helps your thought process...
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Old 06-18-2016, 07:31 AM   #7
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I was a PacBrake distributor for 20 years... a diesel engine has no ability to retard the speed of the vehicle as it goes down hill... an exhaust brake is like sticking a potato in the tail pipe... thus the piston comes up on exhaust gas in the cylinder and absorbs energy... to make this more efficient you need to lock the torque converter up with pressure and put the transmission in a lower gear... thus the features on the transmission...

Of course this is all done with electronics on the newer/modern engines/transmissions.. but the key here is to absorb the energy when traveling down a hill and add life to the brake when you need them...

I tow with a pickup/trailer at a combined weight of 23K lbs... and with the exhaust brake can go down hills like the Teton Pass west out of Teton National Park in 2nd or 3rd gear and never touch the brakes....

I'd go see why the shop didn't re-install the exhaust brake.... BTW as suggested above.. you should use it all the time...

Hope this helps your thought process...
"removed a pacbrake" I used poor wording. It was sitting on the bench with the worn out exhaust pipe and other things. It is being replaced because it was not operating due to corrosion. However, while it was "not working" the coach still slowed down due to the tranny downshifting.

That is why I asked the question.

I also thought that the annual service was supposed to lubricate things that moved.
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Old 06-18-2016, 10:29 AM   #8
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I also thought that the annual service was supposed to lubricate things that moved.
Because motor homes usually sit for long periods of time between uses, it's easy for parts to rust in place . In the case of the exhaust brake , once the main shaft is rusted , just lubing the correct points might not be enough to free it back up without disconnecting the activation cylinder and moving the valve shaft through its range of motion ; multiple times ; then road testing the coach to confirm proper, exhaust brake operation.

I used to have a diagram showing the Pac Brake lube points and can't find it at the moment , when I do I'll re-post.
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Old 06-19-2016, 10:03 AM   #9
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Hope this attachment works.


Maintenance | Pacbrake
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Old 06-19-2016, 09:36 PM   #10
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The exhaust gas temperatures can reach 1000 *F when the engine is pulling a steep hill... with that in mind... PacBake makes a specific lubricant to lube the moving parts including the butterfly pivot when the brake housing is off the vehicle... I know of no other lubricant that can withstand that temperature.... the newer PacBrakes are nitrated to limit the rust/corrosion from the moisture...

Yes a diesel engine will have some retarding ability... about 10% of its rated HP... which is almost non existent on a 6-7% grade... a functioning exhaust brake will produce about 80-85% of engine HP as retarding HP... usually sufficient enough to hold the weight of the coach on that 6-7% grade..

The brake on my truck, with the truck in the correct gear will hold my combined weight, roughly 22K lbs down the steep Teton Pass highway that leaves the park heading West...

Hope this helps..
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Old 06-23-2016, 03:01 PM   #11
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I strongly suggest that you use the PAC brake most all the time. Have to keep it active so it does not rust up & not function at all. I bet a new valve is big bucks.

I go down the grade as fast as I went up with the pac brake on! 90% of the downgrades I don't even touch the brakes! It's the people that go flying over the summit that find out real quick how important is is NOT to overheat the brakes.
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Old 06-24-2016, 11:42 AM   #12
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I strongly suggest that you use the PAC brake most all the time. Have to keep it active so it does not rust up & not function at all. I bet a new valve is big bucks.

I go down the grade as fast as I went up with the pac brake on! 90% of the downgrades I don't even touch the brakes! It's the people that go flying over the summit that find out real quick how important is is NOT to overheat the brakes.
I use the engine retarder system all the time. My actual learning was that I had a pacbrake. I suppose it was originally working when the trans retarder switch was on which was most of the time. The pacbrake was so corroded that I am not sure what environment it was designed to operate in. Same with the turbo gate valve. Both were rusted shut.

What that means to me is that the advice to keep the engine shutdown during storage is faulty. I will now run the engine monthly to exercise the turbo and the pacbrake. At least it will heat things up and dry them out.

So far it is about 3 grand in parts lost to corrosion. I expect the bill to be about 5 grand.
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Old 06-25-2016, 11:20 AM   #13
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[QUOTE=pcurt23;3130278]

What that means to me is that the advice to keep the engine shutdown during storage is faulty. I will now run the engine monthly to exercise the turbo and the pacbrake. At least it will heat things up and dry them out.

/QUOTE]
JMHO;
Water vapor is a product of combustion , and unless the coach is taken out on the road and driven for 25 miles + , the engine and exhaust system will not get to full operating temperature, running for a shorter time could just load the muffler with water and rust it from the inside. My coach sits for two extended periods a year , at home and in the south , because , many times the trip to and from the south means driving in snow and over roads that have been salted, I lube the exhaust brake and turbo waste gate linkages when I arrive in the south and again when I arrive home. I doubt that the two previous owners; of this coach; did this , but the turbo and exhaust brake are original, as is the muffler.
Due to assembly line damage of the turbo's waste gate activation pressure hose , the waste gate hadn't moved in the first 55,000 miles of the engines operation, I had to remove the turbo to extract the broken fitting for the hose and the waste gate was not seized, even though it had never moved in 12 years. Don't know where the previous owners stored the coach, maybe even inside.
I believe , storage environment ; high humidity; is a bigger factor in the rusting of items on coaches than any other factor, and I don't start the engine unless I intend to drive the coach for an extended run.
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