PRXB problem...

[polar76]

My 1997 CC Allure has a PRXB Pacbrake, exhaust brake which replace the original sometime around 2005.
The coach now has 67000 miles on it of which I have put about the last 36,000 on it.
The engine is an 8.3L Cummins (mechanical) with an Allison 3060 tranny.

The problem: I am getting almost no breaking from the PRXB.
It operates fine every time the key is turned on it cycles once and I have observed this.
The cylinder extends fully and the butterfly valve appears to close. (viewing from the outside of course).

I ran a test to see if it was getting a good signal when driving , by running a wire from the switch on the throttle linkage all the way to the drivers seat and took a test drive.

When the switch was engaged my meter would show 12 + volts. and I can hear a slight change in the engine noise but hardly any breaking if at all. Tried this from 5th all the way down to 2nd gear… essentially the same results.

Any suggestions before I pull the brake of to inspect it further???

[polar76]

UPDATE….
OK When activating the PRXB with a bypass switch .with the engine off nothing was leaking. So I started the engine and tested it again , air was pouring out from the seal between turbo and PRXB.
This was after I had removed it and checked for any damage to the unit. But it sounded like the air I have been hearing escaping prior to "working on it"
So I took it loose 3 more times trying to seat the brake against the turbo. Success on the last time, no more leak.
Still not happy with the amount of breaking power but it is an improvement for sure.
I will drive it as is for a couple of trips and see if I like it any better, then I will make the decision if I want to replace it or not.

Guys, Thanks for all your help…
and I am still open to suggestions and opinions as why it lost part of it's breaking power over the 5 years I have owned it.

[sjholt]

It might be the Quick release valve leaking. I had one go bad.

PacBrake Quick release valve C-10559

[wolfe10]

The sound of air escaping when the exhaust brake is applied could certainly indicate a leak upstream of the exhaust brake.

Don't understand how a solenoid or cylinder could cause that sound. If either were defective (or wiring to them) the exhaust brake would not close and there would be no back pressure and reasonably no sound of air escaping.

[polar76]

Update…
Did everything that was suggested could tell no difference.
So I just called Pacbrake and told him what was going on…
But something I haven't mentioned here is that I can here air escaping when PRBX is engaged.
I told him that and right off he said, "that is a problem"
This is my first exhaust brake and I could always here some air escaping even when it seemed to be working well. So I thought it was supposed to.

So this should be easy now, I will isolate the 12V circuit and apply a direct 12V source to make it operate, should be easy from there.
Either solenoid or cylinder.

Will update again later…
Thanks for your input.

[wolfe10]

Most turbo to exhaust brakes have a "V" clamp. On the Cummins C, turbo to PRXB clamp is torqued to 12 ft-lbs. PRXB to header pipe 15 lb-ft.


But,if the two mating surfaces have had exhaust leaking past, you may need to separate the exhaust brake from turbo and surface both surfaces. Easy to do with a large very flat piece of glass or similar very smooth item with a piece of wet dry sandpaper across it so absolutely even pressure applied to the turbo/exhaust brake mating surfaces. Start with 220 grit sandpaper and end up with 2000.

Yes, if this just started, you might just try to reseat the exhaust brake and torque the clamp to 15 lb-ft and see if that helps. Me, I would pull it apart and KNOW it is right. Cost you an extra hour and $4 worth of sandpaper.

[polar76]

Found what may be a leak ….turbo to exhaust brake.
The bolt on the clamp has some discolorazation and looks like a thin coating of soot on it.
If I can figure out how to post pictures I'll do so.

[polar76]

Thanks Brett
Will give a followup report when I know more….

[wolfe10]

Transmission problems on an Allison 3000 transmission are very rare.

If no exhaust leaks (i.e. no sign of black soot as discussed above), my next step would be to contact PacBrake and get the back pressure specs for your brake. Then temporarily install a gauge on a 6'+ hose connected to the pressure port on the PRXB, route the hose through a bedroom window or up through the bed and verify actual back pressures while driving.

[polar76]

Hey Scott and Brett
Thanks for the responses,
As far as the transmission goes , yes it downshifts and you can feel a little holding back but not a lot.
I will inspect the exhaust manifold and pipes today.
I did not see any signs of obvious leaks, but I wasn't really looking for them either.

I think an exhaust leak would be preferred offer a transmission problem for sure… LOL

[ScottG]

Brett brings up a good point.
It's common for the 8.3 to crack its exhaust manifold. If it opened up much you will be loosing a lot of pressure there.

[wolfe10]

Does the transmission downshift toward the pre-select gear?

Have you checked for exhaust leaks-- if a leak before the exhaust brake, it will NOT build back pressure? Look CAREFULLY for black exhaust trails: between head and exhaust manifold, exhaust manifold itself, exhaust manifold to turbo, turbo to exhaust brake.

If all the above is OK, there is a test port on the PacBrake where you can connect a pressure gauge to see what back pressure it is providing.

[ScottG]

It is possible for them to wear out to the point they leak so much that they dont do much but that is very rare.
I would be more inclined to make sure the torque converter is still locking up.