Please help PacBrake 05 Knight 330 ISC

Flagelpater · 11-23-2009, 07:31 PM

I have asked this question before but not had an answer.

I know my PacBrake valve is operating, open and close, properly. The Allison 3000 does downshift. But, when on a 7% downgrade the Allison shifts back up to a higher gear as it reaches 2200 RPM. I know this is the Gov RPM on the Allison ECB. Now I end up at 1800 RPM and need to use the service brakes a lot.

Question: Is there a higher RPM Limit for Exhaust Brake operation while on a down grade?

On a previous coach I was sure it went to 2600 RPM for Exhaust Brake operation.

Can someone tell me what RPM their unit operates at? Thank you.

Sammie · 11-23-2009, 07:46 PM

Why don't you try downshifting manually using the Allison keypad? Find a gear that holds you back to the speed you want and let the pacbrake do its job then apply your brakes as needed.

Sammie

Flagelpater · 11-23-2009, 08:03 PM

As I remember it, my 04 Knight, I could just let the Allison take care of the shifting, it did a great job. But I think the RPM limit was much higher, around 2600 RPM. I have been talking to Allison, Cummins, PacBrake and Monaco. They all tell me to ask the other guy! What I would really like is for an operator of a 330 ISC to tell me what their unit maintains for RPM while going down a steep grade.

Diplomat Don · 11-23-2009, 08:27 PM

Flagelplater......Here is my experience with my Pacbrake. My exhaust brake will turn on, but won't engage below 62 mph. On a 7% grade at 55 mph with my Pacbrake on, my coach will pick up speed and eventually upshift. The exhaust will not hold it back. If I want to stay around 55 mph I have to stab the brakes every so often to bring the speed down. If I want to descend that same 7% grade, without braking, I have to be under 45 mph (I believe it drops into third at this speed) for my exhaust brake to hold. It will then continue to slow me down even more.

The exhaust brakes are just not strong or adjustable enough to really hold a desired speed on a grade. The only way you can do that is to have an engine brake that has 2 or 3 positions.

Downhill speed with an exhaust brake has many variables that you just have to get use to and know what it will do.

Here is a tidbit I learned on a summer trip. We found ourselves descending a 10% grade with our toad in tow. I couldn't slow the coach enough and hold a gear. I started to overheat the brakes. I stopped the coach on the grade in a turnout and waited ofr them to cool. Later, I started off again in first gear and hoped it would hold long enough before it started upshifting. To my surprise, the Allison won't upshift out of first and the exhaust brake/trans held me all the way down the hill.

Hope this answers your question.

Flagelpater · 11-23-2009, 08:37 PM

Thank you Don. Can you tell me if the RPM limit is higher while unloaded with the pack brake on? What RPM does your unit shift at?

Diplomat Don · 11-23-2009, 08:58 PM

Flagelpater....I can tell you that mine will rev higher with the exhaust brake on than the engine will allow under acceleration, but I couldn't tell you the numbers. We'll be climbing/descending the I-5 grade out of L.A. (Grapevine) tomorrow night. I'll try and make a mental note.

Flagelpater · 11-23-2009, 09:07 PM

I will be forever indebted to you Don. I can also remember being in a seminar by Allison while at the Monaco Elkhart facility, and the presenter told us about a higher RPM. The point was that after the transmission shifted into 6th gear it was our job to maintain MAX RPM with the service brakes. I can't find the RPM listed anywhere! You would think that if it is "Our Job" it would be in Bold Print! As you know 6th gear and Max RPM is going mighty fast, unless you are on a very steep grade. And that is what I am worried about. Thank you.

Rex · 11-23-2009, 10:16 PM

You may be confused about WHEN you are supposed to maintain higher RPM. It is when going UP a grade. The coach will naturally slow and you should manually downshift it to maintain 2000 to 2200 RPM to keep the engine from overheating. Going down hill, you should downshift to whatever gear you need using the pack brake that will hold your speed so you don't have to use the service break. That may in fact be first gear, depending on the grade down. You never want high RPM going downhill.

Happycarz · 11-23-2009, 11:47 PM

My 8.3, my ISC & my ISL all would hold to 2650 rpm before upshifting to protect the engine. So, your memory is correct about your previous coach. Yours upshifting at 2200 is not normal. It should be higher, giving you more braking horsepower.
I would think either Allison or Cummins could handle the reprogramming of the software. Call a couple of other Cummins and Allison shops and ask if they can reprogram the "engine over speed upshift, as it is 2200 rpm instead of 2650 rpm."

Flagelpater · 11-24-2009, 10:13 AM

Thank you happycarz, and all others that responded. I am thinking that the check box, if there is such a thing, for Exhaust Brake "Yes" or "No" is checked NO. Monaco said they just install the computer, they don't program it. I know the Pacbrake valve opens and closes and the Allison downshifts. It is just that the Allison seems to be protecting 2200 RPM, with an up-shift, and not 2650 RPM. So far I have not found a person at either Allison or Cummins that can tell me what the official name for the 2650 RPM Limit is. If I had that name I could probably make more headway. One other question for happycarz. My Allison controller is located in the bay forward of the roadside front tire. It has the value 2200 listed for GOV RPM, does your unit have 2200 or 2650?

Happycarz · 11-24-2009, 04:43 PM

Mine is the same as yours, 2200 RPM. If you were to push the throttle to the floor in neutral this is the highest RPM you could attain. Same for the upshift.

I just called Massey's RV and Diesel Repair, but Tom, the Service Mgr., had left for the day. I'll call him in the morning and get his take on the matter.