Exhaust Brake performance

[boguslaw]

Can someone answer what seems to be an easy question? When I use my exhaust brake, the transmission shifts to 4th (from 6th), causing a lot of slowing. This is great when I need to go 40 mph or slower, but not so great when I'm on the freeway going down a grade and want to hold around 60 mph. Is there a way to have the exhaust brake do its thing without the big downshift? I don't want to use the service brakes all the time, but I can't seem to work the exhaust brake into a usable role while on the freeway.

I just got this coach late last summer and all of this is fairly new to me. I have a '07 Country Coach Tribute with a Cat C9 with the Allision 3000 transmission.

Jim Boguslaw

[tbutler]

Welcome Jim,

You'll get a more complete answer from others I'm sure. In general, what you are describing is exactly what should happen with the exhaust brake. With our coach the hard downshift doesn't occur immediately, there is usually a period of time before the transmission downshifts. When you take the brake off the transmission will upshift in response to engine RPM. You can downshift without using the exhaust brake, just use the down arrow on the transmission. If the engine RPM is in a range that allows the transmission will downshift. You can not damage the transmission by using the downshift arrows as the transmission will not allow the engine RPM to go higher than is safe for the engine. Downshifting from 6th gear to 5th gear will slow acceleration going down hill without the engine brake.

In ordinary hills, I seldom use the engine brake. As long as your speed remains in a safe and legal range, let the coach accelerate and then let the next up hill stretch slow the coach to your desired speed. To use the brakes excessively is a waste of energy. Try downshifting to control the speed and then use the engine brake if necessary to hold speed where you want. If you do use the engine brake, release it before you reach the very bottom of the hill to make the best of the acceleration the hill can give you. With a little practice you can use this technique to improve your fuel mileage.

[wolfe10]

Country Coach told Allison what "Pre-select" gear they wanted programmed into the transmission ECU. The pre-select gear it that gear that the transmission will shift toward when the exhaust brake is selected. Actually, it is the gear toward which the transmission will downshift, one gear at a time as soon as speed/engine RPM are low enough that a downshift will not overspeed the engine. The Allison will never downshift in such a manner to harm the drivetrain.

2nd and 4th are typical pre-select gears. Any Allison dealer can re-program the ECU with any pre-select gear you choose (around $100). I had mine programmed for 5th gear for the reason you are describing. In fact, coming down the 5% grade into Benson Az last week 5th gear with exhaust brake on was just perfect.

There is no "perfect" choice for the pre-select gear. When coming to a stop, 2nd is great (but is not ideal for highway driving). On the freeway, a 5th gear pre-select is excellent, but requires more driver input (use of down arrow) in slower conditions. So it is an individual decision.

As Tom said, you can also use the transmission down arrows without engaging the exhaust brake. Again, you can not mess anything up-- down arrow 5 times to select 1st gear at 60 mph and the ECU will interpret this as "Please downshift one gear at a time until 1st gear is reached, but only downshift when it will not overspeed the engine in the next lower gear".

Brett Wolfe

[boguslaw]

Thanks to both of you for your responses. I think I'll feel more confident using the equipment the way it was intended. Jim B