Cat C-9/Allison 3000MH TX Will Not Downshift On Very Steep Grades

[dicknkay]

2005 Country Coach pulls fairly steep grades OK. When doing 65mph the engine will increase rpm when speed drops about 3mph but on very steep hills will not downshift until speed drops under 40 mph. Down shifting manually does not work either until 2nd gear. All tests show everything OK. Boost pressure, fuel pump pressure,etc. all where they should be. Allison says it's an engine problem. The engine guys can't find anything wrong. What are they failing to check?

[wolfe10]

Welcome to the FMCA Forum.

Let me see if I understand: If you use the down arrow to select a lower gear, what happens? What SHOULD happen when you hit the down arrow once is that the transmission downshifts to the next lower gear (6th to 5th) assuming the shift would not over-speed the engine in the lower gear. If you hit the down arrow 4 times to select 2nd gear at highway speeds, it should drop one gear at a time until second is reached, but each downshift ONLY when it would not overspeed the engine in the next lower gear.

If you have the two window Allison shift pad, the left indicates the gear SELECTED, the right the actual gear you are in.

Is this a new problem or has it always existed?

Have you used the shift pad to check for diagnostic codes? Instructions in your Allison manual.

Brett

[dicknkay]

My shift pad is a single window. The problem has been going on for years and no one can fix it. The closest hill that will cause the problem is over 1 hour away so it is not easy to get a tech to spend about 3 hours or more to hook up a computer and take a ride with me.

To clarify what is happening: Traveling at 65mph with cruise control on, rpm about 1750. Start up the very step, long grade, about 7%min. Speed will drop quickly while rpm will drop but no shifting occurs until speed goes below 40mph. If I arrow down while speed is dropping the TX will not down shift until around speed gets to under 40 mph and 2nd kicks in. I then start to arrow up but I can't build up any speed until the grade starts to level out towards the top of the hill.

I've lived with this since most of the time the hills are no problem EXCEPT for the 7% or better grades. But, it annoys me to no end knowing that something is not right and no one can find it.

Thanks for your input

[wolfe10]

So, how low will engine RPM go before the downshift?

As you are driving other than on steep grades, does it shift properly (i.e. keeps RPM between peak torque and peak HP RPM)?

And again, have you used the shift pad to check for diagnostic codes?

[richbinkley]

curious what C-9 you have? 40 mph on the 7% grade seems about correct depending on the hp/torque you have. It might be frustrating but if the vehicle decels on that grade quickly the trans may skip down a few gears and go from 6th to 4th for example. I suggest contacting someone at Allison application engr ask if they can run what is called a scan performance run and that should include downshift performance.

good luck

rich

[dicknkay]

It's been a while since I've had a steep enough grade to cause the problem but I believe that rpm went as low as 1500. On all other hills the shifting works great. I lose only 1 to 3mph before rpm increases to keep climbing the grade. There are no codes in the TX.

[dicknkay]

my c-9 is 400hp with 1200ft-# of torque. I've talked to Allison before and they feel it's an engine problem. For some reason it will not do any downshifting on the 7% type of grade until under 40 mph.

[wolfe10]

my c-9 is 400hp with 1200ft-# of torque. I've talked to Allison before and they feel it's an engine problem. For some reason it will not do any downshifting on the 7% type of grade until under 40 mph.

40 MPH in 6th gear HAS to be well under the 1500 RPM you mention above. How low does engine RPM get at WOT (Wide Open Throttle) as you climb those steep grades?

[dicknkay]

This weekend I'll be traveling on a road that has the grade that gives me the trouble and will get accurate data on engine performance. Will post this info asap.