Re: Trouble in MI. and then again in SD.
Reply #37 –
My 2 cents worth on the Allison World Transmission, which many of us have and some of us don't seem to understand. You should have an Allison Operators manual and have read it. Allison will also mail you a combination CD/DVD about the operation of their transmission, if you request it. It used to be free. Not very much about technique though. Try this link, or 800-252-5283. Ask for the driver training video DV5459EN Allison Transmission
Almost all of us have driven cars with automatic transmissions and that may be the cause of the misunderstanding about "what happens on downshifts". On most cars the transmission does not downshift until the engine RPM is just above idle unless the driver uses the shift lever to select a lower gear. This provides almost no engine braking effect, but it does help improve fuel economy by allowing one to coast when you take your foot off the gas.
The Allision World Transmission is designed for use in large trucks and other heavy vehicles. If you had been trained as a large truck driver on a truck with a manual transmission one of the first things you would do when slowing down is to downshift the transmission and use the engine to provide some of the braking. You would keep downshifting as the speed is reduced. Large diesel engines have a smaller RPM range than cars and need a transmission with many forward speeds in order to move a heavy load and still reach a decent road speed. The Allison World Transmission is designed to do what a truck driver with a manual transmission would do, but do it automatically. The driver still has some responsibility to know what is happening. Going up a mountain is no problem, select D and let the transmission do the work. The only reason for selecting a lower gear range would be for engine overheating or hunting between gears. Going down the mountain is a different situation and the transmission will need some driver input. The transmission has a throttle sensor and when you take your foot off the throttle this is sensed, it thinks that you want to slow down, and at the appropriate speed it will downshift. This actually aids in slowing your coach whether you have stepped on the brake or not. If you wanted the downshift to happen sooner you could use the down arrow to select a lower gear range. The transmission stays in this lower gear until you apply throttle and then it will shift up if not already in the highest selected gear. The transmission should not downshift though if doing so would cause the engine to over rev. As soon as a safe speed is reached the downshift should occur. Going down a mountain in a lower gear if you allow RPM to build to an engine overspeed condition, the transmission should upshift above the selected gear range to protect the engine. This will come as a big surprise as suddenly you have less engine braking and are picking up speed. About the only thing you can do is get on the brakes and slow down. A good driver will pay attention to engine RPM and not let it happen. When selecting a lower gear is necessary, I think the best procedure is to slow first and then select the lower gear. That way there is less of a chance of engine overspeed should the downshift occur when it should not.
Let's talk about the retarder. Not many Allison transmissions have it, most large trucks don't. Emergency vehicles like fire trucks and Foretravels seem to be the major users. Except for the fluid and the fact that it is bolted to the transmission it is almost a totally separate device. Internal rotational speed of the retarder is directly controlled by the road speed of the rear drive wheels. If the wheels aren't turning, it's not turning. The retarder provides braking to the rear drive wheels only. At slow speed even full on, this is almost no braking. The faster the wheels are turning the more effect it has. At 80 mph full on, it might have enough effect to cause the rear wheels to lose traction or skid, they wouldn't lock up as with regular brakes though. This is my imagination at work. I don't know if this would happen and don't plan to find out. Anyway you would definitely notice a strong braking effect. I have found that coming down a mountain that I can generally apply about 2 or 3 notches of retarder on the joy stick and downshift to a range that will kept the speed from increasing generally works well. Transmission temperature increases to about 225 and stays there. If speed begins to build, I can add a notch of retard for a moment or tap the brakes to slow down, but I don't ride the brakes. I try to keep the transmission temperature below 250. If I suddenly find the engine is about to over rev the last thing I would want to do is turn off the retarder. That would reduce braking and just make the situation worse. There is a safety device built into retarder operation; if the fluid temperature gets too high, retarding ceases as the fluid returns to the accumulator. Again this will come as a big surprise as braking suddenly decreases.
There has been so much discussion about brake fade that some of us may have become reluctant to use the brakes. Certainly this is a problem with large trucks with heavy loads and drum brakes. We aren't quite as heavy as those guys (does not apply to full timers towing a garage) and we have disk brakes that can dissipate heat faster and are not as likely to fade. This doesn't mean ride the brakes, but an occasional stab or application of the brakes is okay.
Topic: Trouble in MI. and then again in SD.