Hill Driving Question

Zephod · August 20, 2017, 12:38:36 PM

As you know, I drive a schoolbus. When I learned to drive a bus, nobody taught us how to drive in hills because there just weren't any worth mentioning. Having been driving I have found some undulating terrain which I feel would be beneficial to use engine control rather than braking. The hilly bits aren't long - I don't get brake fade but I know if my foot was off the brake, the bus would get out of control very quickly and with the sharp bends, turnovers etc are a possibility.

So, I have (on both my own and the work bus) an automatic transmission. I've never even used other than D, N and R is my car let alone a bus.

How does it work? If I'm going up a hill, can I with my foot still on the gas, slip the transmission from D to say 3 or even 2? Same for downhill, while I'm braking, can I slip the transmission from D to 3 or even 2? How about when I'm back on the flat and accelerating, can I slip it from 2 to 3 or 3 to D with my foot on the gas?

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Iceni John · August 20, 2017, 01:50:26 PM

If you have a non-lockup transmission such as an AT545, downshifting when descending won't help much because there's no direct mechanical connection between engine and wheels. However, even with the transmission locked, any diesel engine has less inherent braking than a gasoline engine because its throttle is effectively wide open all the time, unlike a gasoline engine that's trying to overcome pumping resistance against the partial vacuum created by the closed throttle.

My transmission locks in 2nd gear, i.e. it runs 1st C, 2nd C, 2nd L, 3rd L and 4th L. In 1st and 2nd C it doesn't hold the bus much on downgrades, but if I manually shift into 3rd and back to 2nd so it's then locked up in 2nd L its ability to hold against downhills is noticeably better. The Jake brake doesn't do much when in 1st and 2nd C, but works pretty well when the transmission is locked up.

The old advice was to descend in no higher a gear than you climbed (assuming the gradient is the same), and if in doubt just downshift to whatever gear holds your speed with minimal use of the brakes. If there's a line of impatient cars behind you when you descend, then that's their bad luck. I even use this technique now when driving my car - I can manually shift it in normal or Sport modes, so when descending the northbound Grapevine's 6% I use whatever gear will hold my speed to 60 MPH without touching the brakes at all. Every time I drive there I see cars with their brake lights on almost the whole way down, and by the bottom I can smell their brakes!

(Place your bets now if this thread will become a "discussion" between the merits of stab braking and continuous light braking!)

John

Oonrahnjay · August 20, 2017, 02:14:15 PM

Quote from: Iceni John on August 20, 2017, 01:50:26 PM... (Place your bets now if this thread will become a "discussion" between the merits of stab braking and continuous light braking!)

John

The first kernels are just beginning to go off in my popper ...

Zephod · August 20, 2017, 02:38:33 PM

According to Wikipedia, it could be AT545, MT643 or A2000. I did find the transmission warranty but no indication as to which transmission it is. I'd have to get underneath and have a look to see if I can find the maker's plate. That will have to wait until the critters have hibernated though. There are wasps underneath, ants and spiders - all of which are hazardous to the health until they decide to hibernate.

As far as stab vs continuous braking are concerned, I don't believe it makes a lot of odds in terms of heat build up or heat dissipation (which is what it's all about anyway).

On the dashboard before I mounted my camera monitor, there was a little sheet that didn't make a whole lot of sense about when to downshift. I have no idea where that sheet has got to. There was another sheet too that was totally illegible. Heaven knows what that said.

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Iceni John · August 20, 2017, 03:00:49 PM

The important thing is to manually shift the transmission when climbing and descending, to prevent lugging the engine and to better control one's speed. If you let the transmission shift all the time, you'll end up with an overheated engine or overheated brakes. I even keep my transmission in 3rd around town to prevent hunting between 3rd and 4th, and only shift it into Drive (4th) when going onto the freeway.

John

Zephod · August 20, 2017, 03:05:16 PM

Quote from: Iceni John on August 20, 2017, 03:00:49 PM
The important thing is to manually shift the transmission when climbing and descending, to prevent lugging the engine and to better control one's speed. If you let the transmission shift all the time, you'll end up with an overheated engine or overheated brakes. I even keep my transmission in 3rd around town to prevent hunting between 3rd and 4th, and only shift it into Drive (4th) when going onto the freeway.

John

I think the sheet said something about using it in 3 in town and 2 on grades, come to mention it. Why 3 in town baffles me.

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windtrader · August 20, 2017, 09:26:14 PM

Check the speedometer for shift/gear indications. The sppedo in my MC-8 with HT740 auto has shift points for the gears. Especially useful when going up and down hills and also in situations where it wants to hunt, you can take manual control over the shift point.

TomC · August 20, 2017, 11:48:55 PM

What year is your bus? The 2000 series came out around the year 2002 replacing the AT545. AT545 is an excellent transmission for around town, but with no torque converter lockup, is not very good at freeway speeds-that's why the 2000 series was brought out-it is a little brother of the 3000 and 4000 series. Good Luck, TomC

Zephod · August 21, 2017, 04:41:42 AM

Quote from: TomC on August 20, 2017, 11:48:55 PM
What year is your bus? The 2000 series came out around the year 2002 replacing the AT545. AT545 is an excellent transmission for around town, but with no torque converter lockup, is not very good at freeway speeds-that's why the 2000 series was brought out-it is a little brother of the 3000 and 4000 series. Good Luck, TomC

My own is a 1994 Carpenter. Dt466 and I have an Allison transmission booklet in the glove compartment so I'm assuming I have an Allison transmission.

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Branderson · August 21, 2017, 08:15:20 AM

Quote from: Iceni John on August 20, 2017, 03:00:49 PM
The important thing is to manually shift the transmission when climbing and descending, to prevent lugging the engine and to better control one's speed. If you let the transmission shift all the time, you'll end up with an overheated engine or overheated brakes. I even keep my transmission in 3rd around town to prevent hunting between 3rd and 4th, and only shift it into Drive (4th) when going onto the freeway.

John

I do the same thing. I keep it in 3rd around town and once I hit 52mph I shift into drive. Also if I go under 40 up a hill, I will shift into 2. If I'm on the free way in drive and I hit a hill and go under 52, I shift down to 3. My bus is slow as hell but she's steady. I assume everyone with a 6/92 knows my pain.

Branderson · August 21, 2017, 08:16:44 AM

Quote from: Zephod on August 20, 2017, 03:05:16 PM
I think the sheet said something about using it in 3 in town and 2 on grades, come to mention it. Why 3 in town baffles me.

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I was told my engine likes to run at higher rpm which is why I'm in 3 around town also, I turn on the higher idle switch as well.